JP3892345B2

JP3892345B2 - Microcurrent measuring device

Info

Publication number: JP3892345B2
Application number: JP2002167099A
Authority: JP
Inventors: 直司鈴木
Original assignee: Advantest Corp
Current assignee: Advantest Corp
Priority date: 2002-06-07
Filing date: 2002-06-07
Publication date: 2007-03-14
Anticipated expiration: 2022-06-07
Also published as: JP2004012330A

Description

【０００１】
【発明の属する技術分野】
この発明は例えば被測定対象として絶縁体の絶縁抵抗を測定することに用いられる微小電流測定装置に関し、特に測定回路が被測定対象に電気的に接触していることを判定することができるコンタクトチェック機能を簡素な構成で提供しようとするものである。
【０００２】
【従来の技術】
図１６に特開平５−８７８５８号公報に開示されたコンタクトチェック機能を装備した微小電流測定装置２の構成を示す。この微小電流測定装置２では被測定対象３の抵抗値Ｒ_xを測定する場合にはスイッチＳＷをオフにし、交流信号発生源６を測定回路から切り離し、微小電流計４の計量値ＭＶ₁と直流電圧源１の値Ｅとから被測定対象３の抵抗値Ｒ_xを得る。
被測定対象３と接触子Ｇ₁、Ｇ₂との接触を判定する場合には、交流信号発生源６を測定回路に投入する。ここで、被測定対象３が正常に接触子Ｇ₁とＧ₂に接触していれば、回路電流はトランス７、Ｉ／Ｖ変換器８１、波高検出器８２を介して電圧計８３により回路電流が測定され、この電流値と交流信号発生源６の電圧値とから、被測定対象３の静電容量Ｃ_xが測定される。もし、被測定対象３が接触子Ｇ₁、Ｇ₂と非接触であれば、トランス７にはほとんど電流は流れず、接続判定回路５は被測定対象３と接触子Ｇ₁、Ｇ₂とが非接触であると判定する。
【０００３】
【発明が解決しようとする課題】
図１６に示したコンタクトチェック機能を装備した微小電流測定装置２ではコンタクトチェック時にオンの状態に操作するスイッチＳＷが差動増幅器４１の入力端子に接続されているため、このスイッチＳＷとしてはオフ抵抗が大きい例えばリレーのような機械式接点スイッチである必要がある。つまり、このスイッチＳＷに半導体スイッチを用いたとすると、半導体スイッチはオフ抵抗が有限であるため、スイッチがオフの状態でもリーク電流が流れ、特に被測定対象３の抵抗を測定する場合の測定精度を悪化させる。
【０００４】
また、機械式接点スイッチを用いた場合、切替え動作が遅いことと、寿命の点で問題がある。
更に、図１６に示した測定装置ではトランス７、Ｉ／Ｖ変換器８１、波高検出器８２等のコンタクトチェックのための回路構成が大きいため、コストが掛かる欠点がある。
また、図１６に示した微小電流測定装置２は等価的に図１７に示す回路構成で被測定対象３を流れる電流ｉ_xを電圧値ＭＶ₁に変換している。図１７に示す回路構成によれば、直流電圧源１の電圧をＥ、被測定対象３の抵抗値をＲ_x、電流検出用抵抗器４０の抵抗値をＲ_sとした場合、被測定対象３を流れる電流ｉ_xは
ｉ_x＝Ｅ／（Ｒ_x＋Ｒ_s）
で求められる。
【０００５】
被測定対象３を流れる電流ｉ_xは本来ｉ_x＝Ｅ／Ｒ_xで求めるべきであるが、分子に電流検出用抵抗器４０の抵抗値Ｒ_sが加算されてしまうため、これが誤差となる欠点がある。
この発明の目的はコンタクトチェックを高速化するために交流電圧印加用のスイッチを半導体スイッチを用いても被測定対象の抵抗値を精度良く測定することができ、また、誤差が発生することなく微小電流を測定することができ、然もコンタクトチェックのための回路構成が簡素な微小電流測定装置を提供しようとするものである。
【０００６】
【課題を解決するための手段】
この発明の請求項１では、演算増幅器の反転入力端子に抵抗器を介して、バッファ増幅器を接続し、上記演算増幅器の出力端子と上記バッファ増幅器の入力端子との間に帰還用抵抗器と、帰還用コンデンサを並列接続し、被測定対象の抵抗値をＲ_x、帰還用抵抗器の抵抗値をＲ_f、被測定対象を流れる電圧をｉ_x、上記演算増幅器の直流出力電圧をＶ_DCとした場合、Ｖ_DC＝−Ｒ_f・ｉ_xにより被測定対象を流れる電流ｉ_xを求める微小電流測定装置において、
上記演算増幅器の反転入力端子にスイッチを通じて交流電流を印加し、この交流電流の印加状態で上記演算増幅器の出力端子に出力される交流電圧Ｖ_ACが、予め定めた所定の値より大きいことを検出して上記被測定対象にこの被測定対象の絶縁抵抗を測定するための直流電圧源及び上記バッファ増幅器の入力端子が接触していると判定する微小電流測定装置を提案する。
【０００７】
この発明の請求項２では、演算増幅器の反転入力端子に抵抗器を介して、バッファ増幅器を接続し、上記演算増幅器の出力端子とバッファ増幅器の入力端子との間に容量値Ｃ_fを持つ積分用コンデンサを接続し、この積分用コンデンサに所定の時間Ｔに積分される直流電圧Ｖ_DCを測定し、これら直流電圧Ｖ_DC、積分用コンデンサの容量値Ｃ_f、時間Ｔから上記被測定対象を流れる電流ｉ_xをＶ_DC＝−ｉ_x・Ｔ／Ｃ_fにより求める微小電流測定装置において、
上記演算増幅器の反転入力端子にスイッチを通じて交流電流を印加し、この交流電流の印加状態で上記演算増幅器の出力端子に出力される交流電圧Ｖ_ACが、予め定めた所定の値より大きいことを検出して上記被測定対象にこの被測定対象の絶縁抵抗を測定するための直流電圧源及び上記バッファ増幅器の入力端子が接触していると判定する微小電流測定装置を提案する。
【０００８】
この発明の請求項３では、演算増幅器の反転入力端子に抵抗器を介して、バッファ増幅器を接続し、上記演算増幅器の出力端子と上記バッファ増幅器の入力端子との間に帰還用抵抗器と、帰還用コンデンサを並列接続し、被測定対象の抵抗値をＲ_x、帰還用抵抗器の抵抗値をＲ_f、被測定対象を流れる電圧をｉ_x、上記演算増幅器の直流出力電圧をＶ_DCとした場合、Ｖ_DC＝−Ｒ_f・ｉ_xにより被測定対象を流れる電流ｉ_xを求める微小電流測定装置において、
上記演算増幅器の反転入力端子にスイッチと抵抗器を通じて直流バイアス源を接続し、スイッチオフ時の上記演算増幅器の出力電圧Ｖ_DC1とスイッチオン時の上記演算増幅器の出力電圧Ｖ_DC2との差Ｖ_DC2−Ｖ_DC1の値が予め定めた値より大きい状態で上記被測定対象にこの被測定対象の絶縁抵抗を測定するための直流電圧源と上記バッファ増幅器の入力端子が接続されていると判定する微小電流測定装置を提案する。
