JP4054655B2

JP4054655B2 - Hot wire flow meter

Info

Publication number: JP4054655B2
Application number: JP2002310018A
Authority: JP
Inventors: 亮安藤; 聖智井手
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 2002-10-24
Filing date: 2002-10-24
Publication date: 2008-02-27
Anticipated expiration: 2022-10-24
Also published as: JP2004144617A

Description

【０００１】
【発明の属する技術分野】
本発明は、空気及びガス等の流量を計測する熱線式流量計に係り、特に、流量を計測する発熱抵抗体の過剰加熱を防止することのできる流量検出回路を備えた熱線式流量計に関する。
【０００２】
【従来の技術】
空気及びガス等の流体の流量を計測する熱線式流量計には、従来から発熱抵抗体と流量検出抵抗の直列接続回路と、温度補償抵抗と固定抵抗の直列接続回路と、を並列に接続するホイートストンブリッジ回路を有する流量検出回路（熱線駆動回路）が用いられており、前記発熱抵抗体を空気及びガス等の流体の流れに曝して流れる流体の流量を計測している。
【０００３】
このような熱線式流量計を用いて空気及びガス等の流体の流量を計測する時、前記流量検出回路が不具合を生じて故障等した場合に、前記ホイートストンブリッジ回路の発熱抵抗体が過剰加熱されることがあり、例えば、計測する流体が可燃性ガスのような流体である場合には、前記発熱抵抗体が過剰加熱されると発火する等の危険性も生じることが予想される。
【０００４】
前記の如き発熱抵抗体が過剰加熱を防止し、高い安全性を実現するために、先行技術としては、流量検出回路にツェナーダイオードなどを用い、該ツェナーダイオードにより前記ホイートストンブリッジ回路の発熱抵抗体に印加される電圧を制限し、該発熱抵抗体が過剰に加熱されるのを防いでいる（例えば、特許文献１参照）。
【０００５】
【特許文献１】
特開平１０−２８１８３５号公報（４頁６欄４８行〜５頁７欄１９行、図１）
【０００６】
【発明が解決しようとする課題】
通常のホイートストンブリッジ回路の発熱抵抗体の温度は、発熱抵抗体が流体の流れに曝して流れる流体の流量を計測しているので、前記発熱抵抗体の発熱量と該発熱抵抗体からの放熱量との熱収支によって決まる。このため、前記発熱抵抗体に流れる電流が少なくても、測定される流体の流れが少ない場合は、前記発熱抵抗体は高温になり、該発熱抵抗体に流れる電流が少なくても、測定される流体の流れが多いと、発熱抵抗体の温度は、測定される流体の温度程度の低い温度に保たれるものである。
【０００７】
ところで、前記先行技術は、ツェナーダイオードにより前記ホイートストンブリッジ回路の発熱抵抗体に印加される電圧を制限するものであるが、発熱抵抗体に印加される制限電圧の設定が難しく、制限電圧が低すぎると高流量までの測定ができず、制限電圧が高すぎると低流量時に故障が生じた場合、発熱抵抗体の過剰加熱を防ぐことができないと云う問題を生じさせてしまうものであって、熱式流量計の測定流量の範囲が狭くなってしまうと云う解決すべき課題を備えている。
【０００８】
本発明の熱線式流量計は、前記課題を解決すべくなされたものであって、その目的とするところは、熱線式流量計の流量検出回路（熱線駆動回路）の故障等の異常時においても該流量検出回路（熱線駆動回路）の発熱抵抗体の過剰加熱を防止し、流量検出回路の損傷等を未然に防ぐと共に、広範囲の流量計測が可能な熱線式流量計を提供することにある。
【０００９】
【課題を解決するための手段】
前記目的を達成するべく、本発明の熱線式流量計は、基本的には、流体通路内に配置され温度により抵抗値が変化する発熱抵抗体を有する熱線駆動回路と、該発熱抵抗体の抵抗値を検出し該発熱抵抗体の抵抗値変化を制限する過剰加熱防止回路と、を有する、流体の流量を検出するための流量検出回路を備えた熱線式流量計であって、前記熱線駆動回路は、発熱抵抗体と流量検出抵抗の第一直列接続回路と温度補償抵抗と固定抵抗（第一、第二）の第二直列接続回路とを並列に接続してホイートストンブリッジ回路を構成して前記発熱抵抗体の抵抗値を検出し、前記過剰加熱防止回路は、比較用固定抵抗（第一）と比較用固定抵抗（第二）と比較用固定抵抗（第三）との第三直列接続回路を備えると共に、比較器と該比較器の出力状態により活性状態となる第二パワートランジスタとを備え、前記第三直列接続回路の前記比較用固定抵抗（第一）と前記比較用固定抵抗（第二）とは、前記熱線駆動回路の前記発熱抵抗体と前記流量検出抵抗との前記第一直列接続回路に並列に接続してホイートストンブリッジ回路を構成し、前記比較器は、前記第一直列接続回路と前記第三直列接続回路とのブリッジ中間点の電圧の差を比較し、前記第二パワートランジスタは、前記電圧差がＨｉｇｈレベルになると活性状態となって該第二パワートランジスタから前記比較用固定抵抗（第三）に電流を流して前記発熱抵抗体に流れる電流を制限することを特徴としている。
また、本発明の熱線式流量計の具体的な態様は、前記第二パワートランジスタは、前記発熱抵抗体と並列に設けられていることを特徴としている。
【００１０】
前記の如く構成された本発明の熱線式流量計は、流量検出回路に熱線駆動回路とは別に、新たに過剰加熱防止回路を備え、該過剰加熱防止回路により、熱線駆動回路の発熱抵抗体の抵抗値を検出する構成とし、該抵抗値の値の状態により発熱抵抗体の加熱状態を判断し、加熱状態が所定以上と判断された場合には、発熱抵抗体の抵抗値変化を制限するようにしたので、熱線式流量計の流量検出回路（熱線駆動回路）の故障（加熱電流を制御する回路やパワートランジスタ）等の異常時における発熱抵抗体の過剰加熱を防止し、流量検出回路の損傷等を未然に防ぐことができると共に、流量検出回路に新たに過剰加熱防止回路を備え構成としても、広範囲の流量計測が可能である。
【００１１】
更に、前記の如く構成された本発明の熱線式流量計は、過剰加熱防止回路を比較用の固定抵抗（第三）と固定抵抗（第四）と固定抵抗（第五）を直列接続した回路とし、該直列接続した回路と前記熱線駆動回路の前記発熱抵抗体と前記流量検出抵抗の直列接続回路とを並列に接続してホイートストンブリッジ回路に構成して、該ブリッジ回路のバランスにより発熱抵抗体の抵抗値を検出できるようにしたので、発熱抵抗体の抵抗値検出の構成が簡単であると共に、検出電流と電圧を測定して抵抗値を算出することや発熱抵抗体の温度を検出すること等他の手段に比べてより簡単に発熱抵抗体の抵抗値を検出することができ、該検出構成をホイートストンブリッジ回路としたので、該検出した電流値（抵抗値）により直ちに比較器とパワートランジスタにより発熱抵抗体に流れる電流を制限して発熱抵抗体の過剰加熱を防止することができる。
