# Voltage Difference between Two Power Supplies without Any Connection to Each Other and Earth

#### alexfrey

Joined Feb 23, 2019
22
Let’s think we have two 24V power supply. These power supplies doesnt have connection to earth and each other. How much voltage would I see if I measure voltage difference between ‘+’ of first power supply and ‘-‘ of second one ? Once I tried it and I saw something like 7-8V but I didn’t think it was a logical value. I mean I shouldn’t see any value but because of structure of digital multimeter I saw some pointless value but then my collegue told me that we can measure a value and As he said if we measure 15V and if we placed a lamp operating with 15V between ‘+’ of first ps and ‘-‘ of second ps the lamp will flash.

Is that correct ? If yes , why there is a voltage difference since there is no referance to each other ?

Joined Jul 18, 2013
28,779
The only voltage you should see is due to any capacitive/inductive coupling that may be present, otherwise it is considered an open circuit and as such should not support any current.
Or at least one of any significance.
The reading is also due to modern very high impedance meters, compared to the old moving coil versions.
Max.

#### shteii01

Joined Feb 19, 2010
4,644
Because components are not perfect. Each component has tolerance. The more components you have, the more tolerances you have, the more different the final results will be.

#### alexfrey

Joined Feb 23, 2019
22
The only voltage you should see is due to any capacitive/inductive coupling that may be present, otherwise it is considered an open circuit and as such should not support any current.
Or at least one of any significance.
The reading is also due to modern very high impedance meters, compared to the old moving coil versions.
Max.
Thanks for respond
So my thought was correct that in normal case we should't see any voltage (or any logical current flow there) ?

#### crutschow

Joined Mar 14, 2008
34,700
Thanks for respond
So my thought was correct that in normal case we should't see any voltage (or any logical current flow there) ?
Yes.
In theory there is no measurable voltage since there is not a path for the current to volt.
But it is possible to have a difference in static charge between the two if they are completely isolated that would give a difference in the electrical field, which can be measured with a electrostatic voltage meter.

Joined Mar 10, 2018
4,057
Do I remember my physics correctly that two bodies in free space with a different
charge on each of them establishes a field between them, a potential gradient ?

No current is flowing.

Regards, Dana.

PS : Just put my glasses on and see crutschow has already addressed this. A day
late and a dollar short, the story of my life.

#### mvas

Joined Jun 19, 2017
539
Let’s think we have two 24V power supply. These power supplies doesnt have connection to earth and each other. How much voltage would I see if I measure voltage difference between ‘+’ of first power supply and ‘-‘ of second one ? Once I tried it and I saw something like 7-8V but I didn’t think it was a logical value. I mean I shouldn’t see any value but because of structure of digital multimeter I saw some pointless value but then my collegue told me that we can measure a value and As he said if we measure 15V and if we placed a lamp operating with 15V between ‘+’ of first ps and ‘-‘ of second ps the lamp will flash.

Is that correct ? If yes , why there is a voltage difference since there is no reference to each other ?
Another term that may apply is ... Leakage Current.
If those two Power Supplies are plugged in to the AC Mains then there is a connection between them.
And if you actually get "a flash on bulb", then that is something other than a simple Static Electricity Charge.

#### BobTPH

Joined Jun 5, 2013
9,177
Connecting the two isolated power supplies through a load is, essentially, discharging a very small capacitor.

Bob