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Author Topic: Old Computer Power Supply  (Read 1756 times)
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Lacey, Washington, USA
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You'd hardly need a 10W resistor if it is 10k on 5V. The point is to draw about 500mA, hence 10 ohms on 5V is 500mA, 5V x .5A  = 2.5W. 10W resistors are simpler to find than 5W and give you a bit more headroom.
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Steve Greenfield AE7HD
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The point is to draw about 500mA

Well I just used the resistor that the article said: it (and all others I've seen) were silent on the current requirement to switch the supply on. So the info on the 500mA is good for future reference.
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@JimboZA
Check the next time you take the power supply apart as 10 ohms sounds like the correct value to me.

Could be your power supply had one already built in though.
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The way you have it in your schematic isn't the same as how you have it wired up! That goes for me too.

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@JimboZA
Could be your power supply had one already built in though.

Nope- it didn't fire up without it.
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That's odd.
I had to put a 10 ohm 5 watt on mine to get it to work.
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The way you have it in your schematic isn't the same as how you have it wired up! That goes for me too.

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That's odd.
I had to put a 10 ohm 5 watt on mine to get it to work.

I didn't try 10ohms, but yeah if it needs 500mA then 10K does seem waaaay too big. But it works....

(I do have a 10ohm one, so I might try it- it's just connected with terminal blocks so it's easy enough to swap out)
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No. I used a 10K having read somewhere (no idea where, now... )
That would explain it.  That ever-reliable source of knowledge.   smiley-eek
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Sherbrooke - Québec - Canada
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Oh thanks everybody for the answers! So I guess the best value for the resistor would be 10 Ohms 10W. But I'm wondering why the cd drive was not enough to start the power supply. For now, I use the power supply still connected to the motherboard smiley-lol but I will look for those resistors.
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CD drives draw most of their power from the 12V rail.
Only the low level logic uses the 5V rail, so if the drive isnt doing anything , like reading or writing
it wont draw power from the 5V rail , which is what you need.
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CD drives draw most of their power from the 12V rail.  Only the low level logic uses the 5V rail, so if the drive isn't doing anything , like reading or writing, it wont draw power from the 5V rail , which is what you need.
Come to think of it, since as you say, the motors run from the 12V, it probably makes little difference to the logic current draw whether or not the drive is in fact reading or writing.
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Sherbrooke - Québec - Canada
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Oh great explanation, it makes sense when you know that kind of stuff. Anyway, I'm going to the electronic store to buy a 10 Ohms 10W resistor today and I will give it a try. I will keep you posted! smiley-grin
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Lacey, Washington, USA
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Hang on, the computer power supply is still plugged into the motherboard? Well, there is your 5V load.
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Sherbrooke - Québec - Canada
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Quote
Hang on, the computer power supply is still plugged into the motherboard? Well, there is your 5V load.
I still use my motherboard for now because it's the only way the power supply starts but I'm trying to make the power supply start without it.

I tried with a 10 Ohms 10W resistor I bought this morning but surprisingly it's still not working. As you can see on the photo, I plugged the resistor with a red and a black wires and I also have a jumper wire from the grey wire to a brown wire. When I plug the green wire with the white one, the power supply starts for a second and stops. I don't understand what's wrong, probably wrong wiring. Someone as an idea?


* 2014-08-16 14.25.43.jpg (45.37 KB, 648x388 - viewed 8 times.)
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Not so obvious, but just try plugging it between the black and orange (3.3V) wires.
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On ATX type power supplies, I just jumper PS_ON# to common (ground) so it turns on without the motherboard ... no load resistor required.

« Last Edit: August 16, 2014, 11:37:15 pm by dlloyd » Logged

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