a little help in analysing this circuit

[maicael]

Hello everyone.
I have constructed this circuit but I want to clarify something which is the fact that I am looking at the output + and - and I am wondering if the output - can be connected to ground as you can see from the circuit, ground is different from output - .
Looking at the website
**broken link removed**
And looking at the instructions for calibration I ran into problems when I decided to use small digital display volt meter which can be attached to the circuit instead of the conventional voltmeters but the issue here is that that voltmeter power supply and its input for displaying measured voltage share a common ground which means that when I connect the output - to the voltmeter which has been powered I am effectively shorting the output - to ground and this affects my circuit as from the circuit ground and output - are different.

 

[audioguru]

Many electronic voltmeters must use a battery or power supply that is completely isolated from the circuit they are measuring.

 

[maicael]

Many electronic voltmeters must use a battery or power supply that is completely isolated from the circuit they are measuring.

hello,so basically i will have to power the volt meter seperately.
the volt meter has the following +9v input, GND-this serves as power to the volt meter. then it has IN+ and IN- and this is where we connect our circuit to be measured but it turns out the the IN- and GND have continuity(shorted) and i tried to power thw voltmeter fr0m this circuit by using an extra 7809 regulator which works ok but then when i put the output - to IN- then an issue occurs as output - is now effectively GND.

 

[Nigel Goodwin]

You're not listening to what AudioGuru said - you must have an entirely separate PSU for the voltmeter module - you can't just use another regulator from the same supply, they must be unconnected in any way.

 

[maicael]

hello,
yes audioguru is right i have just tested it.thanks for the help.

 

[audioguru]

Can't you power the voltmeter from a little battery? Then it will be completely isolated from the circuit.
Oh, does your voltmeter have high current old fashioned LEDs instead of a very low current LCD display? Only my battery tester uses LEDs. They are the load for a battery test.