【０００９】
この発明の請求項４では、演算増幅器の反転入力端子に抵抗器を介して、バッファ増幅器を接続し、上記演算増幅器の出力端子とバッファ増幅器の入力端子との間に容量値Ｃ_fを持つ積分用コンデンサを接続し、この積分用コンデンサに所定の時間Ｔに積分される直流電圧Ｖ_DCを測定し、これら直流電圧Ｖ_DC、積分用コンデンサの容量値Ｃ_f、時間Ｔから上記被測定対象を流れる電流ｉ_xをＶ_DC＝−ｉ_x・Ｔ／Ｃ_fにより求める微小電流測定装置において、
上記演算増幅器の反転入力端子にスイッチと抵抗器を通じて直流バイアス源を接続し、スイッチオフ時の上記演算増幅器の出力電圧Ｖ_DC1とスイッチオン時の上記演算増幅器の出力電圧Ｖ_DC2との差Ｖ_DC2−Ｖ_DC1の値が予め定めた値より大きい状態で上記被測定対象にこの被測定対象の絶縁抵抗を測定するための直流電圧源と上記バッファ増幅器の入力端子が接続されていると判定する微小電流測定装置を提案する。
【００１０】
この発明の請求項５では、請求項１又は２記載の微小電流測定装置の何れかにおいて、バッファ増幅器の交流出力電圧と演算増幅器の交流出力電圧を差動増幅器に入力し、この差動増幅器から出力される交流の差電圧を測定し、この差電圧により直流電圧源とバッファ増幅器の入力端子が被測定対象に接触しているか否かを判定する構成とした微小電流測定装置を提案する。
【００１１】
この発明の請求項６では、請求項３又は４記載の微小電流測定装置において、バッファ増幅器の出力電圧と演算増幅器の出力電圧を差動増幅器に印加し、この差動増幅器からスイッチのオン時とオフ時の電圧を測定し、この電圧の差が予め定めた値より大きい状態で直流電圧源とバッファ増幅器の入力端子が被測定対象に接触していると判定する微小電流測定装置を提案する。
【００１２】
この発明の請求項７では請求項１乃至６記載の微小電流測定装置の何れかにおいて、バッファ増幅器の入力端子と被測定対象との間を接続する導線部分に絶縁層を介して外部導体を被覆し、この外部導体をバッファ増幅器の反転入力端と出力端子の共通接続点に接続した微小電流測定装置を提案する。
【００１３】
作用
この発明によれば演算増幅器の前段にバッファ増幅器と抵抗器を介挿し、バッファ増幅器によりインピーダンス変換して演算増幅器の反転入力端子に抵抗器を介して接続すると共に、演算増幅器の反転入力端子にスイッチを介してコンタクトチェック用の交流電流を印加する構成としたから、演算増幅器の入力端子は低インピーダンスに整合されている。この結果、コンタクトチェック用の交流電流を印加するためのスイッチに多少のオフ抵抗が存在しても、その影響は微小値となる。
【００１４】
【発明の実施の形態】
図１にこの発明の一実施例を示す。図１において１は既知の直流電圧Ｅを発生する直流電圧源、３は被測定対象、１００はこの発明による微小電流測定装置、Ｇ₁、Ｇ₂は被測定対象３に直流電圧源１と微小電流測定装置１００を電気的に接続する接触子を示す。因みに接触子Ｇ₁、Ｇ₂と被測定対象３との関係を図２に示す。被測定対象としては多くの場合絶縁材料とされ、この絶縁材料の絶縁抵抗を測定するために、微小電流測定装置１００が用いられる。図２に示す例では接触子Ｇ₁が上下に可動自在に保持され、Ｇ₁とＧ₂の間に被測定対象３を配置し、接触子Ｇ₁を押し下げて被測定対象３に接触させた状態で被測定対象３を流れる電流ｉ_xを測定する構造とされる。
【００１５】
この発明による微小電流測定装置１００は演算増幅器１０１の前段側にバッファ増幅器１０２を配置した構成を特徴とするものである。バッファ増幅器１０２としては反転入力端子と非反転入力端子とを持つ演算増幅器の反転入力端子と出力端子との間を直結して構成することができる。この構造によれば利得が「１」で入力インピーダンスが高く、出力インピーダンスが低いバッファ増幅器を得ることができる。
バッファ増幅器１０２の非反転入力端子に接触子Ｇ₂を接続し、バッファ増幅器１０２の出力端子を抵抗器１０３を通じて演算増幅器１０１の反転入力端子に接続する。演算増幅器１０１の出力端子とバッファ増幅器１０２の非反転入力端子との間には帰還用抵抗器１０４と帰還用コンデンサ１０５を並列接続し、演算増幅器１０１の出力端子と共通電位との間に直流電圧計１０６と交流電圧計１０７とを並列に接続する。
【００１６】
更に、この発明では演算増幅器１０１の反転入力端子と共通電位との間にコンタクトチェック回路１０８を接続する、コンタクトチェック回路１０８はスイッチ１０９と抵抗器１１１と、交流電圧源１１２とから成る直列回路で構成される。この直列回路は交流電圧源１１２と直列に抵抗器１１１を接続していることから、交流電流源として見ることができ、演算増幅器１０１の反転入力端子には等価的に電流源が接続されていると見ることができる。
被測定対象３を流れる微小電流ｉ_xの値を測定する場合スイッチ１０９はオフ、コンタクトチェックを行う場合はスイッチ１０９をオンの状態に制御する。
【００１７】
図３に図１に示した回路の等価回路を示す。バッファ増幅器１０２は非反転入力端子に信号を入力し、出力端子からその入力信号を出力する構成であるため、入力と出力端子間は同相である。従って、等価的には演算増幅器１０１のみが見える。直流電圧源１が発生する電圧をＥ、被測定対象３の抵抗値をＣ_X、帰還用抵抗器１０４の抵抗値をＲ_f、帰還用コンデンサ１０５の静電容量値をＣ_f、抵抗器１０３と１１１の抵抗値をそれぞれＲ₁、Ｒ₂、演算増幅器１０１の直流出力電圧をＶ_DC、交流電圧源１１２が発生する交流電圧をｅ、演算増幅器１０１の出力端子に出力される交流出力電圧をＶ_ACとすると、
１．スイッチ１０９がオフのとき（直流電流測定時）
Ｖ_DC＝−（Ｒ_f／Ｒ_X）Ｅ＝−Ｒ_f・ｉ_x
Ｖ_DCを測定することで被測定対象３を流れる電流ｉ_xを求めることができる。電流ｉ_xが求められることにより絶縁抵抗Ｒ_xはＲ_x＝Ｅ／ｉ_xで求めることができる。従って、誤差を発生させることなく絶縁抵抗Ｒ_xを測定することができることになる。Ｒ₁＝Ｒ₂とすると、
２．スイッチ１０９がオンのとき（コンタクトチェック時）
Ｖ_AC＝−（１＋Ｚ_f／Ｚ_x）ｅ
ここで、Ｚ_x＝Ｒ_x／（１＋ｊωＣ_x・Ｒ_x）
Ｚ_f＝Ｒ_f／（１＋ｊωＣ_f・Ｒ_f）
ωＣ_x＜＜Ｒ_x、ωＣ_f＜＜Ｒ_fとなるように交流電圧源１１２が発生する交流電圧の周波数を設定することにより、演算増幅器１０１の出力端子に出力される交流電圧Ｖ_ACは、
Ｖ_AC＝−（１＋Ｃ_x／Ｃ_f）ｅ
で表わされる。
【００１８】
つまり、コンタクトチェックは被測定対象３が接触子Ｇ₁、Ｇ₂の間に接続されていない状態の静電容量Ｃ_stを予め測定しておき、被測定対象３を接続した状態の静電容量と比較して判定する。
