Problems joining two PSU's (old parts) is a 300w PSU enough for a 5770?

[marvinmartian]

Have some older spare parts around that I thought I could build into a machine.  Here are the basics:

Dell Optiplex 620 (older P4 based Dell)
ATI HD5770
built in Dell PSU (I think around 300w)
extra PSU (300w rated)

So I've got the two PSU's jumped together (green -> green and black -> black).  I've also tried this by simply jumping the 2nd PSU in standalone (always on) mode.

No dice either way.  I would have thought that a dedicated 300w PSU would be enough for a 5770, no?

Or am I missing something?  I would have thought that the machine would at least boot up to the system BIOS.

If I disconnect the 2nd PSU and boot up using the machine's built-in internal GPU (and PSU) then everything is fine, but of course that GPU won't do squat for BTC mining.

Comments / input appreciated.

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[cicada]

Does that 2nd PSU fire up if you just short the black and green without connecting it to anything else?

If not, that's pretty weird, I'd plug a multimeter between the green and black pins, you should see ~5V.  If not, the PSU may need some other load (like the 5V motherboard standby) in order to power on.  A heavy resistor on that pin to ground should do the trick if so, but I don't know which pin that'd be offhand, just the 5V rail I'd guess?

[ArtForz]

Yep, does it power on with just green/black shorted?
Also, it's generally a VERY bad idea to only load old/cheap 300/350W PSUs on +12.
Those are pretty much all group-regulated designs taking feedback from +5V or +3.3V -> load +12 significantly with +5/+3.3 unloaded and +12 ends up *way* low, I've personally seen < 9.5V with 60% rated load on +12 and nothing on +5/+3.3.

[cicada]

I've personally seen < 9.5V with 60% rated load on +12 and nothing on +5/+3.3.

I've been running into this a lot lately, I wasn't aware it was a problem at all but it could explain the odd locks I get every once in a while..  my multimeter vanished so i'm waiting for a new one in the mail to check this out

[pekv2]

Maybe try one of these?

http://www.add2psu.com/

Ironically, I just browsed this article this morning.

http://www.hardocp.com/article/2011/08/24/black_art_dual_psus_in_your_enthusiast_pc/1

Which your thread reminded me of the add2psu component.

http://www.hardocp.com/article/2011/08/24/black_art_dual_psus_in_your_enthusiast_pc/3

[deslok]

I've personally seen < 9.5V with 60% rated load on +12 and nothing on +5/+3.3.

I've been running into this a lot lately, I wasn't aware it was a problem at all but it could explain the odd locks I get every once in a while..  my multimeter vanished so i'm waiting for a new one in the mail to check this out

If you didn't order a new meter i picked up one of these and it's awesome http://www.harborfreight.com/5-in-1-digital-multimeter-98674.html includes a temperature probe very useful.

On topic I wouldn't muck about with psu's that are too old and i would avoid dell psu's of that age period they're known for being wired in a non standard maner

[cicada]

If you didn't order a new meter i picked up one of these and it's awesome http://www.harborfreight.com/5-in-1-digital-multimeter-98674.html includes a temperature probe very useful.

No shit, one of these stores opened up just around the corner a few months ago, I had no idea they carried this kind of stuff.  Might have to go 'browse' later

[deslok]

I worked at one back in 08 so if you have any questions about any of their products don't hesitate to pm me. they're a bargin bin tool store it's like going to toys are us for grown men. on the same note odds are your local paper has one of their flyers in it it's worth buying one next sunday just to get the 20% off coupon if you're planing on buying anything expensive