Eventhough a common multimeter is appropriate for almost all DC voltage measurement it may at times demonstrate to be insufficient.
This can be the situation when designing measurements on higher impedance circuits which are unable to provide current needed to work perhaps a subtle moving coil meter of the type usually utilized in a multimeter.
The loading result of the meter in that case leads to the voltage in the test point to considerably drop, providing an deceptive reading.
The issue gets over with this FET voltmeter circuit which includes six ranges through 0.5V to 100V FSD having an input impedance of a little more than 11 meg on all ranges.
This provides a sensitivity of more than 22 meg/V on the 0.5V range falling to a little over 11010 on. the 100V range (many multimeters possess a sensitivity of 20k/V).
A FET unity voltage gain buffer amplifier depending on Q1 is employed to have the required higher input impedance, which can be an inherent element of a FET.
An easy FET voltmeter circuit is fed from its output, which includes a FSD value of 0.5V within the 'X1' position of SW2, or 1V in its 'X2' ,position. R1 and R8 correspondingly are actually modified to have the circuit the right FSD values.
You will find a small quiescent output voltage from the buffer stage and thus a bridge circuit is utilized to provide zero quiescent voltage over the meter circuitry.
To provide good stability another source follower is employed to create another portion of the bridge which leads to no visible meter drift. RV1 is utilized to electrically zero the meter.
D1 and D2 merely secure the meter activity towards severe overloading. An input attenuator enables you to slow up the simple sensitivity by a factor of 10 or 100, providing FSD values of OV5, 5V and 50V with SW2 in the 'X1' position, or 1V, 10V and 100V if it is fixed at the 'X1' position.
The circuit will not have access to get constructed like a accomplish instrument within its own right, also it can make an outstanding add on unit for almost any multimeter which includes a 50uA range.
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