被測定対象３が接続されていない状態の交流出力電圧をＶ_AC1とすると、交流出力電圧Ｖ_AC1は
Ｖ_AC1＝−（１＋Ｃ_st／Ｃ_f）ｅ
被測定対象３を接続した状態の交流出力電圧をＶ_AC2とすると、交流出力電圧Ｖ_AC2は、
Ｖ_AC2＝−（１＋（Ｃ_st＋Ｃ_x）／Ｃ_f）ｅ
Ｖ_AC2とＶ_AC1との差が被測定対象３の静電容量Ｃ_xとなるから、
Ｖ_AC＝Ｖ_AC2−Ｖ_AC1＝−（Ｃ_x／Ｃ_f）ｅ
が得られる。
【００１９】
現実には、交流出力電圧Ｖ_AC2がＶ_AC1より大きく、更に予め設定した値（例えば交流出力電圧Ｖ_AC2の値より数％程度小さい値）より大きければ接触子Ｇ₁とＧ₂が被測定対象３に接触していると判定することができる。
ここでスイッチ１０９に半導体スイッチを用いた場合、そのオフ抵抗値（オフ状態にある半導体スイッチの抵抗値）をＲ_off、抵抗器１０３の抵抗値をＲ₁、抵抗器１１１の抵抗値をＲ₂、演算増幅器１０１の反転入力端子に発生するオフセット電圧をＶ_offとした場合、コンタクトチェック回路１０８が接続されたことにより微少電流測定時に演算増幅器１０１の出力側に発生する電圧成分Ｖ_NOisは、
Ｖ_NOis＝（Ｒ₁・Ｖ_off／（Ｒ₂＋Ｒ_off））（１＋Ｒ_f／Ｒ_x）
で表わされる。
【００２０】
オフセット電圧Ｖ_offは元々数ｍボルト程度であり、Ｒ₁＜＜Ｒ₂＋Ｒ_offであることが一般的であるため、電圧成分Ｖ_NOisはＶ_NOis≒０となる。
従って、この発明によればスイッチ１０９を半導体スイッチに置換しても微小電流測定に大きな誤差を発生させることがなく、精度よく微小な直流電流を測定することができ、またコンタクトチェックのための回路も簡素に構成することができる利点が得られる。
図４はこの発明の他の実施例を示す。この実施例では演算増幅器１０１の出力端子とバッファ増幅器１０２の入力端子との間に積分用コンデンサ１１３を接続し、この積分用コンデンサ１１３に積分される直流電圧Ｖ_cにより被測定対象３を流れる微小電流ｉ_xを測定する構成とした場合を示す。積分用コンデンサ１１３には並列にリセットスイッチ１１４が接続され、このリセットスイッチ１１４により積分用コンデンサ１１３に積分された積分電圧Ｖ_cを放電させ、リセット状態にする。
【００２１】
リセット状態から一定の時間Ｔ（図５Ａ参照）を定め、この時間Ｔの間に積分される電圧Ｖ_cを測定する。電圧Ｖ_cは、
Ｖ_c＝−ｉ_x・Ｔ／Ｃ_f
で表わされ、電圧Ｖ_cが直流電圧計１０６で測定することにより時間Ｔと積分用コンデンサ１１３の容量値Ｃ_fが既知であるから被測定対象３を流れる電流ｉ_xを求めることができる。
【００２２】
コンタクトチェック時にはスイッチ１０９をオン、リセットスイッチ１１４をオフに制御し、演算増幅器１０１の反転入力端子に交流電圧源１１２から交流電圧ｅを印加する。演算増幅器１０１の出力端子にはスイッチ１０９がオンの期間中図６Ａに示すように交流信号ＡＣが出力される。この交流信号ＡＣの電圧Ｖ_ACを交流電圧計１０７で測定し、その電圧値が設定値より大きければ被測定対象３に接触子Ｇ₁とＧ₂が接触していると判定する。
尚、直流電流ｉ_xが極端に大きい場合のコンタクトチェック時には図７に示すように、直流出力電圧Ｖ_DCが飽和状態に至り、交流電圧計１０７では交流電圧Ｖ_ACを測定できない状態に至るが、この場合は直流電流ｉ_xが充分流れていることから、接触子Ｇ₁とＧ₂が被測定対象３に接触していることが解る。
【００２３】
図８はこの発明の更に他の実施例を示す。この実施例ではコンタクトチェック時に演算増幅器１０１の出力とバッファ増幅器１０２の出力を差動増幅器１１５で差を求める構造とした場合を示す。差動増幅器１１５で差の電圧Ｖ_aを求めることにより、差の電圧Ｖ_aは、
Ｖ_a＝（Ｃ_x／Ｃ_f）ｅ
で求められる。
【００２４】
この式から明らかなように、この実施例によれば交流電圧Ｖ_aと被測定対象３の静電容量Ｃ_xとの間の関係が比例関係となり、交流電圧Ｖ_aから静電容量Ｃ_xの値を読取るように構成する場合に有効である。
図９は図８に示した実施例を積分型の微小電流測定装置に適応した実施例を示す。この場合もスイッチ１０９と１１４をオフに制御して被測定対象３を流れる電流ｉ_xを測定し、スイッチ１０９をオン、リセットスイッチ１１４をオフに制御してコンタクトチェックを行う。
コンタクトチェック時に交流電圧計１０７には差動増幅器１１５からバッファ増幅器１０２の出力と演算増幅器１０１の出力の差の電圧Ｖ_aを印加する。この場合も、差の電圧Ｖ_aはＶ_a＝（Ｃ_x／Ｃ_f）ｅで求められるため、電圧Ｖ_aの値と、静電容量値Ｃ_xの値が比例関係となり、電圧Ｖ_aから静電容量Ｃ_xを読み取るような場合に適用して好適である。
【００２５】
図１０はこの発明の更に他の実施例を示す。この実施例ではコンタクトチェック時に静電容量Ｃ_xを求めるために用いた交流電圧源１１２の代わりに直流バイアス源１１６を接続し、スイッチ１０９がオフの状態の直流出力電圧Ｖ_DC1と、スイッチ１０９をオンにした状態の直流出力電圧Ｖ_DC2を測定し、その差の電圧Ｖ_aDCからコンタクトチェックを行う構成とした場合を示す。
微小電流測定時は上述と同様にスイッチ１０９をオフにし、このとき演算増幅器１０１の出力に発生する直流出力電圧Ｖ_DCを測定して被測定対象３を流れる電流ｉ_xを、
Ｖ_DC＝−Ｒ_f／ｉ_x
により求める。
【００２６】
一方コンタクトチェック時は、スイッチ１０９を図１１Ａに示すようにオフの状態とオンの状態に切り替える。つまり、スイッチ１０９がオフの状態（時点ｔ₁）で直流電圧計１０６が指し示す直流出力電圧Ｖ_DC1を測定する。次に、スイッチ１０９がオンの状態（時点ｔ₂）で直流電圧計１０６の直流出力電圧Ｖ_DC2を測定する。
これらの直流出力電圧Ｖ_DC1とＶ_DC2の差の電圧Ｖ_aDCは、
Ｖ_aDC＝Ｖ_DC2−Ｖ_DC1＝−（１＋Ｃ_x／Ｃ_f）ｅ_DC
で求められる。この式から被測定対象３の静電容量Ｃ_xを求めることができる。この場合、差の電圧Ｖ_aDCの値が大きい程、静電容量Ｃ_xが大きいことを表わしており、Ｃ_xの値が設定値より大きいことにより、接触子Ｇ₁とＧ₂が被測定対象に接触していると判定することができる。
【００２７】
図１２は積分型の微小電流測定装置に図１０に示した実施例を適用した場合を示す。この場合にもスイッチ１０９がオフの状態と、オンの状態で演算増幅器１０１の直流出力電圧Ｖ_DC1とＶ_DC2の直流電圧計１０６で測定し、その差Ｖ_DC2−Ｖ_DC1を演算し、被測定対象３の静電容量Ｃ_xを求め、この静電容量Ｃ_xの値が設定値より大きければ接触良と判定する。
