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Description:
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...
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Description:
While detecting discontinuities on a circuit board, it is
probable to include resistors, semiconductors or other elements in
measurements. This situation may cause wrong results. On the other hand
sometimes the voltage or current of the multimeter may defect some
circuit components.
Our circuit overcomes this inconvenient
conditions. The circuit determines greater than 1 ohm values as
discontinuity. Measure voltage is not more than 2mV. So no kind of
diode, IC or other component is bypassed. Maximum current output of
the circuit is about 200uA.
Indicator of the circuit is a LED. Voltage supply may be two 9 Volt batteries.
Voltage
adjustment of the amplifier is done by P1 potentiometer. To do this
process, first short circuit the probes and then turn P1 until LED
brights. When you separate the probes again LED will fade out.
This is a cheap and very useful circuit. You can build it on a PCB to use more easily.
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