JPH0682501A - Megohmmeter - Google Patents

Abstract

PURPOSE:To realize a megohmmeter in which safety of guard terminal is ensured under AC voltage measuring state and internal circuit is protected. CONSTITUTION:The megohmmeter comprises an earth terminal, a line terminal, a guard terminal G, a DC high voltage generating circuit 1, a measuring/ indicating circuit 2, and a section for selecting a plurality of measuring functions including a megohm measuring function. The earth terminal is connected with the DC high voltage generating circuit 1 and the measuring/indicating circuit 2, the line terminal is connected with the measuring/indicating circuit 2, and the guard terminal G is connected through a protective resistor R and the measuring function selecting section with a common. Two diodes D1, D2 are connected with the path for the guard terminal G in order to limit current in both directions thus making the guard terminal G effective only when megohm measurement is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an insulation resistance tester, and more particularly to ensuring safety of guard terminals and improving internal circuit protection.

[0002]

2. Description of the Related Art FIG. 2 is a block diagram showing an example of a conventional insulation resistance meter. In the figure, reference numeral 1 is a DC high voltage generating circuit, which is connected to a ground terminal E. Reference numeral 2 is a measurement display circuit, which is connected to the ground terminal E and the line terminal L. Reference numeral 3 is an encoder that outputs control data for selecting a measurement function to the DC high voltage generation circuit 1 and the measurement display circuit 2. The input terminal side is “insulation resistance (MΩ)”, “AC voltage (AC)”, “resistance ( Ω) ”, which is connected to the common via the switch 4 for selecting one of the measurement functions and is also connected to the power supply line of the voltage V _C via the pull-up resistor R _PU . G is a guard terminal,
It is connected to the common via a protection resistor R.

In such a configuration, the measured resistance R _X or the measured AC voltage source is connected between the ground terminal E and the line terminal L. In measuring the resistance, a predetermined voltage is output from the DC high voltage generation circuit 1 according to the control data from the encoder 3, and a current I _X flows through the measured resistance R _X. The measurement display circuit 2 measures and displays the resistance value of the measured resistance R _X based on the current I _X.

On the other hand, in measuring the AC voltage, the encoder 3 controls the output of the DC high voltage generating circuit 1 to zero. The measured AC voltage is input so that the ground terminal E side has a high voltage and the line terminal L side has a low voltage. As a result, the measurement display circuit 2 measures and displays the voltage value of the measured AC voltage. By the way, in measuring the resistance R _X to be measured, a current may leak on the surface of the resistance R _{X to} be measured and enter the measurement display circuit 2 through the line terminal L to cause a measurement error. Therefore, for example, when measuring the insulation resistance of the insulator between the inner conductor and the outer conductor of the coaxial cable, the inner conductor of the coaxial cable is connected to the ground terminal E, the insulator is connected to the line terminal L, and the guard terminal is connected. An external conductor is connected to G so that a leak current flowing on the surface of the resistance R _{X to} be measured is passed to the guard terminal G so that the insulation resistance of the insulator can be accurately measured.

[0005]

However, in such a conventional circuit configuration, since the guard terminal G is only connected to the common via the protection resistor R, the line terminal L is connected to the high voltage side of the measured AC voltage. Measurement display circuit 2 if connected incorrectly
Then, a high voltage is output to the guard terminal G through the encoder 3 and there is a risk of electric shock.

If the measurement voltage is applied to the guard terminal G by mistake, the internal circuit may be damaged. The present invention solves such a problem, and an object thereof is to realize an insulation resistance meter in which the safety of the guard terminal is ensured in the AC voltage measurement state and the internal circuit is protected.

[0007]

In order to solve such problems, the present invention provides a plurality of ground terminals, line terminals, guard terminals, DC high voltage generating circuits, measurement display circuits, and insulation resistance measuring functions. It has a measurement function selection section to select the measurement function, the DC terminal high voltage generation circuit and the measurement display circuit are connected to the ground terminal, the measurement display circuit is connected to the line terminal, the protective resistance and the measurement function selection to the guard terminal. In an insulation resistance tester in which a common is connected via a section, two diodes are connected to the path of the guard terminal so as to limit bidirectional current so that the guard terminal is effective only when measuring insulation resistance. It is characterized by being configured.

[0008]

The one diode allows a common leak current to flow into the guard terminal when measuring the insulation resistance and prevents the current from flowing out to the guard terminal when measuring the AC voltage. Further, the other diode prevents a leak current flowing into the guard terminal from flowing into the internal circuit at the time of measuring the insulation resistance, and also prevents a current from flowing into the internal circuit when a voltage is improperly applied to the guard terminal.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a configuration explanatory view of an embodiment of the present invention.
The same reference numerals are given to the parts common to the above. In the figure, two diodes D1 and D2 are connected to the path of the guard terminal G so as to limit a bidirectional current.
That is, the anode of the diode D1 is connected to one end of the protection resistor R, and the cathode is connected to the MΩ contact of the switch 4. The anode of the diode D2 is the encoder 3
Connected to the connection point between the input terminal of and the pull-up resistor R _PU ,
The cathode is connected to the MΩ contact of the switch 4 together with the cathode of the diode D1.

The operation of such a configuration will be described when each measurement mode is selected. [MΩ measurement mode] The leak current flowing into the guard terminal G will flow to the common via the diode D1.
The guard terminal G functions as a guard. The diode D1 also prevents the current flowing from the pull-up resistor R _PU from flowing to the guard terminal G.

On the other hand, the diode D2 prevents the leak current flowing into the guard terminal G from flowing into the encoder 3 and also prevents the current from flowing into the encoder 3 when a voltage is improperly applied to the guard terminal G. [ACV, Ω measurement mode] The guard terminal G is separated from the encoder 3 and the pull-up resistor R _PU by the diodes D1 and D2. Therefore, even if a high voltage is input to the line terminal L, no current will flow to the guard terminal G, so there is no electric shock even if the guard terminal G is touched, and it is safe. It is possible to protect the internal circuits such as the encoder 3 without the current flowing into the encoder 3.

[0012]

As described in detail above, according to the present invention, since the guard terminal G is isolated by the diodes D1 and D2 so as to effectively function only in the MΩ measurement mode, it is possible to measure in the AC voltage measurement state. It is possible to realize an insulation resistance meter in which the safety of the guard terminal is secured and the internal circuit is protected.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a block diagram of an example of a conventional insulation resistance meter.

[Explanation of symbols]

 1 DC high voltage generation circuit 2 Measurement display circuit 3 Encoder 4 switch

Claims (1)

[Claims] 1. A grounding terminal, a line terminal, a guard terminal, a DC high voltage generating circuit, a measurement display circuit, and a measurement function selecting section for selecting a plurality of measurement functions including an insulation resistance measuring function, and the ground terminal has a DC high voltage. In the insulation resistance tester, in which the generation circuit and the measurement display circuit are connected, the measurement display circuit is connected to the line terminal, and the guard terminal is connected to the protection resistor and the common via the measurement function selection unit, the path of the guard terminal An insulation resistance meter characterized in that a diode that limits bidirectional current is connected to and the guard terminal is enabled only when measuring insulation resistance.