図１３は図１０に示した実施例に差動増幅器１１５を付加した実施例を示す。図１３に示す実施例では微小電流の判定を演算増幅器１０１と帰還用抵抗器で構成される電流測定装置で測定し、コンタクトチェックをスイッチ１０９のオンとオフ時の直流出力電圧Ｖ_DC1とＶ_DC2で判定する。
【００２８】
ここでの特徴は、コンタクトチェック時に直流出力電圧の差Ｖ_DC2−Ｖ_DC1が
差動増幅器１１５の作用により
Ｖ_DC2−Ｖ_DC1＝−（Ｃ_x／Ｃ_f）ｅ_DC
で得られる点を特徴とするものである。
図１４は図１２に示した積分型電流測定装置で微少電流を測定し、コンタクトチェックをスイッチ１０９のオンとオフ時の直流電圧Ｖ_DC1とＶ_DC2で判定する場合を示す。この実施例でもＶ_DC2−Ｖ_DC1は
Ｖ_DC2−Ｖ_DC1＝−（Ｃ_x／Ｃ_f）ｅ_DC
で求められる。
【００２９】
図１５はこの発明の更に他の実施例を示す。この実施例では微小電流測定装置１００の入力端子ＩＮと接触子Ｇ₂との間を接続する導線１１７と共通電位間に寄生する静電容量Ｃ_cbの影響を除去しようとする実施例を示す。
つまり、図１６に示すように微小電流測定装置と接触子Ｇ₂との間はケーブルによって接続されるが、このケーブルとなる導線１１７と共通電位間に静電容量Ｃ_cbが形成されると、この静電容量Ｃ_cbは直流電圧源１が交流的にはショート状態であるため被測定対象３の静電圧容量Ｃ_xに並列接続されることになる。
このために、静電容量Ｃ_cbが大きくなると被測定対象３の静電容量Ｃ_xの測定値に誤差が発生し、コンタクトチェックの信頼性が低下する恐れがある。
【００３０】
このため、この図１５に示す実施例では導線１１７に絶縁層を介して外部導体１１８を被覆し、この外部導体１１８をバッファ増幅器１０２の反転入力端子及び出力端子の共通接続点に接続した構造とするものである。
この構造にすることにより導線１１７と外部導体１１８との間に静電容量Ｃ_cbが形成されるが、この場合には導線１１７と外部導体１１８との間には電位差が与えられないから、静電容量Ｃ_cbによる影響は全く受けることはない。
つまり、バッファ増幅器１０２の反転入力端子と非反転入力端子間の電位は演算増幅器の機能により常に同電位に維持されるから、導線１１７と外部導体１１８との間の電位も同電位に維持される。この結果、導線１１７と外部導体１１８との間に静電容量Ｃ_cbが存在しても、この静電容量には導線１１７から充電電流及び放電電流が流れることはない。この結果として静電容量Ｃ_cbが無い状態と等価であり、導線１１７が長くなってもコンタクトチェックの信頼性を確保することができる。図１５に示す実施例は上述したこの発明の全ての実施例に適用することができる。
【００３１】
【発明の効果】
以上説明したように、この発明によればスイッチ１０９を半導体スイッチに置換しても微小電流を精度よく測定することができる。更に、少ない部品数でコンタクトチェックを行なう回路１０８を構成することができる。この結果、安価なコストでコンタクトチェック機能を具備した微小電流測定装置を提供することができる利点が得られる。
【図面の簡単な説明】
【図１】この発明の一実施例を説明するための接続図。
【図２】図１に示した実施例に用いられる接触子の部分の構造を説明するための側面図。
【図３】図１に示した実施例の動作を説明するために等価回路に置き換えた接続図。
【図４】この発明の変形実施例を説明するための接続図。
【図５】図４に示した実施例の動作を説明するための波形図。
【図６】図５と同様の波形図。
【図７】図５と同様の波形図。
【図８】この発明の更に他の実施例を説明するための接続図。
【図９】この発明の更に他の実施例を説明するための接続図。
【図１０】この発明の更に他の実施例を説明するための接続図。
【図１１】図１０に示した実施例の動作を説明するための波形図。
【図１２】この発明の更に他の実施例を説明するための接続図。
【図１３】この発明の更に他の実施例を説明するための接続図。
【図１４】この発明の更に他の実施例を説明するための接続図。
【図１５】この発明の更に他の実施例を説明するための接続図。
【図１６】従来の技術を説明するための接続図。
【図１７】従来の技術の欠点を説明するための接続図。
【符号の説明】
１直流電圧源１０７交流電圧計
３被測定対象１０８コンタクトチェック回路
Ｇ₁,Ｇ₂接触子１０９スイッチ
１００微小電流測定装置１１１抵抗器
１０１演算増幅器１１２交流電圧源
１０２バッファ増幅器１１３積分用コンデンサ
１０３抵抗器１１４リセットスイッチ
１０４帰還用抵抗器１１５差動増幅器
１０５帰還用コンデンサ１１６直流バイアス源
１０６直流電圧計[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a minute current measuring apparatus used for measuring an insulation resistance of an insulator as a measurement target, for example, and particularly a contact check capable of determining that a measurement circuit is in electrical contact with the measurement target. It is intended to provide functions with a simple configuration.
[0002]
[Prior art]
FIG. 16 shows the configuration of the minute current measuring apparatus 2 equipped with the contact check function disclosed in Japanese Patent Laid-Open No. 5-87858. In this minute current measuring device 2, when measuring the resistance value R _x of the measurement object 3, the switch SW is turned off, the AC signal source 6 is disconnected from the measuring circuit, and the measured value MV ₁ of the minute ammeter 4 and the direct current are measured. From the value E of the voltage source 1, the resistance value R _x of the object to be measured 3 is obtained.
