# Two DC power supplies with large voltage difference between their earths

#### brid971

Joined Feb 26, 2020
4
I recently purchased a Riden RD6006 power supply and attempted to use it in conjunction with my homemade power supply made from a computer PSU. Sparks literally flew when I attempted to connect the grounds of each supply together. I discovered that the voltage difference between the earths was about 66 volts which happens to be the max power supply to the Riden unit. My background is mechanical engineering design with some electronics but not enough to understand this problem.

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#### AlbertHall

Joined Jun 4, 2014
10,777
The PC supply negative is connected to the supply earth internally. The Riden supply output is not so connected, the output is floating. This supply is provided with an earth terminal on the front panel (the green one) so you can earth the output if you want to. The supply will have capacitance between the output and the supply (e.g. between the windings in the transformer) which will mean that there will be some voltage between the output and earth but at very low current. This is normal.

#### brid971

Joined Feb 26, 2020
4
The PC supply negative is connected to the supply earth internally. The Riden supply output is not so connected, the output is floating. This supply is provided with an earth terminal on the front panel (the green one) so you can earth the output if you want to. The supply will have capacitance between the output and the supply (e.g. between the windings in the transformer) which will mean that there will be some voltage between the output and earth but at very low current. This is normal.
I'm not sure I fully understand. When I measure the voltage between the black terminal on the Riden unit and its casing, it measures 62 volts. The green terminal to the casing is the same at 62V. Also, the voltage from either the green or the black terminals to the ground on my homemade supply is 62V. So if this is nomal, I guess that means I cannot connect the grounds of both power supplies. Is that correct.

#### Dodgydave

Joined Jun 22, 2012
9,854
Looks like it's a Switchmode psu,

Probably not Earthed to ground...

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#### AlbertHall

Joined Jun 4, 2014
10,777
The green terminal to the casing is the same at 62V.
Then there is a problem. The case should be grounded, the green terminal should be grounded, and both should connect to the earth pin of the supply plug.

#### brid971

Joined Feb 26, 2020
4
Thanks for the video link. My problem was exactly as described in the video where a screw holding the power supply in position was was causing a short

#### brid971

Joined Feb 26, 2020
4
Then there is a problem. The case should be grounded, the green terminal should be grounded, and both should connect to the earth pin of the supply plug.
Having solved the problem caused by a screw holding the casing in place I find that the case is indeed grounded to the supply plug. However neither the black or green terminals on the Riden unit are grounded to the supply plug or casing. There is also no connection between the green and black terminal. I'm guessing this is all normal but I hope someone might clarify.

#### AlbertHall

Joined Jun 4, 2014
10,777
The green terminal should be connected to the case and supply earth. It is there so you can ground either output terminal if you wish.

#### MikeA

Joined Jan 20, 2013
239
The green terminal on the RD6006 is actually a battery charging terminal. It's like the red (+) terminal, but only used for charging batteries. So it's not a ground terminal of any type.

#### MikeA

Joined Jan 20, 2013
239
I have the same power supply and don't understand this type of behavior ...

Say I have the output set at 10V and 5A.

When the output terminals are shorted with very low resistance, the display shows say 0.01V and 5A, and whatever is connected is not heating up.

With a traditional power supply I think it should send 10V and 5A across the "short" and heat it up. Is that correct?

#### AlbertHall

Joined Jun 4, 2014
10,777
No. A 'short circuit' means zero ohms, or very close to that. From ohms law, 5A in zero ohms is zero volts.

#### MikeA

Joined Jan 20, 2013
239
No. A 'short circuit' means zero ohms, or very close to that. From ohms law, 5A in zero ohms is zero volts.
I get what you are saying.

I think my mistake was overestimating how much a short piece of 32awg copper wire at 6A should warm up. The answer is not detectable to touch.

#### DNA Robotics

Joined Jun 13, 2014
596
I would expect 10 volts at 6 amps to melt a short piece of 32awg copper wire.

#### MikeA

Joined Jan 20, 2013
239
I would expect 10 volts at 6 amps to melt a short piece of 32awg copper wire.
That's what I thought also. But I did a more accurate experiment.

Got out the calipers and verified that a 4" piece of wire was exactly 34 awg. That piece of wire should have a resistance of 0.08539196 ohms at room temperature.

The piece of wire was barely glowing red. So that's about 500°C. Then the resistance should have been around 0.24524570912 ohms at that temperature. So using Ohm's law it should have been around 8.83W of heat dissipation through the hair thin wire.