【００１２】
そして、発熱抵抗体に流れる電流を制限する手段として、比較器とパワートランジスタを用いたので、電磁石のリレースイッチやヒューズ等を前記ホイートストンブリッジ回路に組み込んで、発熱抵抗体に流れる電流を制限するものに比べて、誤差の程度を低くでき精度の良い電流制限を実現できる。前記第二パワートランジスタを前記発熱抵抗体と並列に設けることにより、該第二パワートランジスタが活性状態になった場合には、前記発熱抵抗体を迂回して過剰加熱電流を流すことができ、第一パワートランジスタが故障して遮断されない通電状態のままになっているような場合にも、保護回路として機能する。
【００１３】
また、本発明の熱線式流量計の他の具体的な態様は、前記熱線駆動回路は、前記発熱抵抗体に電源から電流を供給する第一パワートランジスタを備え、前記過剰加熱防止回路は、前記発熱抵抗体に流れる電流の制限を、前記比較器と前記第二パワートランジスタとの作動により前記第一パワートランジスタを遮断するべく構成されていることを特徴としている。
【００１４】
前記の如く構成された本発明の熱線式流量計は、第二パワートランジスタを前記発熱抵抗体と並列に設けたものに比べて、該第二パワートランジスタを小さなもので、前記発熱抵抗体に流れる電流の制限することができる。しかし、この場合は、第一パワートランジスタが故障して遮断されない通電状態のままになっているような場合には、保護回路として働かない虞がある。
【００１５】
更に、本発明の熱線式流量計の更に他の具体的な態様は、前記過剰加熱防止回路は、比較器と、ＩＧＢＴもしくはサイリスタと、を備え、該比較器と、該ＩＧＢＴもしくはサイリスタとにより前記発熱抵抗体に流れる電流を制限することを特徴としている。
【００１６】
更にまた、本発明の熱線式流量計の更に他の具体的な態様は、前記流量検出回路は、前記熱線駆動回路に流れる電流を制限する保護抵抗を、電源側に備えていることを特徴とし、前記流量検出回路は、前記熱線駆動回路に許容最大定格電流以上の電流が流れた場合に、該電流を遮断するヒューズを電源側に備えていることを特徴としている。
【００１７】
前記の如く構成された本発明の熱線式流量計は、発熱抵抗体への過剰加熱電流を回避した後も、大電流が第一パワートランジスタを流れ続けることになると、第一パワートランジスタや第二パワートランジスタ、比較用固定抵抗等が発熱する可能性があるので、これを回避するために、熱線駆動回路に流れる電流を制限する保護抵抗を電源側に設けて、入力電流を低減制御させることができる。また、ヒューズを電源側に設けることで、過剰加熱電流を回避した後も更に過剰電流が流れた場合に、該ヒューズが切断して電流を遮断することもできる。
【００１８】
更にまた、本発明の熱線式流量計の更に他の具体的な態様は、前記流量検出回路は、前記発熱抵抗体の過剰加熱を診断し、故障診断信号を出力する構成を備えていることを特徴としている。
【００１９】
前記の如く構成された本発明の熱線式流量計は、熱線式流量計の流量検出回路の故障等の異常時における発熱抵抗体の過剰加熱を防止することと併せて、発熱抵抗体の過剰加熱を診断し、故障診断信号を出力することで、熱線式流量計の使用者に発熱抵抗体の過剰加熱や熱線式流量計の故障を知らせることができる。
【００２０】
【発明の実施の形態】
以下、図面を参照して本発明の熱線式流量計の実施形態を詳細に説明する。
図１は、本発明の第一の実施形態の熱線式流量計の流量検出回路１００を示したものであり、該流量検出回路１００は、基本流量検出回路（熱線駆動回路）１０１と過剰加熱防止回路１０２とを備えている。
【００２１】
基本流量検出回路（熱線駆動回路）１０１は、発熱抵抗体１と流量検出抵抗２の直列接続回路と、温度補償抵抗９と第一、第二固定抵抗１０、１１の直列接続回路とを並列に接続するホイートストンブリッジ回路を備えると共に、該ホイートストンブリッジ回路の前記二つの直列接続回路の各ブリッジ中間点ａ、ｂの電位を入力する演算増幅器１３と、該演算増幅器１３の出力に応じて前記ホイートストンブリッジ回路に供給する電流量を制御する第一パワートランジスタ５とを備えた構成とされている。
【００２２】
また、過剰加熱防止回路１０２は、比較用第一固定抵抗３と比較用第二固定抵抗４と比較用第三固定抵抗４４を直列接続し、該直列接続した回路が、前記基本流量検出回路（熱線駆動回路）１０１の発熱抵抗体１と流量検出抵抗２の直列接続回路に並列に接続してホイートストンブリッジ回路が構成されていると共に、該ホイートストンブリッジ回路の前記二つの直列接続回路の各中間点ａ、ｃの電位を入力する比較器１２と、前記第一固定抵抗３と第二固定抵抗４と並列に接続されて前記比較器１２の出力が高レベルになると活性化して発熱抵抗体１への電流量を制限するパワートランジスタ６と、比較器１２の出力端子に接続されている電流制限抵抗14と、を備えた構成とされている。
【００２３】
流量検出回路１００は、その基本流量検出回路（熱線駆動回路）１０１の前記ホイートストンブリッジ回路に、第一パワートランジスタ５を介して電源ＶＢから電流（電圧Ｖ１）を供給するようになっている。前記基本流量検出回路（熱線駆動回路）１０１の前記ホイートストンブリッジ回路は、電源ＶＢからの電流供給により、そのブリッジ中間点ａ、ｂで電圧Ｖ２、Ｖ３となるが、演算増幅器13と第一パワートランジスタ５との作用によって、該ブリッジ中間点ａ、ｂでの電圧Ｖ２、Ｖ３が等しくなるように発熱抵抗体１に流れる電流を調整している。
【００２４】
発熱抵抗体１は、温度上昇によりその抵抗値Ｒｈを増加させるものであり、該抵抗値Ｒｈは、発熱抵抗体の温度Ｔｈに比例しており、該温度Ｔｈは、測定する空気の温度Ｔａより所定の温度ΔＴｈだけ高くなるように、温度Ｔｈ＝Ｔａ＋ΔＴｈに設定されている。即ち、抵抗値Ｒｈは、次の式（１）で示される。
Ｒｈ＝Ｒｈｏ（１＋αＴｈ）（１）
ここで、Ｒｈｏは、温度０℃のときの発熱抵抗体１の抵抗値、αは、温度係数である。
次に、前記の如く構成された本実施形態の熱線式流量計の流量検出回路１００の機能について説明する。
【００２５】
基本流量検出回路（熱線駆動回路）１０１の発熱抵抗体１を通過する空気の流量が増えると、該発熱抵抗体１から熱が奪われ、発熱抵抗体１の温度Ｔｈは下がり、ブリッジ中間点ａの電圧V2の値が上昇する。すると、演算増幅器１３の出力が上がり、第一パワートランジスタ５が発熱抵抗体１に供給する電流を増加させ、発熱抵抗体１の温度Ｔｈが所定の値に戻るように作用する。