When determining the contact between the measurement object 3 and the contacts G ₁ and G ₂ , the AC signal source 6 is input to the measurement circuit. Here, if the object 3 to be measured is normally in contact with the contacts G ₁ and G ₂ , the circuit current is generated by the voltmeter 83 via the transformer 7, I / V converter 81, and wave height detector 82. Is measured, and the capacitance C _{x of the} object to be measured 3 is measured from the current value and the voltage value of the AC signal generation source 6. If the measurement target 3 is not in contact with the contacts G ₁ and G ₂ , almost no current flows through the transformer 7, and the connection determination circuit 5 determines that the measurement target 3 and the contacts G ₁ and G ₂ are connected. Determined to be non-contact.
[0003]
[Problems to be solved by the invention]
In the minute current measuring apparatus 2 equipped with the contact check function shown in FIG. 16, the switch SW that is turned on at the time of contact check is connected to the input terminal of the differential amplifier 41. It is necessary to be a mechanical contact switch such as a relay. In other words, if a semiconductor switch is used for this switch SW, the semiconductor switch has a finite off-resistance, so that a leakage current flows even when the switch is off, and the measurement accuracy is particularly high when measuring the resistance of the object 3 to be measured. make worse.
[0004]
Further, when a mechanical contact switch is used, there are problems in that the switching operation is slow and the life is short.
Further, the measuring apparatus shown in FIG. 16 has a drawback in that the circuit configuration for contact check of the transformer 7, the I / V converter 81, the wave height detector 82, and the like is large, and thus costs are increased.
Also converted into a voltage value MV ₁ the current i _x flowing in the measurement object 3 by a circuit arrangement shown in low current measuring device 2 is equivalent to Figure 17 shown in FIG. 16. According to the circuit configuration shown in FIG. 17, when the voltage of the DC voltage source 1 is E, the resistance value of the measurement target 3 is R _x , and the resistance value of the current detection resistor 40 is R _s , the measurement target 3 The current i _x flowing through is i _x = E / (R _x + R _s )
Is required.
[0005]
The current i _x flowing through the object 3 to be measured should originally be obtained by i _x = E / R _x , but since the resistance value R _s of the current detection resistor 40 is added to the numerator, this is an error. There is.
The object of the present invention is to accurately measure the resistance value of an object to be measured even if a semiconductor switch is used as an AC voltage application switch in order to speed up the contact check, and it is possible to make a minute measurement without causing an error. An object of the present invention is to provide a minute current measuring apparatus that can measure current and has a simple circuit configuration for contact check.
[0006]
[Means for Solving the Problems]
According to a first aspect of the present invention, a buffer amplifier is connected to the inverting input terminal of the operational amplifier via a resistor, and a feedback resistor is connected between the output terminal of the operational amplifier and the input terminal of the buffer amplifier, A feedback capacitor is connected in parallel, the resistance value of the measurement target is R _x , the resistance value of the feedback resistor is R _f , the voltage flowing through the measurement target is i _x , and the DC output voltage of the operational amplifier is V _DC If you, the minute current measuring device for determining the current i _x flowing in the object to be measured by _{_{_{V DC = -R f · i x}}} ,
An AC current is applied to the inverting input terminal of the operational amplifier through a switch, and it is detected that the _AC voltage VAC output to the output terminal of the operational amplifier in a state where the AC current is applied is greater than a predetermined value. Then, a minute current measuring device for determining that a DC voltage source for measuring the insulation resistance of the measurement object and the input terminal of the buffer amplifier are in contact with the measurement object is proposed.
[0007]
According to a second aspect of the present invention, a buffer amplifier is connected to the inverting input terminal of the operational amplifier via a resistor, and an integration having a capacitance value C _f between the output terminal of the operational amplifier and the input terminal of the buffer amplifier. A capacitor for measurement is connected, and a DC voltage _VDC integrated at a predetermined time T is measured on the integration capacitor, and the measured object is determined from the DC voltage V _DC , the capacitance value C _{f of the} capacitor for integration, and time T. In a minute current measuring apparatus for obtaining a flowing current i _x by V _DC = −ix _x T / C _f ,
An AC current is applied to the inverting input terminal of the operational amplifier through a switch, and it is detected that the _AC voltage VAC output to the output terminal of the operational amplifier in a state where the AC current is applied is greater than a predetermined value. Then, a minute current measuring device for determining that a DC voltage source for measuring the insulation resistance of the measurement object and the input terminal of the buffer amplifier are in contact with the measurement object is proposed.
[0008]
According to a third aspect of the present invention, a buffer amplifier is connected to the inverting input terminal of the operational amplifier via a resistor, and a feedback resistor is connected between the output terminal of the operational amplifier and the input terminal of the buffer amplifier, A feedback capacitor is connected in parallel, the resistance value of the measurement target is R _x , the resistance value of the feedback resistor is R _f , the voltage flowing through the measurement target is i _x , and the DC output voltage of the operational amplifier is V _DC If you, the minute current measuring device for determining the current i _x flowing in the object to be measured by _{_{_{V DC = -R f · i x}}} ,
A DC bias source is connected to the inverting input terminal of the operational amplifier through a switch and a resistor, and the difference V _DC2 between the output voltage V _{DC1 of the} operational amplifier when the switch is turned off and the output voltage V _DC2 of the operational amplifier when the switch is turned on. A minute value for determining that the DC voltage source for measuring the insulation resistance of the object to be measured and the input terminal of the buffer amplifier are connected to the object to be measured in a state where the value of −V _DC1 is larger than a predetermined value. A current measuring device is proposed.
[0009]
According to a fourth aspect of the present invention, a buffer amplifier is connected to the inverting input terminal of the operational amplifier via a resistor, and an integration having a capacitance value C _f between the output terminal of the operational amplifier and the input terminal of the buffer amplifier. A capacitor for measurement is connected, and a DC voltage _VDC integrated at a predetermined time T is measured on the integration capacitor, and the measured object is determined from the DC voltage V _DC , the capacitance value C _{f of the} capacitor for integration, and time T. In a minute current measuring apparatus for obtaining a flowing current i _x by V _DC = −ix _x T / C _f ,
A DC bias source is connected to the inverting input terminal of the operational amplifier through a switch and a resistor, and the difference V _DC2 between the output voltage V _{DC1 of the} operational amplifier when the switch is turned off and the output voltage V _DC2 of the operational amplifier when the switch is turned on. A minute value for determining that the DC voltage source for measuring the insulation resistance of the object to be measured and the input terminal of the buffer amplifier are connected to the object to be measured in a state where the value of −V _DC1 is larger than a predetermined value. A current measuring device is proposed.
[0010]
According to claim 5 of the present invention, in any of the minute current measuring apparatuses according to claim 1 or 2, the AC output voltage of the buffer amplifier and the AC output voltage of the operational amplifier are input to the differential amplifier, and the differential amplifier is used. The present invention proposes a minute current measuring apparatus configured to measure an output AC differential voltage and determine whether or not a DC voltage source and an input terminal of a buffer amplifier are in contact with an object to be measured based on the differential voltage.
[0011]
According to a sixth aspect of the present invention, in the minute current measuring apparatus according to the third or fourth aspect, the output voltage of the buffer amplifier and the output voltage of the operational amplifier are applied to the differential amplifier, and when the switch is turned on from the differential amplifier, A micro-current measuring device is proposed that measures the off-time voltage and determines that the DC voltage source and the input terminal of the buffer amplifier are in contact with the object to be measured in a state where the voltage difference is larger than a predetermined value.
