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Mains Switch-Mode Power Bricks In Series-Parallel

FireMyLaser

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I need a DC 24V power supply that can deliver at 5A. I first got one from Ebay, but it crapped out instantly. I knew it was a long shot..

Anyway, I got four identical 12V, 3.3A power bricks. They are double insulated, mains isolated (as far as I can tell) power bricks, similar to one for a laptop. I wonder if it is possible/safe to wire these in series-parallel to get 24V, 6.6A.

Has anyone done this before, or know of any reason why it wouldn't work? I'd like to know of any issues before I blow up my project/set the apartment on fire.
 
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FireMyLaser

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I did some reading on it and found that it is not recommended due to balancing issues. I'll just have to find a proper power supply elsewhere.
 

Cyparagon

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You can put constant voltage supplies in series (provided they're mains ground isolated), but not in parallel. Since the voltage of the supplies won't be identical, one will force current into the other and destabilize it. Current sharing will also not be identical for this reason.
 

FireMyLaser

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Yeah that's what I found out after some googling. 24V, 5A was the highest rated power brick I could find on Ebay, but now I ordered a 24V, 10A PSU in a grounded case. I would just have preferred a double insulated power brick instead.
 
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Benm

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It's preferable to get a single supply that does the job.

As you found putting them in parallel can give problems if their outputs don't match exactly. This can be mitigated a bit using a diode and low value resistor to spread the load better but it's not foolproof.

Using them in series is less problematic, but not without issues either: If you create an overload there is a chance one of them will cut out, and then be driven in reverse from the other one through the overload/short circuit.

Protection against overcurrent and temperature are common features, but against reverse drive at the output is not. You could put a big crowbar diode over each output to help with that.
 

Benm

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Those seem cheap indeed, but sometimes they are still good quality.

You could obviously test if they deliver output power as advertised, and they just might.

Mains isolation safety is another thing though. You can open it up and look for proper design (distances, solder mask blanks, slots) but you can't really see into the transformer without taking it apart.

One way to find out is to fire a kilovolt or two between in and output with a megger, but i don't have one. This also risks blowing up interference surpressing capacitors intentionally installed between primary and secondary. Those should be rated to a few kV, but sometimes are not, despite being fine with the 320 volts or so of rectified (non-US) mains.
 

FireMyLaser

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I got no megger, but I'll look out for the things mentioned, as well as the soldering. I took apart the 5A power brick and found three dodgy joints, but fixing those made no improvement. Starting it with a few amps on the output makes it instantly drop to 7V. Slowly ramping up the load makes it hold 24V for a few minutes, but then drops again. It has very few components and weights nothing.
 
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Benm

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That's odd, perhaps a thermal problem. You could check for components that run hot. If they have dodgy solder joints in there perhaps they forgot to put in some thermal paste or something like that as well.

Good thing about ebay is that you'll probably get your money back if it turns out to be defective, often no questions asked as they fear negative feedback.
 

FireMyLaser

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Well I cut off the DC plug and have cracked it open now, so I don't think it's fair to ask for a refund.
 

Benm

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Yeah, that'd void the warantee ;)

Although it actually has nothing to do with the reason it's defective... they may still send you a replacement but that's likely of equal quality.
 




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