【００２６】
つまり、流量検出回路１００の基本流量検出回路（熱線駆動回路）１０１のみの制御システムにおいては、空気の流量が増加する前と後とでは、発熱抵抗体１の温度Ｔｈは同じであるが、発熱抵抗体１に流れる電流、即ち、電圧Ｖ１と電圧Ｖ２の値は増加することとなる。このことは、基本流量検出回路（熱線駆動回路）１０１のみからなる流量検出回路１００では、発熱抵抗体１に流れる電流や電圧Ｖ１の値を制限することでは、発熱抵抗体１の過剰加熱を防止することが困難であるを意味する。
【００２７】
過剰加熱防止回路１０２は、基本流量検出回路（熱線駆動回路）１０１の発熱抵抗体１が正常な抵抗値Ｒｈのときは、比較用固定抵抗３と固定抵抗４とのブリッジの中間点ｃの電圧V4が、発熱抵抗体１と流量検出抵抗２の直列接続回路のブリッジ中間点ａの電圧Ｖ２より低く設定されているために、比較器12の出力はLowレベルの状態にあり、第二パワートランジスタ６は、遮断状態にある。
【００２８】
基本流量検出回路（熱線駆動回路）１０１の温度補償抵抗９が断線したり、第一パワートランジスタ５がショートしたりして、基本流量検出回路（熱線駆動回路）１０１が故障して、発熱抵抗体１に過大電流が流れてその温度Ｔｈが上がって、発熱抵抗体１の抵抗値Ｒｈが所定値Rhuを超えると、ブリッジ中間点ａの電圧Ｖ２がブリッジ中間点ｃの電圧Ｖ４より低くなる。このため、比較器12の出力がHighレベルとなり、第二パワートランジスタ６が活性状態になる。
【００２９】
これにより、発熱抵抗体１に流れていた過大電流は、第二パワートランジスタ６に流れ、発熱抵抗体１の過剰加熱を避けることができる。第二パワートランジスタ６に流れた電流は、比較用固定抵抗4４に流れ込み電圧Ｖ４が上がる。これにより、発熱抵抗体１への過大電流が回避され、発熱抵抗体１の温度が下がっても比較器１２の出力は、Highレベルを保ったままなので、第二パワートランジスタ６も活性状態を保持する。
【００３０】
このように、本実施形態の過剰加熱防止回路１０２は、比較用の固定抵抗３と固定抵抗４と固定抵抗４４を直列接続し、該直列接続した回路が、基本流量検出回路（熱線駆動回路）１０１の発熱抵抗体１と流量検出抵抗２の直列接続回路に並列に接続してホイートストンブリッジ回路を構成し、発熱抵抗体１に過大電流が流れた場合には、第二パワートランジスタ６を活性状態とし、発熱抵抗体１に流れていた過大電流をパワートランジスタ６に流れるようにしたので、発熱抵抗体１の過剰加熱が防止される。
【００３１】
本実施形態の過剰加熱防止回路１０２は、前記構成とすることにより、発熱抵抗体１に流れる電流値が電圧値によらずに、発熱抵抗体１の抵抗値の値、つまり、発熱抵抗体１の抵抗値が過剰加熱されたときの値、もしくは過剰加熱される少し手前の値になったときに、前記パワートランジスタ６が活性状態となり、発熱抵抗体１の過剰加熱が防止できる措置を取ることができる。
本実施形態の過剰加熱防止回路１０２のより具体的な動作について説明する。
【００３２】
基本流量検出回路（熱線駆動回路）１０１の発熱抵抗体１の許容できる加熱上限温度を温度Ｔhuとすると、この時の抵抗値Rhuは、次の式（２）のようになり、流量検出抵抗２の抵抗値R2と比較用第一、第二、第三固定抵抗３、４、４４の抵抗値R3、R4、R44との抵抗比は、次の式（３）のようになる。
Rhu = Rh0(1+αThu) （２）
R3/(R4+R44) = Rhu/R2 （３）
【００３３】
ただし、比較用第一、第二、第三固定抵抗３、４、４４に流れる電流は、空気流量測定に影響を与えない程度に少なくする必要があるため、R3+R4+R44≫Rh+R2とする。また、比較器１２の出力端子に接続されている電流制限抵抗１４の抵抗値は、該電流制限抵抗１４に流れる電流がコンパレータのシンク電流より大きくならないような値に設定する。
【００３４】
図1に示す本実施形態の第二パワートランジスタ６は、バイポーラトランジスタであるが、MOSトランジスタなどの電界効果型トランジスタでも良い。
また、第二パワートランジスタ６の代わりに、IGBTやサイリスタを用いても本実施形態の流量検出回路１００は、実現できる。
次に、本発明の第二の実施形態の熱線式流量計について説明する。
【００３５】
図２は、第二の実施形態の熱線式流量計の流量検出回路１００’を示したものであって、前記第一の実施形態の熱線式流量計の流量検出回路１００とは、電源VB側に電流制限抵抗７を付加した点でのみ相違し、その他の構成は同じであるので、同じ部材には同じ符号を付してある。
【００３６】
前記第一の実施形態の熱線式流量計の流量検出回路１００において、発熱抵抗体１への過剰加熱電流を回避した後の動作において、その後も、大電流が第一パワートランジスタ５を流れ続けることになると、第一パワートランジスタ５や第二パワートランジスタ６、比較用固定抵抗4４等が発熱する可能性がある。
【００３７】
第二の実施形態の熱線式流量計の流量検出回路１００’は、このため、これを回避するために、図２に示すように、電流制限抵抗７を電源VB側に設けて、入力電流を低減制御したものである。
【００３８】
本実施形態の流量検出回路１００’において、第二パワートランジスタ６の最大定格電流Ipmax、ベース・エミッタ間電圧Vbe6、バッテリーの最大電圧Vbmax、及び、比較用固定抵抗4４の抵抗値R44と、電流制限抵抗７の抵抗値R7との関係は、次の式（４）のようになる。
R7 ＞ (VBmax−Vbe6)／Ipmax−R44 式（４）
電流制限抵抗７の抵抗値R7の上限値は、熱式空気流量計の動作が確保できるように選ぶようにする。
【００３９】
例えば、熱線式流量計の動作要求範囲が、“電源電圧VBのとき、空気流量Qを測定できること”である場合、空気流量Qを測定するのに必要な電流をＩｈ、第一パワートランジスタ5のベース・エミッタ間電圧をVbe5、発熱抵抗体１の抵抗値をRh、流量検出抵抗２の抵抗値をR2とすると、電源電圧VBは、次の式（５）のようになり、電流制限抵抗７の抵抗値R7の上限値は、次の式（６）のようになる。
VB ＞Ｉh*(Rh+R2+R7)+Vbe5 式（５）
Ｒ7 ＜ (VB-Vbe5)/Ih-(Rh+R2) 式（６）
【００４０】
また、電流制限抵抗７で消費される電力Wr7は、式（７）のように、比較用固定抵抗4４で消費される電力Wr44は、式（８）となる。
Wr７ = (VBmax-Vbe)２R7/(R4+R7)２式（７）
Wr４４ = (VBmax-Vbe)２R44/(R44+R7)２式（８）
