On rare occasions I need positive and negative rails. But I only have two identical single rail power bench supplies. I would like to connect them in series with the center point grounded. Just to be sure, is this as simple as drawn in the attached diagram? It shows four supplies but I would only use two, with a reverse-biased diode across the output of each. Are there any other measures that should be taken to protect the supplies when used in this way?

 

Make sure supply ground is isolated from earth ground.

 

The green "earth" socket on both power supplies is tied to the center pin of the power cables. However, the black "negative" socket is not, so I guess I am OK. This particular model also has a short circuit alarm.

Please correct me if I am wrong, but I assume I need at least a 10A diode to match the supply's maximum current rating.

 

Please correct me if I am wrong, but I assume I need at least a 10A diode to match the supply's maximum current rating.

You don't need the diodes.

 

So then, the PS's have internal reverse polarity protection. I heard most modern ones do, but I just wanted to be safe. Thanks for your replies.

 

So then, the PS's have internal reverse polarity protection

How is this reverse polarity generated?

 

As I understand it, in the case described above, if one PS is switched on before the other.

 

As I understand it, in the case described above, if one PS is switched on before the other.

Okay, diodes would protect against that.
Schottky diodes would be preferred, and they should be able to carry the maximum load load.

Better would be to use a DPST switch to connect the load after both supplies have been turned on.

 

Better would be to use a DPST switch to connect the load after both supplies have been turned on.

Great idea!

 

If you have two bench PSUs with + GND - terminals, remove any grounding strap on the PSU. In other words, make both supplies "floating". Connect the -ve terminal of the positive supply to the +ve terminal of the negative supply. Now you have a "floating" dual supply, or a single supply giving the sum of two voltages.

If you want the COM node to be referenced to GND, then connect COM to GND. The supplies are no longer "floating".

Turn on both supplies first before connecting your load.



You can do the same thing with two 9 V batteries.

 

Thank you. I have made a screen shot of your diagram.

 

Certainly using dual supplies as discussed here is fairly common. And the cautions suggested all have made sense. The diode protection against reversed polarity is an interesting consideration. Possibly meriting additional discussion.

 

I have since learned that many modern bench PS's have this type of diode protection already built in. The adventurous could open theirs up and have a look to confirm. But I prefer to err on the side of caution.