The circuit diagram of the ohm meter in this site is very usefull for measuring the low resistance range form 0 to 1 and and 0 to 10. You can adjust the range according to your wise. The circuit for a low Ohm meter described here is simple and has the following advantages over other meters:
1. Do don't need to see it again and again just set it once and forget it forever).
2. Scale reading capacity of this circuit is from zero to a fixed value rather than infinity.
3. This meter is low power consuming as it uses a 1.5-volt penlight cell, two scales (0-1 ohms and 0-10 ohms) over a dial and a push-to-on switch large power consumption by the circuit.
The circuit diagram that you can see below is the ohm meter which can measure the resistance for 0 to 10 ohm . You can see the selector switch over there the circuit diagram which can select the measuring rang form 0 to 1 ohm and 0 to 10 ohms. Transistor T1 works as a constant current generator which passes a know current through the resistors which resistance is to be measured. If the maximum drop of the voltage across the emitter of the transistor T1 will be more than 100 mV and the ground is displayed on the meter whose internal resistance is much higher than the testing resistance that is 10 ohms. Because of which this ohm meter can not load the circuit.
There is a diode D3 across the micro ammeter which is use to protect the oh meter form the overload during the the absence of the testing resistor which resistance is to be measured.
Resistors R1, VR1 , R2 , R3 , D1 , D2 and R4 are biased by the transistor T1. Diodes D1 and D2 are use for holding the bias level constant inspite of the decaying battery.
The scale of the meter in this project should have 0-500 μA . The shunt resistance in this project can be any general propose meter. Transistor T1 is the silicon npn with a high gain factor.
Now the meter should be adjust by shorting probes A and B. If the meter is adjust before the it shows a zero resistance. You only have to adjust in 0 to 10 ohm scale first and other adjustments will be follow automatically. This can be easily built within a few minutes. This is the very useful project for the electronics beginners.
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