電流制限抵抗７と比較用固定抵抗４４の最大定格消費電力は、それぞれ電力Wr７、電力Wr４４より大きいものを選ぶのがよい。
過剰加熱電流を回避した後、熱線式流量計が故障していることを知らせる信号を比較器１２から得ることができる。
【００４１】
前記のように、熱線式流量計が正常に動作しており、発熱抵抗体１が正常な値のとき、比較器１２の出力は、Lowレベルである。しかし、熱線駆動回路１０１が故障して過剰加熱電流が流れると、比較器１２の出力は、Highレベルになる。比較器１２の出力がHighレベルになった場合を異常信号と捕らえれば、故障診断を行なうことができる。
【００４２】
図３は、本発明の第三の実施形態の熱線式流量計の流量検出回路１００”を示したものであって、前記第一の実施形態の熱線式流量計の流量検出回路１００とは、電源VB側にヒューズ８を付加した点でのみ相違し、その他の構成は同じであるので、同じ部材には同じ符号を付してある。
【００４３】
図３に示すように、ヒューズ８を設け、過剰加熱電流を回避した後に、更に過剰電流が流れた場合にヒューズ８が切断して、熱式空気流量計の流量検出回路１００”への電流供給を切断する方法がある。この場合、パワートランジスタ６はヒューズ８を遮断するのに必要な電流を所定の時間流せば良いことになる。
【００４４】
図４は、本発明の第四の実施形態の熱線式流量計の流量検出回路１００”’を示したものであって、前記第一の実施形態の熱線式流量計の流量検出回路１００とは、第二パワートランジスタ６への配線を、演算増幅器１３の出力側から接続した点でのみ相違し、その他の構成は同じであるので、同じ部材には同じ符号を付してある。
【００４５】
第四の実施形態の熱線式流量計の流量検出回路１００”’の構成とすることにより、第二パワートランジスタ６を活性状態とし、発熱抵抗体１に流れていた過大電流を第二パワートランジスタ６に流れるようにすることで、第一パワートランジスタ５を遮断して電流を制限し、発熱抵抗体１へ流れる過剰電流を制限することができる。
【００４６】
以上、本発明の四つの実施形態について説明したが、本発明は、前記実施形態に限定されるものではなく、本発明の精神を逸脱しない範囲で、設計において種種々の変更ができるものである。
【００４７】
【発明の効果】
前記説明から理解されるように、本発明の熱線式流量計は、流量検出回路（熱線駆動回路）の故障（加熱電流を制御する回路やパワートランジスタ）等の異常時における発熱抵抗体の過剰加熱を防止し、流量検出回路の損傷等を未然に防ぐことができると共に、流量検出回路に新たに過剰加熱防止回路を備え構成としても、広範囲の流量計測が可能である。
【図面の簡単な説明】
【図１】本発明の第一の実施形態であって、過剰加熱防止回路を備えた熱線式流量計の流量検出回路構成図。
【図２】本発明の第ニの実施形態であって、電流制限抵抗と過剰加熱防止回路を備えた熱線式流量計の流量検出回路構成図。
【図３】本発明の第三の実施形態であって、ヒューズと過剰加熱防止回路を備えた熱線式流量計の流量検出回路構成図。
【図４】本発明の第四の実施形態であって、変形の過剰加熱防止回路を備えた熱線式流量計の流量検出回路構成図。
【符号の説明】
１・・・発熱抵抗体、２・・・流量検出抵抗、３・・・比較用第一固定抵抗
４・・・比較用第二固定抵抗、５・・・第一パワートランジスタ、６・・・第二パワートランジスタ、７・・・電流制限抵抗、８・・・ヒューズ、９・・・温度補償抵抗、１０・・・第一固定抵抗、１１・・・第二固定抵抗、１２・・・比較器、１３・・・演算増幅器、１４・・・電流制限抵抗、４４・・・比較用第三固定抵抗、１００・・・流量検出回路、１０１・・・基本流量検出回路（熱線駆動回路）、１０２・・・過剰加熱防止回路。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a hot-wire flow meter that measures the flow rate of air, gas, and the like, and more particularly, to a hot-wire flow meter that includes a flow rate detection circuit that can prevent overheating of a heating resistor that measures the flow rate.
[0002]
[Prior art]
Conventionally, in a hot-wire flow meter that measures the flow rate of fluids such as air and gas, a series connection circuit of a heating resistor and a flow rate detection resistor and a series connection circuit of a temperature compensation resistor and a fixed resistor are connected in parallel. A flow rate detection circuit (heat line drive circuit) having a Wheatstone bridge circuit is used, and the flow rate of the fluid flowing by exposing the heating resistor to the flow of fluid such as air and gas is measured.
[0003]
When the flow rate of fluid such as air and gas is measured using such a hot wire type flow meter, the heating resistor of the Wheatstone bridge circuit is overheated when the flow rate detection circuit malfunctions and fails. For example, in the case where the fluid to be measured is a fluid such as a flammable gas, it is expected that there is a risk of ignition when the heating resistor is overheated.