[0012]
According to a seventh aspect of the present invention, in any one of the minute current measuring apparatuses according to the first to sixth aspects, an outer conductor is covered via an insulating layer on a conductor portion connecting between the input terminal of the buffer amplifier and the object to be measured. Then, a minute current measuring apparatus is proposed in which this external conductor is connected to a common connection point between the inverting input terminal and the output terminal of the buffer amplifier.
[0013]
Action <br/> interposed a buffer amplifier and a resistor in front of accordance when the operational amplifier to the present invention, as well as connected through a resistor to the inverting input terminal of the operational amplifier and the impedance converter by the buffer amplifier, the operational amplifier Since an AC current for contact check is applied to the inverting input terminal via a switch, the input terminal of the operational amplifier is matched to a low impedance. As a result, even if there is some off-resistance in the switch for applying the alternating current for contact check, the effect is a minute value.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an embodiment of the present invention. In FIG. 1, 1 is a DC voltage source for generating a known DC voltage E, 3 is an object to be measured, 100 is a minute current measuring device according to the present invention, and G ₁ and G ₂ are a DC voltage source 1 and a minute voltage on the object to be measured 3. The contact which electrically connects the electric current measurement apparatus 100 is shown. Incidentally, the relationship between the contacts G ₁ and G ₂ and the measurement target 3 is shown in FIG. In many cases, the object to be measured is an insulating material, and the minute current measuring device 100 is used to measure the insulation resistance of the insulating material. In the example shown in FIG. 2, the contact G ₁ is held movably up and down, the measurement target 3 is arranged between G ₁ and G ₂ , and the contact G ₁ is pushed down to contact the measurement target 3. It is a structure for measuring the current i _x flowing in the measurement target 3 in a state.
[0015]
The minute current measuring apparatus 100 according to the present invention is characterized by a configuration in which a buffer amplifier 102 is arranged in front of the operational amplifier 101. The buffer amplifier 102 can be configured by directly connecting an inverting input terminal and an output terminal of an operational amplifier having an inverting input terminal and a non-inverting input terminal. According to this structure, it is possible to obtain a buffer amplifier having a gain of “1”, a high input impedance, and a low output impedance.
The contact G ₂ is connected to the non-inverting input terminal of the buffer amplifier 102, and the output terminal of the buffer amplifier 102 is connected to the inverting input terminal of the operational amplifier 101 through the resistor 103. A feedback resistor 104 and a feedback capacitor 105 are connected in parallel between the output terminal of the operational amplifier 101 and the non-inverting input terminal of the buffer amplifier 102, and a DC voltmeter is connected between the output terminal of the operational amplifier 101 and the common potential. 106 and AC voltmeter 107 are connected in parallel.
[0016]
Further, in the present invention, the contact check circuit 108 is connected between the inverting input terminal of the operational amplifier 101 and the common potential. The contact check circuit 108 is a series circuit comprising a switch 109, a resistor 111, and an AC voltage source 112. Composed. Since this series circuit has a resistor 111 connected in series with an AC voltage source 112, it can be viewed as an AC current source, and an equivalent current source is connected to the inverting input terminal of the operational amplifier 101. Can be seen.
Switch 109 for determining values of small current i _x flowing in the measurement object 3 is off, when performing contact check to control the switch 109 to ON-state.
[0017]
FIG. 3 shows an equivalent circuit of the circuit shown in FIG. Since the buffer amplifier 102 is configured to input a signal to the non-inverting input terminal and output the input signal from the output terminal, the input and output terminals are in phase. Therefore, equivalently, only the operational amplifier 101 is visible. The voltage generated by the DC voltage source 1 is E, the resistance value of the object 3 to be measured is C _X , the resistance value of the feedback resistor 104 is R _f , the capacitance value of the feedback capacitor 105 is C _f , and the resistor 103 And R ₁ , R ₁ , R ₂ , the DC output voltage of the operational amplifier 101 is V _DC , the AC voltage generated by the AC voltage source 112 is e, and the AC output voltage output to the output terminal of the operational amplifier 101 is V _AC
1. When switch 109 is off (when measuring DC current)
_{_{V DC = - (R f /}} R X) E = -R f · i x
You can obtain a current i _x flowing in the measurement object 3 by measuring V _DC. Insulation resistance R _x by current i _x is required can be obtained by R _{_x} = E / i _x. Therefore, the insulation resistance R _x can be measured without causing an error. If R ₁ = R ₂ ,
2. When switch 109 is on (contact check)
V _AC = − (1 + Z _f / Z _x ) e
Here, Z _x = R _x / (1 + jωC _x · R _x )
Z _f = R _f / (1 + jωC _f · R _f )
.omega.C _x << R _x, by .omega.C _f << alternating voltage source 112 such that the R _f sets the frequency of the AC voltage generated AC voltage V _AC output to the output terminal of the operational amplifier 101,
V _AC = − (1 + C _x / C _f ) e
It is represented by
[0018]
That is, in the contact check, the capacitance C _st in a state where the measurement target 3 is not connected between the contacts G ₁ and G ₂ is measured in advance, and the capacitance in a state where the measurement target 3 is connected. Judged by comparing with.
_Assuming that the AC output voltage without the object 3 to be measured is V _AC1 , the AC output voltage V _AC1 is V _AC1 = − (1 + C _st / C _f ) e
_Assuming that the AC output voltage with the measurement object 3 connected is V _AC2 , the AC output voltage V _AC2 is
V _AC2 = − (1+ (C _st + C _x ) / C _f ) e
Since the difference between V _AC2 and V _AC1 is the capacitance C _{x of the} object 3 to be measured,
V _AC = V _AC2 −V _AC1 = − (C _x / C _f ) e
Is obtained.
[0019]
Actually, if the AC output voltage V _AC2 is larger than V _AC1 and further larger than a preset value (for example, a value about several percent smaller than the value of the AC output voltage V _AC2 ), the contacts G ₁ and G ₂ are measured. 3 can be determined to be in contact.
Here, when a semiconductor switch is used as the switch 109, its off resistance value (resistance value of the semiconductor switch in the off state) is R _off , the resistance value of the resistor 103 is R ₁ , and the resistance value of the resistor 111 is R _2. When the offset voltage generated at the inverting input terminal of the operational amplifier 101 is V _off , the voltage component V _NOis generated on the output side of the operational amplifier 101 when measuring a minute current due to the connection of the contact check circuit 108 is
V _NOis = (R ₁ · V _off / (R ₂ + R _off )) (1 + R _f / R _x )
It is represented by
[0020]
Since the offset voltage V _off is originally about several _{millivolts and} is generally R ₁ << R ₂ + R _off , the voltage component V _NOis becomes V _{NOis ≈0} .
Therefore, according to the present invention, even if the switch 109 is replaced with a semiconductor switch, a minute DC current can be measured with high accuracy without causing a large error in the minute current measurement, and a circuit for contact check The advantage that it can be configured simply is also obtained.
