# 90A out of a PSU ?!

Discussion in 'The Projects Forum' started by Siro Simo, Jun 24, 2015.

1. ### Siro Simo Thread Starter New Member

Jun 24, 2015
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0
Hi guys,

I’m working on transforming a PSU into a benchtop power supply.

I scavenged a Dell N875EF-00 (875W).

There is a ton of project online on how to convert those but I’m very surprised by my power supply.

It says on the sticker that each 12V feeds can supply 18A. There are 5x12V.

Does it means, by connecting all the leads together, I’ll be able to supply (if I’ve the load for) 90A / 12VDC with this power supply !?

The power supply is rated 875W but this would means 1080 W…

I don’t need that much power for now but I try to understand how big I can go without to kill the PSU.

I want to use this PSU to make a car battery charger, an electrolysis system or electronic projects so I want it to be very flexible.

PS: sorry for the bad English… English is not my native language

2. ### AnalogKid AAC Fanatic!

Aug 1, 2013
5,567
1,575
A typical situation with high power switchers is that the total of all output voltages times currents exceeds the max total power available. The total power is limited by the front end electronics and the transformer(s), while the individual output are limited by the secondary regulators. For example, if you draw only 12 A out of the 5V instead of 31 A, the remaining 100 W is available to the other outputs.

It would be nice if you could sum all of the 12 V output together into a single massive one, but that might not be allowed by the internal design.

ak

3. ### paulktreg Distinguished Member

Jun 2, 2008
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Your maximum combined power for +12VA, +12VB, +12VC, +12VD and +12VE is 825W or 68A according to the label.

A typical computer power supply will have all the +12V rails coming from a single high current 12V source on the printed circuit board.

You'll need around 14V to charge a car battery. It's not easy modifying computer supplies and there are some dangerous voltages knocking about. Please be careful!

Last edited: Jun 24, 2015
4. ### tcmtech Distinguished Member

Nov 4, 2013
2,681
2,440
Unfortunately despite what many instructables and other such how to hack a power supply threads may suggest typical computer power supplies are terrible power sources for 12 volt loads plus getting them to work satisfactory for such loads is not an easy modification to do either.

If you need to charge typical 12 volt lead acid batteries you are way further ahead to use a common old fashioned low frequency transformer based one. They can take the most abuse and are the most forgiving to such abuses that very few SMPS based units can handle and survive let alone a modified computer power supply.

5. ### Siro Simo Thread Starter New Member

Jun 24, 2015
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Thank you guys ! I'll not use it as car battery charger anymore. I read more about it and it doesn't seems like a good idea.
I don't know if I've a use for the 68A but I'll definetly try to play with it in some electrolysis experiment.

Thanks again guys

6. ### R!f@@ AAC Fanatic!

Apr 2, 2009
9,616
1,082
Not quite right.
VA, VB and the rest might have different rectifier and filters as I have seen