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# High Voltage Protector for Meters by CA3130

2016-09-25 12:10
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#### Figure 1

Normally, in a digital voltmeter, high input impedance of the preamplifier stage is protected against very high voltages by two diodes. One of them is connected to the positive terminal and the other one is connected to the negative terminal of the power supply. Practically this kind of protection is very sufficient. But the diodes that are used for this process must have very low leakage current values. The main problem is that kind of diodes are expensive and it is sometimes hard to provide them. On the other hand electronic hobbyists commonly choose general purpose diodes like 1N4148. But this situation causes to occur some millivolts on 1M input impedance. Today, it is usually very important to measure the values with high accuracy so this extra voltages will affect our experimental results.

This circuit overcomes this problem by usingFETs ( BF2526A ) instead of diodes. Under reverse bias, leakage current of theFETis only 12uA in spite of the diode’s is about 5.2nA. This means you can increase the input impedance up to 10M with no drawback.

You can see an input stage of a high impedance voltmeter in the schematic. R1 resistor forms the 10 Mohm input impedance. Tr1 and Tr2FETs which are used as diodes do the protection function. ThisFETs can endure maximum 10mA. The other part of the circuit which includes IC1, Tr3 and other components, forms a voltage follower with very low impedance.

Voltage supply (VS) should be any value between 5V to 15V. Zener voltage (VZ) should be chosen 2V less than VS.

Calibration of the circuit is very easy. P1 potentiometer adjusts the output voltage until it is equal to the input voltage and this process finishes.

Theoretically input can be protected up to 1000V with this circuit. But note that input stage resistors must have high watt values to provide this.