potential differences from ground to ground why?

Signal Ground to DC ground was 10 millivolts
Signal Ground to Servo ground was 16.2 millivolts
Signal Ground to Chassis Ground was 1.4 Millivolts

DC ground to Servo Ground was 6 Millivolts
DC ground to Chassis Ground was 11.2 millivolts

Servo Ground To Chassis Ground was 17.6 millivolts


Why is there potential differences? and where does the potential differences come from?

When I measured ohms between each ground , it was in the milliohms , so they are tied together and not isolated? what would I measure in ohms to know if the grounds are isolated?

When I measured ohms between each ground , it was in the milliohms , so they are tied together and not isolated?

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Reasonable conclusion.

I presume you are measuring DC voltages. Most likely you are seeing voltage drops caused by internal circuit currents, e.g. supply current.

what would I measure in ohms to know if the grounds are isolated?

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It depends. In some cases, low ohmic resistors could be intentionally connected between separate ground nodes, e.g. for current measurement. As a first guess, I would consider < 1 ohm as common node.

But there might be also transformer windings or inductors present in the circuit which can't be identified by a DC resistance measurement. Preferably you'll look at a circuit schematic, if not available inspect the layout and try to derive the circuit function.

Yes measuring DC millivolts range on my Fluke DVM 87

Most likely you are seeing voltage drops caused by internal circuit currents, e.g. supply current.

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I thought ground would be zero , but it's in the millivolts compared to other grounds , so there is a potential difference

Where does there internal circuit current come from , they sink to the ground through IC chip ground pins?

transformer windings or inductors present in the circuit which can't be identified by a DC resistance measurement.

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So how would you measure the ground to ground testing? what DVM meter setting AC volts?

I'm trying to measure the potential difference between each different isolated ground or non-isolated ground, how can I do this?

danny davis said:

...Where does there internal circuit current come from ...?

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Note that printed circuit boards cannot be understood as ideal conductors, and each infinitesimal part of copper track has its own resistance. Any advanced simulation tool could be able to model effect of dropping voltage due to PCB geometry, but would become impraticable. Just accept that is expected to note some drop voltage on many parts on energized circuit, even electrically connected to same net.



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Why are you measuring and talking about such tiny DC voltages?
They will not affect AC circuits and they are 44 times or more smaller than the smallest TTL logic (800mV low) voltage.
They are 217 times or more smaller than the smallest Cmos logic 3.9V (low) voltage.

Im just trying to understands why there is millivolt voltage drops from ground to grounds fans where does the voltage come from

The reason for voltage differences has been explained by several contributors.