[0004]
In order to prevent overheating of the heating resistor as described above and realize high safety, as a prior art, a Zener diode or the like is used in the flow rate detection circuit, and the Zener diode serves as a heating resistor of the Wheatstone bridge circuit. The applied voltage is limited to prevent the heating resistor from being excessively heated (see, for example, Patent Document 1).
[0005]
[Patent Document 1]
JP-A-10-281835 (page 4, column 6, line 48 to page 5, column 7, line 19, FIG. 1)
[0006]
[Problems to be solved by the invention]
The temperature of the heating resistor of a normal Wheatstone bridge circuit is measured by measuring the flow rate of the fluid that the heating resistor is exposed to the fluid flow, so the amount of heat generated by the heating resistor and the amount of heat released from the heating resistor It depends on the heat balance. For this reason, even if the current flowing through the heating resistor is small, if the flow of fluid to be measured is small, the heating resistor becomes high temperature, and even if the current flowing through the heating resistor is small, it is measured. When the flow of the fluid is large, the temperature of the heating resistor is maintained at a temperature as low as the temperature of the fluid to be measured.
[0007]
In the prior art, the voltage applied to the heating resistor of the Wheatstone bridge circuit is limited by a Zener diode. However, it is difficult to set the limiting voltage applied to the heating resistor, and the limiting voltage is too low. If the limit voltage is too high and a failure occurs at a low flow rate, it may cause a problem that overheating of the heating resistor cannot be prevented. There is a problem to be solved that the range of the measured flow rate of the flow meter becomes narrow.
[0008]
The hot-wire flow meter of the present invention has been made to solve the above-described problems, and the purpose thereof is to provide an abnormality such as a failure of the flow detection circuit (hot-wire drive circuit) of the hot-wire flow meter. An object of the present invention is to provide a hot-wire flow meter capable of measuring a flow rate over a wide range while preventing overheating of a heating resistor of the flow rate detection circuit (heat wire drive circuit), preventing damage to the flow rate detection circuit and the like.
[0009]
[Means for Solving the Problems]
In order to achieve the above-mentioned object, the hot-wire flow meter of the present invention basically includes a heat-wire driving circuit having a heat-generating resistor that is disposed in a fluid passage and whose resistance value varies with temperature, and the resistance of the heat-generating resistor. An overheating prevention circuit that detects a value and restricts a change in the resistance value of the heating resistor, and includes a flow rate detection circuit for detecting a fluid flow rate, the hot wire drive circuit comprising: The Wheatstone bridge circuit is configured by connecting in parallel the first series connection circuit of the heating resistor, the flow rate detection resistance, the temperature compensation resistance, and the second series connection circuit of the fixed resistance (first and second). A resistance value of the heating resistor is detected, and the overheating prevention circuit is connected in a third series of a fixed resistor for comparison (first), a fixed resistor for comparison (second), and a fixed resistor for comparison (third). A circuit, and a comparator and an output state of the comparator The comparison fixed resistor (first) and the comparison fixed resistor (second) of the third series connection circuit include the heating resistor of the heat ray driving circuit. And a Wheatstone bridge circuit connected in parallel to the first series connection circuit of the flow rate detection resistor, and the comparator is a bridge intermediate between the first series connection circuit and the third series connection circuit The voltage difference between the points is compared, and the second power transistor becomes active when the voltage difference becomes a high level, and a current is passed from the second power transistor to the comparison fixed resistor (third). It is characterized by limiting the current flowing through the heating resistor .
Moreover, the specific aspect of the hot wire type flow meter of the present invention is characterized in that the second power transistor is provided in parallel with the heating resistor.
[0010]
The hot-wire flow meter of the present invention configured as described above is provided with a new overheating prevention circuit in the flow rate detection circuit in addition to the hot-wire driving circuit, and the overheating prevention circuit allows the heating resistor of the heating wire driving circuit to be The resistance value is detected, the heating state of the heating resistor is determined based on the state of the resistance value, and when the heating state is determined to be a predetermined value or more, the resistance value change of the heating resistor is limited. As a result, overheating of the heating resistor is prevented and the flow rate detection circuit is damaged when the flow rate detection circuit (heat line drive circuit) of the hot-wire flow meter malfunctions (heating current control circuit or power transistor). In addition, it is possible to measure a wide range of flow rates even if the flow rate detection circuit is newly provided with an overheating prevention circuit.
[0011]
Furthermore, the hot-wire flow meter of the present invention configured as described above is a circuit in which an overheating prevention circuit is connected in series with a comparative fixed resistor (third), fixed resistor (fourth), and fixed resistor (fifth). The series connected circuit, the heating resistor of the heat ray driving circuit, and the series connection circuit of the flow rate detection resistor are connected in parallel to form a Wheatstone bridge circuit, and the heating resistor is balanced by the balance of the bridge circuit. The resistance value of the heating resistor can be detected, so the resistance value detection configuration of the heating resistor is simple, the resistance value is calculated by measuring the detection current and voltage, and the temperature of the heating resistor is detected. The resistance value of the heating resistor can be detected more easily than other means, and the detection configuration is a Wheatstone bridge circuit. Therefore, the comparator and the power transformer are immediately detected by the detected current value (resistance value). Excess heating of the heating resistor to limit the current flowing through the heating resistor can be prevented by register.
[0012]
And, as a means to limit the current flowing through the heating resistor, a comparator and a power transistor are used. Therefore, a relay switch or a fuse of an electromagnet is incorporated in the Wheatstone bridge circuit to limit the current flowing through the heating resistor. Compared to the above, it is possible to reduce the degree of error and realize a current limit with high accuracy. By providing the second power transistor in parallel with the heating resistor, when the second power transistor is activated, an excessive heating current can be passed around the heating resistor, Even when one power transistor fails and remains in an energized state that is not cut off, it functions as a protection circuit.
[0013]
In another specific aspect of the hot-wire flow meter of the present invention, the hot-wire drive circuit includes a first power transistor that supplies current from a power source to the heating resistor, and the overheating prevention circuit includes the The current flowing through the heating resistor is limited so that the first power transistor is cut off by the operation of the comparator and the second power transistor.
[0014]
The hot-wire flow meter of the present invention configured as described above has a smaller second power transistor than the second power transistor provided in parallel with the heating resistor, and flows to the heating resistor. The current can be limited. However, in this case, if the first power transistor fails and remains in an energized state that is not cut off, it may not function as a protection circuit.
[0015]
Furthermore, in still another specific aspect of the hot-wire flow meter of the present invention, the overheat prevention circuit includes a comparator and an IGBT or thyristor, and the comparator and the IGBT or thyristor are used to It is characterized by limiting the current flowing through the heating resistor.