FIG. 4 shows another embodiment of the present invention. In this embodiment, an integrating capacitor 113 is connected between the output terminal of the operational amplifier 101 and the input terminal of the buffer amplifier 102, and a minute amount flowing through the measurement target 3 by the DC voltage V _c integrated in the integrating capacitor 113. shows the case where the structure for measuring the current i _x. The integrating capacitor 113 reset switch 114 are connected in parallel, the integrated voltage V _c which is integrated in the integrating capacitor 113 is discharged by the reset switch 114 is in the reset state.
[0021]
A fixed time T (see FIG. 5A) is determined from the reset state, and the voltage V _c integrated during this time T is measured. The voltage V _c is
V _c = −i _x · T / C _f
In represented, it is possible to determine the capacitance value C _f is the current i _x flowing in the measurement target 3 from a known integrating capacitor 113 with time T by the voltage V _c is measured by DC voltmeter 106.
[0022]
During the contact check, the switch 109 is turned on and the reset switch 114 is turned off, and the AC voltage e is applied from the AC voltage source 112 to the inverting input terminal of the operational amplifier 101. As shown in FIG. 6A, an AC signal AC is output to the output terminal of the operational amplifier 101 as long as the switch 109 is on. The voltage V _AC of the AC signal AC is measured by the AC voltmeter 107, and if the voltage value is larger than the set value, it is determined that the contacts G ₁ and G ₂ are in contact with the measurement target 3.
Note that, as at the time of contact check if the DC current i _x is extremely large 7 reaches the DC output voltage V _DC is saturated, but to the state which can not measure the AC voltmeter 107 in the AC voltage V _AC, since this case is direct current i _x is sufficiently flowing, it can be seen that the contact G ₁ and G ₂ is in contact with the object to be measured 3.
[0023]
FIG. 8 shows still another embodiment of the present invention. In this embodiment, a case is shown in which the difference between the output of the operational amplifier 101 and the output of the buffer amplifier 102 is obtained by the differential amplifier 115 at the time of contact check. By determining the voltage V _a of the difference in the differential amplifier 115, the voltage V _a of the difference,
V _a = (C _x / C _f ) e
Is required.
[0024]
As is apparent from this equation, the AC voltage V _a and the object to be measured 3 According to this example the relationship between the electrostatic capacitance C _x is a proportional relationship from the AC voltage V _a of the capacitance C _x This is effective when it is configured to read a value.
FIG. 9 shows an embodiment in which the embodiment shown in FIG. 8 is applied to an integral type minute current measuring apparatus. In this case also the measurement of the current i _x flowing in the measurement target 3 by controlling turns off the switch 109 and 114, performs the contact check by controlling the switch 109 on and off the reset switch 114.
The AC voltmeter 107 during contact check for applying a voltage V _a of the difference between the output of the output operational amplifier 101 of the buffer amplifier 102 from the differential amplifier 115. Again, since the voltage V _a of the difference obtained by _{_{V a = (C x / C}} f) e, and the value of the voltage V _a, the value of the electrostatic capacitance value C _x is a proportional relationship from the voltage V _a it is suitable to the case that reading capacitance C _x.
[0025]
FIG. 10 shows still another embodiment of the present invention. This connects the DC bias source 116 in place of the AC voltage source 112 used to determine the capacitance C _x at contact check in the embodiment, the switch 109 and the DC output voltage V _DC1 of off, the switch 109 The case where the DC output voltage V _DC2 in the ON state is measured and the contact check is performed from the difference voltage _VaDC is shown.
At low current measurement turns off the switch 109 in the same manner as described above, the when the current i _x flowing in the measurement object 3 by measuring the DC output voltage V _DC generated in the output of the operational amplifier 101,
V _DC = −R _f / i _x
Ask for.
[0026]
On the other hand, at the time of contact check, the switch 109 is switched between an off state and an on state as shown in FIG. 11A. That is, the DC output voltage V _DC1 indicated by the DC voltmeter 106 is measured in a state where the switch 109 is OFF (time point t ₁ ). Next, the DC output voltage V _DC2 of the DC voltmeter 106 is measured in a state where the switch 109 is on (time point t ₂ ).
The difference voltage V _aDC between these DC output voltages V _DC1 and V _DC2 is
V _aDC = V _DC2 −V _DC1 = − (1 + C _x / C _f ) e _DC
Is required. From this equation, the capacitance C _x of the object 3 to be measured can be obtained. In this case, the larger the value of the voltage V _ADC differential, represent a greater electrostatic capacitance C _x, by the value of C _x is larger than the set value, the contact G ₁ and G ₂ is the object to be measured It can be determined that it is touching.
[0027]
FIG. 12 shows a case where the embodiment shown in FIG. 10 is applied to an integral type minute current measuring apparatus. In this case also, the switch 109 is turned off and on, and the DC output voltage V _DC1 and V _DC2 of the operational amplifier 101 are measured by the DC voltmeter 106, and the difference V _DC2 −V _DC1 is calculated to be measured. 3 obtains a capacitance C _x of the judges that the contact good if the value of the capacitance C _x is larger than the set value.
FIG. 13 shows an embodiment in which a differential amplifier 115 is added to the embodiment shown in FIG. In the embodiment shown in FIG. 13, the determination of a minute current is measured by a current measuring device composed of an operational amplifier 101 and a feedback resistor, and a contact check is performed on the DC output voltages V _DC1 and V _DC2 when the switch 109 is turned on and off. Judge with.
[0028]
The feature here is that the DC output voltage difference V _DC2 −V _DC1 is V _DC2 −V _DC1 = − (C _x / C _f ) e _DC due to the action of the differential amplifier 115 at the time of contact check.
It is characterized by the points obtained in
FIG. 14 shows a case where a minute current is measured by the integral type current measuring device shown in FIG. 12, and the contact check is determined by the DC voltages V _DC1 and V _DC2 when the switch 109 is on and off. In this embodiment, V _DC2 -V _DC1 is V _DC2 -V _DC1 =-(C _x / C _f ) e _DC
Is required.
[0029]
FIG. 15 shows still another embodiment of the present invention. In this embodiment, an embodiment is shown in which the influence of the electrostatic capacitance C _cb parasitic between the conducting wire 117 connecting the input terminal IN of the minute current measuring apparatus 100 and the contact G ₂ and the common potential is shown.
That is, as shown in FIG. 16, the minute current measuring device and the contact G ₂ are connected by a cable, but when a capacitance C _cb is formed between the conducting wire 117 serving as the cable and a common potential, This capacitance C _cb is connected in parallel to the static voltage capacitance C _x of the object to be measured 3 because the DC voltage source 1 is in a short state in terms of AC.
For this reason, when the capacitance C _cb increases, an error occurs in the measured value of the capacitance C _x of the object 3 to be measured, which may reduce the reliability of the contact check.
[0030]
For this reason, in the embodiment shown in FIG. 15, the conductor 117 is covered with an external conductor 118 via an insulating layer, and the external conductor 118 is connected to the common connection point of the inverting input terminal and the output terminal of the buffer amplifier 102. To do.
With this structure, a capacitance C _cb is formed between the conductor 117 and the outer conductor 118. In this case, no potential difference is applied between the conductor 117 and the outer conductor 118. It is not affected at all by the capacitance C _cb .
