piggy back powersupplies?

Hi. If it have 3x 12v power supplies 1x2a 2x1a and put all the positive and negative together would that make it 12v 4a? Cheers

Well, sort of...

Depends on several things...

How accurately can you set and hold the voltage at each supply. You would have to be able to set the voltage of each supply to within mV of each other before paralleling them.

What happens to these supplies if they are overloaded. Do they have build-in current limiting (like lab supplies), or do they blow a fuse? If the latter, don't even try it.

Hi mike, thanks for your reply.
They are actually 3 DC power supplies, the same as you get with say your modem etc. they are all rated 12v and different ampages.
I need about 3 amps for my cctv cameras and thought i could parallel these to make 4amp 12v which would do the job and i get rid of some stuff i have laying around here.
Im guessing each powersupply has some sort of overload / fuse built into them?

Thanks for your help thus far.

Load each supply with ~ 1A of current. Measure the output voltage. The loaded output voltages would have to be within mV so that the supplies would be loaded equally. My guess is that they will be 100 of mV apart, in which case the supply with the highest output voltage will be carrying the entire load until it overheats and blows up, at which point the second highest will get overloaded and so on...

stuee said:

Hi mike, thanks for your reply.
They are actually 3 DC power supplies, the same as you get with say your modem etc. they are all rated 12v and different ampages.
...

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If they are 12v UNREGULATED plugpacks they are just a transformer and diode bridge and a cap, and because of the high sag of the transformer under load they will "share" the load to some extent.

That will work best if the transformers are the same size and the same voltage in each plugpack!

Since you said they are "different amperages" (and likely different voltages) they won't share very well, and it's likely one plugpack will deliver most of the power and soon overheat.

stuee said:

Hi mike, thanks for your reply.
They are actually 3 DC power supplies, the same as you get with say your modem etc. they are all rated 12v and different ampages.
I need about 3 amps for my cctv cameras and thought i could parallel these to make 4amp 12v which would do the job and i get rid of some stuff i have laying around here.
Im guessing each powersupply has some sort of overload / fuse built into them?

Thanks for your help thus far.

Click to expand...

Hello there,


Wall warts seldom have fuses or any kind of protection other than the fact that they are tested for overload before they leave the factory (at least the design) so that they dont burn up the house.

If you have several DC wall warts the chances are they are unregulated, however they do make a lot of regulated wall warts these days. If they are regulated they may not work together, but if they are all just transformers with rectifiers and filter caps then they could work together if they are all the same voltage as you say. There's a change one is slightly higher than the other, but it will quickly load down to the others.

Since there are so many variations though the best thing you can do is a few quick tests.
1. Measure the terminal voltages. If any are exactly 12v or very close, they are probably regulated and might not work well with others.
If they all measure around 15v or higher then they are probably unregulated.
2. After connecting two, check the voltage to make sure it is reasonable.
3. After connecting three, check the voltage again.
4. Load the units down with a resistance that will draw the required current and measure the voltage. If it is close to 12v it should work.
5. You may wish to measure the current from each wall wart once it is loaded down to the required load current to make sure they are all contributing a reasonable portion of the total current.

How about 3 small ohmage balancing resistors in series with each output?

Or better still, buying a 12v 4A transformer off ebay for a few dollars and doing the job right?

If the supplies have current limit and they are all the same kind of regulator then you might be able to just parallel them, noise will be worse than just one supply.
If your paralleling supplies your better doing it with supplies that are eaxctly the same, shoving any old 3 supplies together might work, but also might go bang.
I occaisonally parallel my 2 bench supplies up to get more current, they are both linear regulators but thats about it for similaritys and they work ok together.

If one of the supplies fails, the others will be overloaded and will also eventually fail. You would probably not know that one of them has failed because the power indicator light is usually wired across the output and would be powered by the other two.

Timescope