How to add split-rail outputs to a bench supply that didn't come with them

Thread Starter

kmpres

Joined Jan 24, 2016
21
Hi Guys,

I have a Gophert NPS-1601 variable bench DC power supply and a number of single voltage supplies. They all work well but lack split-rail capability. I have a project coming up that will require plus and minus supply rails so would like to add a minus output post to the Gophert supply. Or maybe make an external circuit with plus-ground-minus posts on it so I don't modify the supply's front panel needlessly. Some supplies on the net (Matrix Technology MPS 3206 for example) have split-rail outputs on their front panels but don't advertise that fact nor even list the capability in their spec sheets, which strikes me as odd.

Given the above I can't imagine that adding split outputs to a variable supply that didn't come with them would terribly difficult. If I added them to the Gophert, which is 0 to 32V, 0 to 5A, CV+CC capable, would it be as easy as placing a simple voltage divider between the plus and ground posts and using the junction between the resistors as the test circuit's virtual ground? What values would I use for resistance and wattage that could pass the supply's maximum power output without consuming it as heat? Is it that simple or am I missing something?

I could, of course, build a split-rail supply just for this project, but that's not a permanent solution. I have also put two 12V server supplies in series in the past (and isolated the DC ground from one of them to prevent sparks) with no problems, but those supplies are not variable nor CC capable.

Thanks!
 

Irving

Joined Jan 30, 2016
957
I would have said the same. Depends on how much current needed. Of course if you have multiple supplies you could always use two, linking - of one to + of other and adding a DPST switch to isolate both simultaneously from your test circuit.

Sometimes, if a linear supply, you'll find the transformer is centre tapped but not used. If so, you can often use that as your ground point, remembering the voltage display is now 2V.

If your circuit is audio related but essentially DC balanced you can sometimes get away with a virtual ground using 2 large electrolytic capacitors and parallel resistors of a few 100ohms to handle DC imbalances of a few mA. Not recommended for supply currents over a few 10s of milliamps.
 

Thread Starter

kmpres

Joined Jan 24, 2016
21
Glad I asked the question as it seems this is not as easy as it looks.

What if I put two Gophert supplies in series? Could I not use the full voltage and current ranges of both with the tie-point between them being the test circuit ground? The DPDT switch would then cut out the series connection and return the supplies to their normal separated outputs. While connected together could I then use different voltage and current settings or would that cause balance issues, and if so, how could those balance issues be mitigated? Never having owned one, I guess I just don't fully understand how purpose-built plus-minus bench supplies are wired.
 

ericgibbs

Joined Jan 29, 2010
10,531
hi,
Can you check that the 0V terminal on the Gophert is Earth and Chassis free.?

Do you mean supplies in Series.??
E
 

Thread Starter

kmpres

Joined Jan 24, 2016
21
I tested it with an ohmmeter. The Gophert's AC and DC grounds are isolated from each other and from the case. And yes, I mean to put them in series, that is: The + terminal on one supply is attached to the - terminal on the other and that junction will serve as the DC ground between them.
 

Irving

Joined Jan 30, 2016
957
Glad I asked the question as it seems this is not as easy as it looks.

What if I put two Gophert supplies in series? Could I not use the full voltage and current ranges of both with the tie-point between them being the test circuit ground? The DPDT switch would then cut out the series connection and return the supplies to their normal separated outputs. While connected together could I then use different voltage and current settings or would that cause balance issues, and if so, how could those balance issues be mitigated? Never having owned one, I guess I just don't fully understand how purpose-built plus-minus bench supplies are wired.
Thats what I said in line 1 of post #3

The DPST is in the + and - feeds to your test circuit as you cant switch both off/on together from 2 PSU. You can have different voltages each side, but that would be unusual. You don't need to switch the earth/common point.

I did see an article some time back on linking two Tenma supplies to track voltage on one by the other. If you have 2 identical supplies this may be possible on yours. I'll see if I can find it again.
 

ci139

Joined Jul 11, 2016
1,684
the "primary" device has a short-circuit protection + a load of specifications that you might want to be present and for the inverted output voltage rail . . . such is not too easy to achieve

it's an SMPS . . . so buying two of these for bi-polar supply must be verified that there would be no unwanted stray current traffic from one SMPS to another . . .
(( by any means such is not a rule https://www.power-supplies-australia.com.au/blog/connecting-mean-well-power-supplies-series ))

 

Thread Starter

kmpres

Joined Jan 24, 2016
21
OK, I have a plan now. Two power supplies, two diodes, a DPDT switch and five banana jacks. The diodes go across each output from negative to positive (anode to cathode) and will prevent each supply from trying to backfeed the other if their voltages are not exact. The switch will allow instantaneous disconnection of the positive and negative ends and restore the supplies to their normal separated outputs. This should allow variable voltage and current adjustments throughout their entire ranges giving me a single output of 0 to 64V at 0 to 5A or dual plus-minus supplies at 0 to +32V at 0 to 5A and 0 to -32V at 0 to -5A. The DC and AC grounds are isolated from each other so there is no risk of ground shorts. Do I have this correct?

I probably could have saved some money by buying a purpose-built plus-minus supply but then where's the challenge in that?
 

ci139

Joined Jul 11, 2016
1,684
I probably could have saved some money by buying a purpose-built plus-minus supply but then where's the challenge in that?
you should take the 1-st option . . . unless you're sure the 2-nd won't surprise you (or you know how to avoid the situations it would)
 

Thread Starter

kmpres

Joined Jan 24, 2016
21
My supplies don't have the green terminals which I assume means case ground. I've done this without diodes in the past, too, but with fixed server supplies. I was just concerned about using variable supplies this time around.

Thanks for your help!
 
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