Making a dual power supply from two plug packs

[Ammar]

Hello all,

I have a friend who is thinking of taking two 15V SMPS plug packs and creating a plus minus 15V dual supply. My gut tells me this is a bad idea and that it would depend a lot on the internals of the plug packs, but i thought it would be best to ask here.

Is there a way to tell whether a plug pack variety would be suitable for this application? Or is it better to buy a proper dual lab power supply.

Thoughts?

[Brumby]

A lot of people will say it's always better to get a proper lab-style supply - and I would not disagree with them, however, sometimes the budget just isn't there.

I don't foresee any basic problems.  The outputs of the plugpacks should be isolated (except for maybe some capacitive coupling, which you should be able to ignore) - so, as long as nobody does anything silly I think it would be OK.  The only immediate reservation I would have is the voltage matching between the two ... but how important that is will depend on the circuit being powered.

[Ian.M]

The outputs of the plugpacks should be isolated.

Easy enough to check:  If the mains in is 3 pin and the Ground pin is metal not plastic, you need to check resistance between Ground and output 0V (unpowered). If you get continuity or anything under 1Meg resistance, don't stack them!

2 pin main input plugpacks should be fine, unless they were made by 'Wun Hung Lo' Power Pack & Electric Chair factory for a cheap-skate client!

However I'd probably take ONE beefy plugpack and an EBAY DC-DC converter module to generate the -ve rail. See https://iw0ffk.wordpress.com/2014/09/19/negative-voltage-from-lm2596-dc-dc-ebay-module/

[Zero999]

It will work, as long as the output isn't earthed. One potential problem is the leakage current from the mains will be doubled, which might give you a tingle (nothing harmful to you though) but it could damage some ESD sensitive components.

[Brumby]

2 pin main input plugpacks should be fine, unless they were made by 'Wun Hung Lo' Power Pack & Electric Chair factory for a cheap-skate client!

Good point.

However I'd probably take ONE beefy plugpack and an EBAY DC-DC converter module to generate the -ve rail. See https://iw0ffk.wordpress.com/2014/09/19/negative-voltage-from-lm2596-dc-dc-ebay-module/

I'd likely go this way myself.

[setq]

I'd skip the two switching wall warts. Switchers are notoriously noisy, explodey if you short them and difficult to use properly if you have more than one.

You can however get straight AC wall warts that will kick out pure 50/60Hz AC. These are much more flexible, are properly isolated and can shift some serious current as well if you buy a large one. It's just a transformer in a box. You can build a small front end with a couple of diodes, caps and a couple of linear regulators that will do the same as two wall warts and give you a dual rail supply:

See the following: http://sound.whsites.net/project05.htm



Also you can derive all sorts of voltages (from Poor Man's Spectrum Analyser, which is incidentally highly sensitive RF stuff):



I've used a similar arrangement on and off for about 10 years before I bought a proper dual rail supply with zero problems at all.

In fact I've gone as far as using AC wall warts as a standard in my designs as it's dead easy to derive almost any voltage you need. You can step up to kv range with another transformer, use the circuit above to get dual rails, get single rail DC, anything. Minimum cost and component count, low noise, low hassle. Also as the transformer isn't integrated in the device, it keeps the device small and allows you to reuse a transformer.

I reckon you can build a dual rail LM317/337 supply for less than £20/$30.

Then again I paid £19 for one of my dual rail supplies (Heath IP-2718) so bargains are to be had if you shop around.

[dentaku]

Ray Wilson's website explains the AC wallwart power supply well.
http://www.musicfromouterspace.com/analogsynth_new/WALLWARTSUPPLY/WALLWARTSUPPLY.php

[setq]

Just watch out though because his design doesn't account for drop out voltage under load.

[macboy]

I'd skip the two switching wall warts. Switchers are notoriously noisy, explodey if you short them and difficult to use properly if you have more than one.

"explodey" 
My new favorite adjective.

[Kleinstein]

An AC wall wart can be rather flexible, but they are relatively difficult to get these days.

The second circuit in reply #5 also has an mistake inside: if the 12 V are not loaded enough, the zener diode form the 24 V stabilization will raise the 12 V to potentially significantly more than 12 V and possible even damage the regulator. So the general Idea is OK, but it depends on the load.

Switched mode supplies can be a little noisy, but so are the ready made DCDC converters to go from something like 5 V to +-12 v. With some filtering this might be acceptable.

[setq]

That's possible. That circuit had a load attached permanently that isn't detailed. I wouldn't run it on it's own. The +24/-6 are opamp rail supplies so they don't need proper regulators as the PSRR of the amp takes care of that to a reasonable degree. Nice cheap way round it.

Got no problems getting AC adapters here in the UK still.

[rob77]

if you need a low current symmetric supply and you don't mind the switcher noise , then build something like i built. it takes 5V input from a micro USB connector and provides +- 12V , +-5V , 3,3V and pass-through 5V from usb. also can be powered from 3-4 AA cells (2pads in the upper left corner are for battery input).
it's a simple 34063 boost + 34063 inverting boost making +-12V , followed by a 78L05 and 79L05 providing the +-5V (from that +-12V)  and a 1117-3.3 providing the 3V3 directly from input.

it's less than $5 in parts and works well on a single sided board

 

[setq]

Nice. I built one similar to that albeit with through hole tech before the wall wart revelation. Also I didn't have any 34063's so I used a 555 for the oscillator and an LM311 controlling the thing via the reset pin. Oh and no over current shut down other than a fried BD139. Sucked but it worked. Managed to use it to charge up some 80v capacitors.

[Ammar]

I forgot to mention a couple points, I wasn't very clear. He wants to use two 3 Amp plug packs and he doesn't actually have them yet, he is looking at purchasing them. Looks like he will give it a go.

So many good suggestions and points to consider, really liking the eevblog forum.