Strange behavior measuring voltage difference between two independent voltage source

Hi, I have meet a confusing problem when try to measuring voltage difference between two independent voltage source. I used two dc voltage sources as separate port for my instrument. Either port could be regulated from 0-100V. The two port then were connected to electrodes that are separated apart from each other. Then I need to know what is the voltage difference between the two electrodes.

The two voltage was set to be 50V and 60V. I use a Fluke multimeter to measure voltage difference between each port and the GND. And I get the read 50V and 60 respectively, no problem here. Then I try to measure voltage difference between the two port. To my surprise, I read 0V from the multimeter. Theoretically, I should have a 10V like that.

I know that phenomenon is arising from the internal resistance of the multimeter (~10Mohm). But to what extent do I have to trust the multimeter? Why the port-to-GND voltage read is much more reasonable than port-to-port voltage read?And in my problem, if I want to know the voltage difference between the two port, what value should I trust? almost 0V or close to 10V?

Thank you.

Hi Toyonlin,

are the two voltage source isolated from each other or have a common ground?

Enjoy your design work!

The two sources are isolated from each other.

Attached is a schematic diagram showing the connection of power supply.

Source 1 is a home-made power supply that grounded to earth. Source 2 is a commercial one. The output of source 2 is relative to its COMM port. But I connected COMM to earth ground.

HTA said:

Hi Toyonlin,

are the two voltage source isolated from each other or have a common ground?

Enjoy your design work!

Click to expand...

I think for the Fluke multimeter the problem is that it has no reference to measure against. Try to connect a 1M ohm between output 2 and 1 and measure.

Enjoy your design work!

Your schematic shows that the two supplies are connected together with the same earth ground. If they are isolated then they do not share the same ground.
If one probe of your voltage meter connects to 50V and the other probe connects to 60V then the meter should show 10V.

Yes, it should be. But I didn't have the right reading now. So it's confusing to me.

I will try to see if connect a 1Mohm between port 1 and 2 would help later, as suggested by HTA.

If you place 1M resistor it may happen that reading will not be 10V but only 9.09V. Who knows...

I connect a ~10M resistor in between the two port, and measured voltage across the resistor. What I read from the multimeter is still 0V. (Voltage of port 1 and 2 was measured to be 50V and 60V against earth ground.) Quite difficult to understand.

Is it possible that current flow into the multimeter is so small that could not induce a meaningful reading?

Interesting! Have you connected the ground of each voltage source together? The impedance of the multimeter should be very high, but you can try also 100k Ohm.

Enjoy your troubleshooting work!

It is possible that current is even 0 so as reading. How it happened? Current should come from somewhere flow around and return to the same point. If it can't there is no current.

Try these things:
1) Measure the two grounds using your multimeter. You should expect it to be 0V ideally. See what does the measurement show you.
2) Instead of 1M ohms or 10 M ohms in between the the port 1 and port 2, put a small value resistor like may be few 1 K ohms or so. Measure port one and port 2.
3) Connect the two grounds of the two power supplies to each other, so they have the same ground. Then definitely when you measure the two ports you should measure 10 V on your multimeter.

I would like to know what does the multimeter read for the above three test cases.

Obviously the 0V of the voltage sources are not connected together.

Here are what I measured so far:

1) Measuring two grounds reads 0V.

2) Inserting a 100k between two ports, voltage between two ports is 0V. Each to its ground is 10.01 and 20.1 V, respectively.

3) Connecting two ground ports, measuring two output ports reads 0V.

4) Only when I increased the voltage difference between two ports higher than 15V, the multimeter starts to give me a readable voltage. And the readings will increase as voltage across two ports thereafter.

I am considering it is the problem of home-made power supply. Since when I changed to another home-made power supply, the strange behavior disappears.

i think its a problem of inappropriate cc setting in the power supply.

By inappropriate cc setting, do you mean a low level limit of output current?

Venkadesh_M said:

i think its a problem of inappropriate cc setting in the power supply.

Click to expand...

yeah,

Hi can you have a try of following ?
set first source as some 10 V using multimeter,
set second source as 20V.
now without any connection between two sources measure the voltage between two negatives and then if it is zero
now connect the positive of first source to the negative of second source and measure the voltage between other two points.

Thank you, Venkadesh_M. The problem now solved.

I think it is due to our engineer has somehow miss connected of ground line to home-made power supply. Since we have one that work correctly, a comparison between those two gives engineer a hint. He reconnected wiring and now the supply works.

Thank you very much for your help.:grin:

Venkadesh_M said:

yeah,

Hi can you have a try of following ?
set first source as some 10 V using multimeter,
set second source as 20V.
now without any connection between two sources measure the voltage between two negatives and then if it is zero
now connect the positive of first source to the negative of second source and measure the voltage between other two points.

Click to expand...