[0016]
Still further, still another specific aspect of the hot-wire flow meter of the present invention is characterized in that the flow rate detection circuit is provided with a protective resistor that limits a current flowing in the hot-wire drive circuit on the power supply side. The flow rate detection circuit is provided with a fuse on the power supply side that cuts off the current when a current exceeding an allowable maximum rated current flows in the hot wire drive circuit.
[0017]
The hot-wire flow meter of the present invention configured as described above is configured such that a large current continues to flow through the first power transistor even after avoiding excessive heating current to the heating resistor. Since power transistors, fixed resistors for comparison, etc. may generate heat, a protective resistor that limits the current flowing in the heat-wire drive circuit is provided on the power supply side to avoid this, and the input current can be controlled to be reduced. it can. In addition, by providing a fuse on the power supply side, when the excessive current flows even after the excessive heating current is avoided, the fuse can be cut off to interrupt the current.
[0018]
Furthermore, according to still another specific aspect of the hot-wire flow meter of the present invention, the flow rate detection circuit has a configuration for diagnosing overheating of the heating resistor and outputting a failure diagnosis signal. It is a feature.
[0019]
The hot-wire flow meter of the present invention configured as described above is used to prevent overheating of the heating resistor in the event of an abnormality such as a failure of the flow detection circuit of the hot-wire flow meter. By outputting the failure diagnosis signal, it is possible to notify the user of the hot-wire flow meter of overheating of the heating resistor or failure of the hot-wire flow meter.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the hot-wire flow meter of the present invention will be described below in detail with reference to the drawings.
FIG. 1 shows a flow rate detection circuit 100 of a hot-wire flow meter according to a first embodiment of the present invention. The flow rate detection circuit 100 includes a basic flow rate detection circuit (heat line drive circuit) 101 and an overheating prevention. Circuit 102.
[0021]
The basic flow rate detection circuit (heat line drive circuit) 101 includes a series connection circuit of the heating resistor 1 and the flow rate detection resistor 2, a temperature compensation resistor 9 and a series connection circuit of the first and second fixed resistors 10 and 11 in parallel. A Wheatstone bridge circuit to be connected, an operational amplifier 13 for inputting the potentials of the bridge intermediate points a and b of the two series connection circuits of the Wheatstone bridge circuit, and the Wheatstone bridge according to the output of the operational amplifier 13 The first power transistor 5 that controls the amount of current supplied to the circuit is provided.
[0022]
In addition, the overheating prevention circuit 102 includes a comparative first fixed resistor 3, a comparative second fixed resistor 4, and a comparative third fixed resistor 44 connected in series, and the circuit connected in series is the basic flow rate detection circuit ( The Wheatstone bridge circuit is configured by connecting in parallel to the series connection circuit of the heating resistor 1 and the flow rate detection resistor 2 of the heat ray drive circuit 101), and each intermediate point of the two series connection circuits of the Wheatstone bridge circuit. A comparator 12 for inputting the potentials a and c, and the first fixed resistor 3 and the second fixed resistor 4 are connected in parallel, and when the output of the comparator 12 becomes high level, the comparator 12 is activated to the heating resistor 1. The power transistor 6 that limits the amount of current and the current limiting resistor 14 connected to the output terminal of the comparator 12 are provided.
[0023]
The flow rate detection circuit 100 supplies a current (voltage V1) from the power source VB to the Wheatstone bridge circuit of the basic flow rate detection circuit (heat wire drive circuit) 101 via the first power transistor 5. The Wheatstone bridge circuit of the basic flow rate detection circuit (heat line drive circuit) 101 is supplied with current from the power source VB, and becomes the voltages V2 and V3 at the bridge intermediate points a and b. The operational amplifier 13 and the first power transistor 5, the current flowing through the heating resistor 1 is adjusted so that the voltages V2 and V3 at the bridge intermediate points a and b are equal.
[0024]
The heating resistor 1 increases its resistance value Rh with a rise in temperature. The resistance value Rh is proportional to the temperature Th of the heating resistor, and the temperature Th is determined from the temperature Ta of the air to be measured. The temperature is set to Th = Ta + ΔTh so as to increase by a predetermined temperature ΔTh. That is, the resistance value Rh is expressed by the following formula (1).
Rh = Rho (1 + αTh) (1)
Here, Rho is a resistance value of the heating resistor 1 at a temperature of 0 ° C., and α is a temperature coefficient.
Next, the function of the flow rate detection circuit 100 of the hot-wire flow meter of the present embodiment configured as described above will be described.
[0025]
When the flow rate of the air passing through the heating resistor 1 of the basic flow rate detection circuit (heat line driving circuit) 101 increases, heat is taken from the heating resistor 1, the temperature Th of the heating resistor 1 decreases, and the bridge middle point a The value of voltage V2 increases. Then, the output of the operational amplifier 13 is increased, the current supplied from the first power transistor 5 to the heating resistor 1 is increased, and the temperature Th of the heating resistor 1 returns to a predetermined value.
[0026]
In other words, in the control system of only the basic flow rate detection circuit (heat wire drive circuit) 101 of the flow rate detection circuit 100, the temperature Th of the heating resistor 1 is the same before and after the increase of the air flow rate, but the heat generation. The current flowing through the resistor 1, that is, the values of the voltage V1 and the voltage V2 increase. This is because the flow rate detection circuit 100 including only the basic flow rate detection circuit (heat line drive circuit) 101 prevents excessive heating of the heating resistor 1 by limiting the value of the current flowing through the heating resistor 1 and the voltage V1. Means difficult to do.
[0027]
When the heating resistor 1 of the basic flow rate detection circuit (heat line driving circuit) 101 has a normal resistance value Rh, the overheating prevention circuit 102 is a voltage at the intermediate point c of the bridge between the comparative fixed resistor 3 and the fixed resistor 4. Since V4 is set lower than the voltage V2 at the bridge intermediate point a of the series connection circuit of the heating resistor 1 and the flow rate detection resistor 2, the output of the comparator 12 is in the low level state, and the second power transistor 6 is in a blocking state.
[0028]
The temperature compensation resistor 9 of the basic flow rate detection circuit (heat line drive circuit) 101 is disconnected, or the first power transistor 5 is short-circuited, causing the basic flow rate detection circuit (heat line drive circuit) 101 to fail and the heating resistor. When an excessive current flows through 1 and the temperature Th rises and the resistance value Rh of the heating resistor 1 exceeds a predetermined value Rhu, the voltage V2 at the bridge intermediate point a becomes lower than the voltage V4 at the bridge intermediate point c. For this reason, the output of the comparator 12 becomes High level, and the second power transistor 6 is activated.
