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playing with power supplied the cheap way

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Simon:
Well I've got myself a 3-15V bench power supply off ebay. Now it only goes to 15V and ideally I wanted 24-30V but I felt that £40 for a 25A supply was a bargain.

Now, suppose i do want to get more voltage, can I put another power supply in series ? say a PC power supply (or even another power supply i could make from the 500VA transformer i just got). the main bench power supply should provide the voltage regulation and any current control /protection ?

Or should i just take my foot out of my mouth and get on my way ?

amspire:
You can, but once you start putting power supplies in series, you have to make sure that a power supply doesn't get damaged by reverse voltages if load forces one or more of the power supplies to go into current limiting.

Now normally power supplies have a built in reverse biased diode across the output. Check, and if one is not there, it is a good idea to add one.

Here is the situation. You have your new PS set at 20V 25A current limit.  You put a second power supply in series - say a 10V which may have a current limit setting of 1A.

Now a faulty load shorts the output.

The 20V power supply will basically be putting 25A at up to 20V in the reverse polarity across the 10V 1A power supply. So even if there is a built in diode across the output of the 1A power supply, it won't be powerful enough to cope with 25A continuously. The end result will probably be bad news for the 1A power supply.

So if you are putting power supplies in series, make sure the current limit settings on the two supplies are similar, and make sure they both have reverse diode protection across the outputs. 

Richard.

metalphreak:
Don't forget that PC power supplies are always mains earthed. I don't see why you can't have an isolated DC supply "on top" to increase voltage but you will always end up with a ground-referenced output. You at least certainly cannot use two PC power supplies in series!  ;)

PeterG:
How much current will you really need at the higher range?
If you only need a few Amps, you may look at building another pwer supply with the wider voltage range but lower current range.

Regards

Simon:
well i was looking for the full 25A at higher voltages, I don't know if the supply I am getting has the negative earthed.

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