Digitial Multiimeter Question

[Vindhyachal.takniki]

1. I am using digital mutilmeter for measuring voltmeter.
It has 9V battery for multimeter operation.

2. My question is when I use digital multimeter for voltmeter purpose, do black probe of multimeter is shorted to its internal 9V battery or both are isolated?

[EEVblog]

Usually connected directly. Certainly not isolated because it doesn't need to be.
Why do you ask?

[electr_peter]

DMM red and black probes are most likely at +/- half voltage of the internal battery. DO NOT try to measure DMM's internal battery with same DMM probes - magic smoke will escape.

[hamdi.tn]

i guess he's trying to power up the DMM from a lab power supply or any thing connected to main, and wondering about the probes being isolated. 

[Vindhyachal.takniki]

@EEVblog , I checked connectivity between black probe & battery GND using another multimter.
They were not shorted together.

I am trying to build a null detetctor as I have attached a circuit.

1. GND of null detector & measuring ckt is different.
2. I am planning to use ADS1248 (24 bit adc) to measure voltage drop across 100ohm resistor.
3. Null detetor ckt is fixed I cannot make changes in it. I have to make my measuring circuit according to it.
4. There are three points I have selcetd as in ckt: P,Q,R
5. This leads to three cases, 2 of them are : +-(0.1uV - 100uV) , +-(0.1mV - 100mV).
6. Third case leads to voltage : +-(100mV - 40V), I have break down it using 1 ohm resistor, so +-(1mV - 400mV)
7. Any of three cases can come during functioning. So I have to do decision while reading data.


I had asked this question as I want to short ground of my measuring circuit with null detector circuit. This will make measurement easy.
That why I had asked if this can be done.

I will switch the two gnds through relay & measure the result.

[dannyf]

I have attached a circuit.

You have lots of symbols on a chart. You don't have is a solid understanding of how you intended the circuit to work and why that requires the understanding of if a multi-meter's negative lead is connected to its battery negative terminal.

I would suggest that you step back and ask yourself what you are trying to do.