[0029]
Thereby, the excessive current that has flowed through the heating resistor 1 flows into the second power transistor 6, and overheating of the heating resistor 1 can be avoided. The current flowing through the second power transistor 6 flows into the comparison fixed resistor 44 and the voltage V4 increases. As a result, an excessive current to the heating resistor 1 is avoided, and even if the temperature of the heating resistor 1 decreases, the output of the comparator 12 remains high, so the second power transistor 6 also remains active. To do.
[0030]
As described above, the overheating prevention circuit 102 of the present embodiment has the fixed resistor 3 for comparison, the fixed resistor 4 and the fixed resistor 44 connected in series, and the circuit connected in series is a basic flow rate detection circuit (heat line driving circuit). The Wheatstone bridge circuit is configured by connecting in parallel with the series connection circuit of the heating resistor 1 and the flow rate detection resistor 101, and when the excessive current flows through the heating resistor 1, the second power transistor 6 is activated. Since the excessive current flowing in the heating resistor 1 is allowed to flow into the power transistor 6, overheating of the heating resistor 1 is prevented.
[0031]
The overheat prevention circuit 102 of the present embodiment is configured as described above, so that the value of the current flowing through the heating resistor 1 does not depend on the voltage value, that is, the value of the heating resistor 1, that is, the heating resistor 1. When the resistance value of the power transistor 6 becomes a value when overheated or a value just before overheating, the power transistor 6 becomes active, and measures to prevent overheating of the heating resistor 1 should be taken. Can do.
A more specific operation of the overheating prevention circuit 102 of this embodiment will be described.
[0032]
Assuming that the allowable heating upper limit temperature of the heating resistor 1 of the basic flow rate detection circuit (heat line drive circuit) 101 is the temperature Thu, the resistance value Rhu at this time is expressed by the following equation (2), and the flow rate detection resistor 2 The resistance ratio between the resistance value R2 and the resistance values R3, R4, and R44 of the first, second, and third fixed resistors 3, 4, and 44 for comparison is expressed by the following equation (3).
Rhu = Rh0 (1 + αThu) (2)
R3 / (R4 + R44) = Rhu / R2 (3)
[0033]
However, since the current flowing through the first, second, and third fixed resistors 3, 4, 44 for comparison needs to be reduced so as not to affect the air flow measurement, R3 + R4 + R44 >> Rh + R2 And The resistance value of the current limiting resistor 14 connected to the output terminal of the comparator 12 is set to a value such that the current flowing through the current limiting resistor 14 does not become larger than the sink current of the comparator.
[0034]
The second power transistor 6 of the present embodiment shown in FIG. 1 is a bipolar transistor, but may be a field effect transistor such as a MOS transistor.
Further, the flow rate detection circuit 100 of the present embodiment can be realized by using an IGBT or a thyristor instead of the second power transistor 6.
Next, the hot wire type flow meter of the second embodiment of the present invention will be described.
[0035]
FIG. 2 shows a flow rate detection circuit 100 ′ of the hot-wire flow meter according to the second embodiment. The flow rate detection circuit 100 of the hot-wire flow meter according to the first embodiment is different from the power supply VB side. The difference is only in the point that the current limiting resistor 7 is added, and the other components are the same. Therefore, the same members are denoted by the same reference numerals.
[0036]
In the flow detection circuit 100 of the hot-wire flow meter of the first embodiment, a large current continues to flow through the first power transistor 5 in the operation after avoiding excessive heating current to the heating resistor 1. Then, there is a possibility that the first power transistor 5, the second power transistor 6, the comparative fixed resistor 44, etc. will generate heat.
[0037]
In order to avoid this, the flow rate detection circuit 100 ′ of the hot-wire flow meter of the second embodiment is provided with a current limiting resistor 7 on the power source VB side as shown in FIG. This is a reduction control.
[0038]
In the flow rate detection circuit 100 ′ of the present embodiment, the maximum rated current Ipmax of the second power transistor 6, the base-emitter voltage Vbe6, the maximum voltage Vbmax of the battery, the resistance value R44 of the comparison fixed resistor 44, and the current limit. The relationship with the resistance value R7 of the resistor 7 is expressed by the following equation (4).
R7> (VBmax−Vbe6) / Ipmax−R44 Formula (4)
The upper limit value of the resistance value R7 of the current limiting resistor 7 is selected so as to ensure the operation of the thermal air flow meter.
[0039]
For example, if the operating range of the hot-wire flow meter is “capable of measuring the air flow rate Q when the power supply voltage is VB”, the current required for measuring the air flow rate Q is Ih, and the first power transistor 5 When the base-emitter voltage is Vbe5, the resistance value of the heating resistor 1 is Rh, and the resistance value of the flow rate detection resistor 2 is R2, the power supply voltage VB is expressed by the following equation (5), and the current limiting resistor 7 The upper limit value of the resistance value R7 is expressed by the following equation (6).
VB> Ih * (Rh + R2 + R7) + Vbe5 Formula (5)
R7 <(VB-Vbe5) / Ih- (Rh + R2) Equation (6)
[0040]
Further, the power Wr7 consumed by the current limiting resistor 7 is expressed by the following equation (8), and the power Wr44 consumed by the comparative fixed resistor 44 is expressed by the following equation (8).
Wr7 = (VBmax-Vbe) 2R7 / (R4 + R7) 2 Equation (7)
Wr44 = (VBmax-Vbe) 2R44 / (R44 + R7) 2 Equation (8)
The maximum rated power consumption of the current limiting resistor 7 and the comparative fixed resistor 44 is preferably selected to be larger than the power Wr7 and the power Wr44, respectively.
After avoiding the overheating current, a signal can be obtained from the comparator 12 indicating that the hot wire flow meter has failed.
[0041]
As described above, when the hot-wire flow meter is operating normally and the heating resistor 1 has a normal value, the output of the comparator 12 is at a low level. However, when the heat ray driving circuit 101 fails and an excessive heating current flows, the output of the comparator 12 becomes High level. If the case where the output of the comparator 12 becomes High level is regarded as an abnormal signal, a failure diagnosis can be performed.
[0042]
FIG. 3 shows a flow rate detection circuit 100 ″ of the hot-wire flow meter according to the third embodiment of the present invention. The flow rate detection circuit 100 of the hot-wire flow meter according to the first embodiment is The difference is only in that a fuse 8 is added to the power supply VB side, and the other components are the same. Therefore, the same members are denoted by the same reference numerals.
