playing with power supplied the cheap way

[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.

[Zero999]

If the output of the power supply isn't earthed, connecting a computer power supply in series would work. You'll obviously need a PSU with a 25A 12V output to get the full power output from your power supply and you'll need a reverse polarity protection diode in parallel with the PC PSU.

[Simon]

If the output of the power supply isn't earthed, connecting a computer power supply in series would work. You'll obviously need a PSU with a 25A 12V output to get the full power output from your power supply and you'll need a reverse polarity protection diode in parallel with the PC PSU.

yep that's what I'd do.

[AlsInd]

when there is a will, there is a way! but this does not mean you should do it. if you are looking for a psu for really high power applications, you should find an old one on ebay, which will be too expensive for shipping, or build one yourself out of surplus equipment. you can serialize psus, contrary to another post here you can serialize computer psus also but you have to do some modifications to allow this, but there are so many things that can go wrong and if you don't know what you are doing and do not understand exactly how your psus are functioning, you are playing Russian roulette! i would highly discourage you from doing this. you can easily build a high power psu yourself out of old surplus electronics as i did. the key is to find a great big old transformer that can handle the current load. old heavy psus are often tossed because people want the dainty little new psus for their workbench. i have found great things in industrial auctions. i found a nice 1:2 transformer that could handle 25A and built a simple psu around it. but it is not reasonable to get these things by mail because that transformer is one heavy mama! and the chassis and other stuff for the psu was not light and small either. my psu is moved on a dolly due to it's weight. anyway... what the heck are you doing that requires so much power?!
cheers,
al...

[Simon]

well i did want to be able to run automotive fans, we have some that use 18 A and if i want to do a stall test that is 50+A. I ptobably won't use over 15V often and will rarely require the full 25A but as i have the actual PSU that can handle that much i wanted to match it. I'm not in a hurry on this one so will give it some thought.

I have a 500VA (45V) transformer but I wanted to put power factor correction into the design so I'm trying to understand first how this works