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I need to draw approx 4 amps for 10-15 hours at a minimum of 18V.

I have hooked up two 300 watt power supplies in series:

  • +12V on one and +5V on the other: the output is only +15.8V rather than the 17 point something that I was expecting
  • +12V from each: then one power supply shuts down.

Why does this happen and how can I get the +18V (or more) that I am after?

Adam Lawrence
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Peter
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    You can connect the PSUs in parallell, but you will get problems if you try them serially. They are not constructed for that kind of use. – Gunnish Feb 04 '13 at 08:15
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    @Gunnish How do you know that, without knowing what they are? I routinely connect my *4* in series and get 4x the voltage drop of one of them. It's a reasonable deduction in this case, given the problem report, but it isn't a universal law. – user207421 Feb 04 '13 at 08:41
  • This sounds like a poor idea, can you not use a laptop supply (usually 17-19v, often quite high power) or tweak the voltage reference point in ONE PSU's switcher to natively give 18v? – John U Feb 04 '13 at 09:11
  • Thanks guys I am using ATX computer power supplies, I needed a quick hook up use, then pull down and use elsewhere. Laptop PSU' not enough current. I need to draw approx 4amps for 10-15 hours. However I am more interested in why it shuts down if 12V is joined to 12V but doesn't shutdown if 12V is hooked up to 5V or 3V.and why it only shuts down one power supply. It still gives 12volt output even with one shut down. – Peter Feb 04 '13 at 09:46
  • @EJP If i remember correctly, SMPS have to be synchronized for that to work(correctly), PSUs of normal computers don't have that feature. – Gunnish Feb 04 '13 at 13:28
  • You should move your extra details out of the comment and into the original question. See my edits. – Adam Lawrence Feb 04 '13 at 13:32
  • HP laptop computer have a 90 watt inline power supply. Outputs 18.5 volts DC @ 4.74 amps. That may be the simple answer. – Optionparty Feb 04 '13 at 14:57

1 Answers1

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Most likely the 12V power supply returns are connected to earth ground, and by putting them in series you're essentially shorting one of the rails out. You can only connect power supply rails in series if one of the outputs is 'floating' with respect to the other (no galvanic connection between them).

Most likely the 5V rail is floating with respect to the 12V, which allows them to be put in series.

You may want to investigate a boost converter, to bring the +12V up to whatever voltage you need. A single power supply may have enough capacity for your target.

Adam Lawrence
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    +1: When we require more than 30V DC for testing, we isolate our two 30V supplies using an isolation transformer and connect those in series to get up to 60V without problems. – Rev Jul 24 '13 at 09:54