[0043]
As shown in FIG. 3, after providing a fuse 8 and avoiding an excessive heating current, if an excess current further flows, the fuse 8 is cut to supply a current to the flow detection circuit 100 ″ of the thermal air flow meter. In this case, the power transistor 6 has only to flow a current necessary for breaking the fuse 8 for a predetermined time.
[0044]
FIG. 4 shows a flow rate detection circuit 100 ″ ′ of the hot-wire flow meter according to the fourth embodiment of the present invention, which is a flow rate detection circuit 100 of the hot-wire flow meter according to the first embodiment. The second member is different only in that the wiring to the second power transistor 6 is connected from the output side of the operational amplifier 13, and the other components are the same. Therefore, the same members are denoted by the same reference numerals.
[0045]
With the configuration of the flow rate detection circuit 100 ″ ′ of the hot-wire flow meter according to the fourth embodiment, the second power transistor 6 is activated, and an excessive current flowing in the heating resistor 1 is converted into the second power transistor 6. As a result, the first power transistor 5 is cut off to limit the current, and the excess current flowing to the heating resistor 1 can be limited.
[0046]
As mentioned above, although four embodiment of this invention was described, this invention is not limited to the said embodiment, In the range which does not deviate from the mind of this invention, a seed | species various change can be made. .
[0047]
【The invention's effect】
As understood from the above description, the hot-wire flow meter of the present invention overheats the heating resistor when a flow rate detection circuit (heat-wire drive circuit) malfunctions (a circuit for controlling the heating current or a power transistor) or the like is abnormal. In addition, the flow rate detection circuit can be prevented from being damaged, and a wide range of flow rate measurement is possible even if the flow rate detection circuit is newly provided with an overheating prevention circuit.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a flow rate detection circuit of a hot-wire flow meter including an overheating prevention circuit according to a first embodiment of the present invention.
FIG. 2 is a configuration diagram of a flow rate detection circuit of a hot-wire flow meter including a current limiting resistor and an overheating prevention circuit according to a second embodiment of the present invention.
FIG. 3 is a configuration diagram of a flow rate detection circuit of a hot wire type flow meter including a fuse and an overheating prevention circuit according to a third embodiment of the present invention.
FIG. 4 is a configuration diagram of a flow rate detection circuit of a hot-wire flow meter including a modified overheating prevention circuit according to a fourth embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Heating resistor, 2 ... Flow detection resistance, 3 ... Comparison 1st fixed resistance 4 ... Comparison 2nd fixed resistance, 5 ... 1st power transistor, 6 ... Second power transistor, 7 ... current limiting resistor, 8 ... fuse, 9 ... temperature compensation resistor, 10 ... first fixed resistor, 11 ... second fixed resistor, 12 ... comparison 13 ... operational amplifier, 14 ... current limiting resistor, 44 ... third fixed resistor for comparison, 100 ... flow rate detection circuit, 101 ... basic flow rate detection circuit (heat wire drive circuit), 102: Overheat prevention circuit.

Claims

A heat ray driving circuit having a heating resistor disposed in the fluid passage and having a resistance value that varies with temperature; and an overheating prevention circuit that detects the resistance value of the heating resistor and limits the resistance value change of the heating resistor; A hot-wire flow meter having a flow rate detection circuit for detecting the flow rate of fluid,
The heat ray drive circuit comprises a Wheatstone bridge circuit in which a first series connection circuit of a heating resistor, a flow rate detection resistor, a temperature compensation resistor, and a second series connection circuit of a fixed resistor (first and second) are connected in parallel. To detect the resistance value of the heating resistor,
The overheat prevention circuit includes a third series connection circuit of a fixed resistor for comparison (first), a fixed resistor for comparison (second), and a fixed resistor for comparison (third), and the comparator and the comparator A second power transistor that is activated by the output state of
The comparison fixed resistor (first) and the comparison fixed resistor (second) of the third series connection circuit are the first series of the heating resistor and the flow rate detection resistor of the heat ray driving circuit. A Wheatstone bridge circuit is configured by connecting in parallel to a connection circuit, and the comparator compares a voltage difference at a bridge midpoint between the first series connection circuit and the third series connection circuit, and The power transistor is activated when the voltage difference becomes a high level, and the current flows from the second power transistor to the comparative fixed resistor (third) to limit the current flowing to the heating resistor. A hot-wire flow meter.

The hot-wire flow meter according to claim 1 , wherein the second power transistor is provided in parallel with the heating resistor.

The hot wire driving circuit includes a first power transistor that supplies current to the heating resistor from a power source, and the overheating prevention circuit limits the current flowing through the heating resistor to the comparator and the second power. 3. The hot-wire flow meter according to claim 1, wherein the first power transistor is configured to be cut off by operation with the transistor.

A heat ray driving circuit having a heating resistor disposed in the fluid passage and having a resistance value that varies with temperature; an overheating preventing circuit that detects the resistance value of the heating resistor and restricts a change in the resistance value of the heating resistor; A hot-wire flow meter having a flow rate detection circuit for detecting the flow rate of fluid,
The heat ray drive circuit comprises a Wheatstone bridge circuit in which a first series connection circuit of a heating resistor, a flow rate detection resistor, a temperature compensation resistor, and a second series connection circuit of a fixed resistor (first and second) are connected in parallel. To detect the resistance value of the heating resistor,
The overheat prevention circuit includes a third series connection circuit of a fixed resistor for comparison (first), a fixed resistor for comparison (second), and a fixed resistor for comparison (third), and the comparator and the comparator IGBT or thyristor which is activated by the output state of
The comparison fixed resistor (first) and the comparison fixed resistor (second) of the third series connection circuit are the first series of the heating resistor and the flow rate detection resistor of the heat ray driving circuit. A Wheatstone bridge circuit is configured by connecting in parallel with a connection circuit, and the comparator compares a voltage difference at a bridge midpoint between the first series connection circuit and the third series connection circuit, and the IGBT or The thyristor is activated when the voltage difference becomes a high level, and the current flows from the IGBT or thyristor to the comparative fixed resistor (third) to limit the current flowing to the heating resistor. Hot wire flow meter.

The flow rate detection circuit, a protection resistor for limiting a current flowing through the hot wire drive circuit, hot-wire flow meter according to claim 1, any one of 4 possible, characterized in that provided on the power supply side.

The flow rate detection circuit, when the allowable maximum rated current or more current to the hot wire drive circuit flows, any one of claims 1 to 5, characterized in that it comprises a fuse for interrupting the electric current to the power supply side The hot-wire flow meter according to one item.

The hot-wire flow meter according to any one of claims 1 to 6 , wherein the flow rate detection circuit has a configuration for diagnosing overheating of the heating resistor and outputting a failure diagnosis signal. .