PSU in parrallel

[Rigalo]

Does anybody know if I can put the output of 2 similar PSU's in parrallel? I draw abou 14-16amps of a 20 amp PSU. I want 3 chargers on two PSU's assuming not all 3 chargers pulls full throtle all the time.

Thanks.

[redgiki]

If you have any difference in voltage or capacity between the two power supplies, the higher-voltage one will bleed into the lower-voltage one. You might burn out a power supply. In fact, this is the symptom I see regularly in the data center of approximately 10,000 machines in which I work: a single power supply tends to be more reliable than dual power supplies. Dual supplies are more "fault tolerant", but more likely to have a fault that needs tolerating :)

Anyway, the worst that can happen is that you smoke a PSU. They are cheap, give it a shot. Best to do it with matched PSUs for the reason mentioned above, though. Make sure their voltage is as close to identical as possible (you can usually adjust this via resistors on the 5v lines if it's a PC power supply), put it somewhere safe where a brief spark and puff of smoke won't ignite anything, then hook them up in parallel. Chances are good, as light-duty as battery charging is, they'll last you for years.

Remember the worst-case scenario though: a smoked PSU. They happen all the time where I work, and sometimes briefly set off a smoke alarm (though more often not, they just die silently). Because they're in an all-metal case, though, the death-puff of a PSU doesn't ignite anything else. Make sure that your environment could tolerate a power supply flash.

[Rigalo]

redgiki,

Thanks for the reply Dude! Well, seems as if I might not try it after all, listening to your experience. Maybe I will look into converting a PC PSU. I see you get some pretty big ones that has 30+ amps on the 12V output.

It seems there ain't many cheap (affordable) PSU's that has higher amp output than 20. IF anybody knows of some in the UK, please let me know.

Thanks.

[redgiki]

Rigalo,

Yeah, my suggestion is to just buy the biggest 12V power supply you can find. I have two identical units -- one is a backup in case the first fails -- and they can handle a staggering 36A on the 12V line. These particular ones are salvage from an old server, and I suggest seeing if you can find a friend tossing out old 1990s-era big computers to see if you can find something similar.

I routinely pump 15A out of the PSU without a hiccup... this one is so good (salvage from a Compaq CL380) that the voltage dips less than a tenth of a volt under load! 12.4v solid, constant. A nice change from the crappy, aging, used power supply I used to have.

The real PSU killer is *heat*. The "rating" on the side doesn't actually mean a whole lot, except that higher-rated PSUs usually have bigger heat sinks and more heat dissipation ability, along with beefier circuits. Fact is, outside a computer case, sitting on a concrete floor (great for heat dissipation!), that PSU might be able to pump the amps you want. The fix I did with my old PSU was to put two automotive light bulbs on the 5v line. This caused the PSU to raise the voltage of both the 5v line and the 12v line to attempt to keep the 5v line steady, resulting in a nice boost on the 12v line enough that I could run several chargers at once. I was driving it way past its rating, but it held up until I finally replaced it with my current "Hah! you call that amperage???" 12V PSUs.

Back on-topic, though, I'm not saying don't do the parallel thing. The flash is usually contained within a PSU if it smokes, and in a data center of 10,000 machines, almost all of them with dual power supplies, we lose about one to two PSUs per week. Do the math on that:
20,000 power supplies
2 failures per week (that means 1 per 10,000 per week)

That's one failure per 1.6 MILLION hours of runtime.

I'd lay even odds, if your PSUs are identical, that you won't hit 1.6M hours of runtime on this dual-PSU setup for a little while with only 8,760 hours in a year. Your risk is very low.

If the two PSUs are not identical, though, I wouldn't do it. You'll absolutely have current leakage from one to the other, and although I'm not an electrical engineer, two power supplies running at dramatically different voltages hooked up in parallel means that one of the two is gonna die early.