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Hi there community, i have a very certain question.
I recently modified an atx supply of 420W. Some of its features are :
5V 32 A

12V 14A

3.3V 22A
-12V 0,5A

Lets say i connect one end to 5v and the other to -12v, what wattage will this give me ? Probbaly not 17x32=544
Is bridging possible? What are the possible wattages ?

Thank you for your time :)

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5 Replies

phevtron (author)2018-02-21

Thank you for the answers kind strangers !
My basic question was the one you both answered. Maximum amperage is therefore the lowest available from a rail, got it

rickharris (author)2018-02-23

Correct. Essentially you can't get something for nothing.

Jack A Lopez (author)2018-02-17

The numbers for current, printed on the nameplate of your power supply, represent the maximum current that rail can comfortably supply.

The actual current drawn by a load, depends on the character of the load.

Anyway, you asked about a hypothetical load, connected between the +5V rail and the -12V rail. What is the maximum current that could comfortably be supplied to this load?

The answer is 0.5 A, because that is the maximum for the -12V rail.

How much power dissipated by this hypothetical load with 17 volts across it, and 0.5 amperes, maximum, flowing through it? Answer: a maximum of (17 V)*(0.5 A) = 8.5 W

By the way, the people who sell power supplies, often try to make the numbers seem big, because big power is sexy, obviously.

I was expecting the number 420 W, to be the sum of all those individual rails, supplying the maximum rated current.

Specifically:

(5V*32A)+(12V*14A)+(3.3V*22A)+(-12V*-0.5A)

= 160W +168W +72.6W + 6.0W

= 406.6W

which is close to 420W, but not exact. I dunno. Maybe they rounded it up to 420, because that number sounds good?

Final note: I do not grok this thing you call "bridging". I just do not get what you are asking about that. So I cannot say if that activity is possible to do that, or not, with your power supply.

I have heard of "planking", but I think that is something different than "bridging".

;-)

phevtron (author)2018-02-21

What i mean by bridging is a process available in common transformers where wiring them in different combinations provide different voltages or amperage. But i suspect this is not possible( asked to be sure).