Split supplies using off-the-shelf switchers

I've been thinking of playing around with some off-the-shelf switching supplies to use for bench testing purposes however none of the readily available dual output supplies are offered with the voltage and current specs that I'm looking for.

I was looking at some of the Elpac open frame supplies but their datashets weren't clear as to whether their outputs were isolated such that two units could be used to create a +/gnd/- split supply.

I called and talked to an Elpac engineer who said that their regular supplies were not isolated and their common was tied to AC safety ground, but that their medical grade supplies were and should be able to be used for creating a split supply.

However the way he answered didn't inspire a whole lot of confidence and I'm just wondering if anyone has actually done this.

se
 
carlosfm said:
If you have a PSU (switching or not) connected to AC safety ground and then join that with the positive voltage of the other PSU you will blow the fuses on your house.
No?

Well yeah, if you have two single supplies and the secondary common is tied to AC safety ground and you tie the V+ of one to the common of the other, you're effectively shorting out that supply.

But it wasn't clear in the datasheets that Eplac had on their website whether the output common was tied to AC safety ground or not.

Also, even if it wasn't, I know enough about switching power supplies to know that they've some idiosycracies that linears typically don't have so I also wanted to try and confirm that there wouldn't be some other issue I'd have to be aware of with regard to tying together two single supplies.

se
 
Stocker said:
That's what the "isolated" part was all about... if the outputs are only relative to each other and isolated from earth, stack them as high as you like to achieve desired voltage. I have heard of this being applied to computer power supplies, even.

Thanks, Stocker.

Yes, that's how it would seem at first glance. But I just don't know enough specifically about switchers to know if there might be something else that may be problematic and was rather hoping to find someone who's actually done it.

se
 
Ok, I ended up ordering a pair of Elpac MSM6524s which are an isolated, medical grade, single output 24 volt, 2.71 amp (65 watt) switching supply.

I wired up their outputs in series and instead of sparks and smoke ended up with a perfectly nice +/-24 volt split supply.

At $71.90 each from Mouser, they ain't exactly cheap, but man, it don't get much easier than this. And at only 3" x 5" x 1.1", a pair of them will fit inside a cigar box with plenty of room to spare.

se

NOTE: THIS CAN ONLY BE DONE WITH ELPAC'S MEDICAL GRADE SUPPLIES, DOING THIS WITH THEIR REGULAR SUPPLIES WILL RESULT IN A SHORTED OUTPUT.
 
Humm,
Well,with a bit of tweaking even a non-isolated supply could possibly be made isolated.
Many times the secondary is completely isolated from the primary
(using opto-feedback) the only thing linking the two sides together is that pesky Earth ground.
Cutting a trace or two in the right spots with an X-acto knife could solve that..
I'm not sure a PSU without optical feedback would work,but I suppose you could tweak that,and add an opto-isolator to it?

Humm,Maybe leave the one supply (For +V) as-is,and just modify one (the one for the -V rail) and earth/safety ground the +V output rail,instead of the -V rail.
Then you could series them up,and have 0V earth/safety grounded,without toasting the one supply.
 
DigitalJunkie said:
Humm,
Well,with a bit of tweaking even a non-isolated supply could possibly be made isolated.
Many times the secondary is completely isolated from the primary
(using opto-feedback) the only thing linking the two sides together is that pesky Earth ground.
Cutting a trace or two in the right spots with an X-acto knife could solve that..
I'm not sure a PSU without optical feedback would work,but I suppose you could tweak that,and add an opto-isolator to it?

Humm,Maybe leave the one supply (For +V) as-is,and just modify one (the one for the -V rail) and earth/safety ground the +V output rail,instead of the -V rail.
Then you could series them up,and have 0V earth/safety grounded,without toasting the one supply.

I'll leave that to someone else to figure out. I haven't got the time. :)

se
 
Somebody near me has more than a few SMPS's looking for a home but unfortunately, they have 12 volt outputs and are not isolated.

So far I have tried just one of them to power an inverted LM3875 GC and the results are surprisingly good. I will get another and try one per channel but the results are certainly good enough for an amp powering efficient speakers! ;)
 
What, do you have the 3875 strapped for single supply operation?

No Steve, it is running on +/-12 volts. The +12v is rated at 6A and the -12v at only 1A but it works. I figure if it works for both cvhannels then using one SMPS per channel will be better.

Stocker, I agree, IF I can modify them safely, it would be a very good project and a chance to recouperate some of my vast outlay on hi-fi! ;)