That is, since the potential between the inverting input terminal and the non-inverting input terminal of the buffer amplifier 102 is always maintained at the same potential by the function of the operational amplifier, the potential between the conductor 117 and the external conductor 118 is also maintained at the same potential. . As a result, even if there is a capacitance C _cb between the conducting wire 117 and the external conductor 118, charging current and discharging current do not flow from the conducting wire 117 to this capacitance. As a result, this is equivalent to the state where there is no capacitance C _cb , and the reliability of the contact check can be ensured even if the conducting wire 117 becomes long. The embodiment shown in FIG. 15 can be applied to all the embodiments of the present invention described above.
[0031]
【The invention's effect】
As described above, according to the present invention, a minute current can be accurately measured even if the switch 109 is replaced with a semiconductor switch. Furthermore, the circuit 108 for performing contact check with a small number of parts can be configured. As a result, there is an advantage that it is possible to provide a minute current measuring device having a contact check function at a low cost.
[Brief description of the drawings]
FIG. 1 is a connection diagram for explaining an embodiment of the present invention.
FIG. 2 is a side view for explaining the structure of a contact portion used in the embodiment shown in FIG. 1;
FIG. 3 is a connection diagram replaced with an equivalent circuit for explaining the operation of the embodiment shown in FIG. 1;
FIG. 4 is a connection diagram for explaining a modified embodiment of the present invention.
FIG. 5 is a waveform chart for explaining the operation of the embodiment shown in FIG. 4;
6 is a waveform diagram similar to FIG.
7 is a waveform diagram similar to FIG.
FIG. 8 is a connection diagram for explaining still another embodiment of the present invention.
FIG. 9 is a connection diagram for explaining still another embodiment of the present invention.
FIG. 10 is a connection diagram for explaining still another embodiment of the present invention.
11 is a waveform diagram for explaining the operation of the embodiment shown in FIG.
FIG. 12 is a connection diagram for explaining still another embodiment of the present invention.
FIG. 13 is a connection diagram for explaining still another embodiment of the present invention.
FIG. 14 is a connection diagram for explaining still another embodiment of the present invention.
FIG. 15 is a connection diagram for explaining still another embodiment of the present invention.
FIG. 16 is a connection diagram for explaining a conventional technique.
FIG. 17 is a connection diagram for explaining the drawbacks of the prior art.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 DC voltage source 107 AC voltmeter 3 Object to be measured 108 Contact check circuit G ₁ , G ₂ contact 109 Switch 100 Microcurrent measuring device 111 Resistor 101 Operational amplifier 112 AC voltage source 102 Buffer amplifier 113 Integration capacitor 103 Resistor 114 Reset switch 104 Feedback resistor 115 Differential amplifier 105 Feedback capacitor 116 DC bias source 106 DC voltmeter

Claims

A buffer amplifier is connected to the inverting input terminal of the operational amplifier via a resistor, a feedback resistor and a feedback capacitor are connected in parallel between the output terminal of the operational amplifier and the input terminal of the buffer amplifier, When the resistance value of the object to be measured is R _x , the resistance value of the feedback resistor is R _f , the voltage flowing through the object to be measured is i _x , and the DC output voltage of the operational amplifier is V _DC , V _DC = −R in low current measurement device for determining the current i _x flowing in the object to be measured by _f · i _x,
An AC current is applied to the inverting input terminal of the operational amplifier through a switch, and it is detected that the _AC voltage VAC output to the output terminal of the operational amplifier in a state where the AC current is applied is greater than a predetermined value. Then, it is determined that the DC voltage source for measuring the insulation resistance of the measurement object and the input terminal of the buffer amplifier are in contact with the measurement object.

A buffer amplifier is connected to the inverting input terminal of the operational amplifier via a resistor, and an integrating capacitor having a capacitance value C _f is connected between the output terminal of the operational amplifier and the input terminal of the buffer amplifier. The DC voltage V _DC integrated in the capacitor for a predetermined time T is measured, and the DC voltage V _DC , the capacitance value C _{f of the} capacitor for integration, and the current i _x flowing through the measurement target from time T are expressed as V _DC = -I _x · T / C _f
An AC current is applied to the inverting input terminal of the operational amplifier through a switch, and it is detected that the _AC voltage VAC output to the output terminal of the operational amplifier in a state where the AC current is applied is greater than a predetermined value. Then, it is determined that the DC voltage source for measuring the insulation resistance of the measurement object and the input terminal of the buffer amplifier are in contact with the measurement object.

A buffer amplifier is connected to the inverting input terminal of the operational amplifier via a resistor, a feedback resistor and a feedback capacitor are connected in parallel between the output terminal of the operational amplifier and the input terminal of the buffer amplifier, When the resistance value of the object to be measured is R _x , the resistance value of the feedback resistor is R _f , the voltage flowing through the object to be measured is i _x , and the DC output voltage of the operational amplifier is V _DC , V _DC = −R in low current measurement device for determining the current i _x flowing in the object to be measured by _f · i _x,
A DC bias source is connected to the inverting input terminal of the operational amplifier through a switch and a resistor, and the difference V _DC2 between the output voltage V _{DC1 of the} operational amplifier when the switch is turned off and the output voltage V _DC2 of the operational amplifier when the switch is turned on. Determining that the DC voltage source for measuring the insulation resistance of the object to be measured and the input terminal of the buffer amplifier are connected to the object to be measured in a state where the value of −V _DC1 is larger than a predetermined value; A minute current measuring device characterized by

A buffer amplifier is connected to the inverting input terminal of the operational amplifier via a resistor, and an integrating capacitor having a capacitance value C _f is connected between the output terminal of the operational amplifier and the input terminal of the buffer amplifier. The DC voltage V _DC integrated in the capacitor for a predetermined time T is measured, and the DC voltage V _DC , the capacitance value C _{f of the} capacitor for integration, and the current i _x flowing through the measurement target from time T are expressed as V _DC = -I _x · T / C _f
A DC bias source is connected to the inverting input terminal of the operational amplifier through a switch and a resistor, and the difference V _DC2 between the output voltage V _{DC1 of the} operational amplifier when the switch is turned off and the output voltage V _DC2 of the operational amplifier when the switch is turned on. Determining that the DC voltage source for measuring the insulation resistance of the object to be measured and the input terminal of the buffer amplifier are connected to the object to be measured in a state where the value of −V _DC1 is larger than a predetermined value; A minute current measuring device characterized by

3. The minute current measuring apparatus according to claim 1 or 2, wherein the AC output voltage of the buffer amplifier and the AC output voltage of the operational amplifier are input to a differential amplifier, and the difference between the AC outputs from the differential amplifier. A minute current measuring apparatus characterized in that a voltage is measured and whether or not the DC voltage source and the input terminal of the buffer amplifier are in contact with the object to be measured is determined based on the difference voltage.

5. The minute current measuring device according to claim 3 or 4, wherein the output voltage of the buffer amplifier and the output voltage of the operational amplifier are applied to a differential amplifier, and the voltage when the switch is turned on and off is applied from the differential amplifier. A minute current measuring device that measures and determines that the DC voltage source and the input terminal of the buffer amplifier are in contact with the object to be measured in a state in which the voltage difference is larger than a predetermined value.

7. The minute current measuring device according to claim 1, wherein an outer conductor is covered via an insulating layer on a conductor portion connecting the input terminal of the buffer amplifier and the object to be measured. Is connected to a common connection point between the inverting input terminal and the output terminal of the buffer amplifier.