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Author Topic: Light bulb 5v PSU load - how much is enough?  (Read 2003 times)
amazingrando
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October 28, 2011, 08:55:06 PM
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I see that many people have done the 5v light bulb mod for their PSU.

I am not doing the light bulb but I do have a SATA hard drive which uses 3.3v, 5v, and 12v according to the pinout.  Is that enough of a power draw?

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bulanula
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October 30, 2011, 08:40:02 PM
 #2

I see that many people have done the 5v light bulb mod for their PSU.

I am not doing the light bulb but I do have a SATA hard drive which uses 3.3v, 5v, and 12v according to the pinout.  Is that enough of a power draw?

Please link this as I have not heard before of it. Seems dumb making light when I sleep next to my rigs.

I use two 10W ceramic resistors in parallel soldered. Is this good enough ?
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November 01, 2011, 11:54:28 PM
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Please link this as I have not heard before of it. Seems dumb making light when I sleep next to my rigs.

I use two 10W ceramic resistors in parallel soldered. Is this good enough ?
so that would be equivalant to a 20W resistor?
what Ohm?
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November 02, 2011, 12:20:55 AM
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Assuming you did the calculations correctly, a resistor dissipating 10W at 5v would be drawing 2 amps. Two in paralell would draw 4 amps and the resistor value of each would be 2.5 ohms.

This seems way more current than would be needed. I would guess - and it is a guess - that loading the power supply to 1 or 2% of its full rated power would be about the size of whats needed.

So One 10W resistor would be correct if it was a 1KW supply, supplying 1KW at 5volts and no other current. That would be a total maximum current of 200 Amps. But the 5v power supply in a 1KW PSU is often only 50W, (REad your PSU specs.)  so in this case you would expect to plan for 0.5 watts of power drain if we assume 1%, and at 5 volts 0.5w is 0.1 amps, The required resistance would be 50 Ohms.

 

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bulanula
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December 31, 2011, 03:03:26 PM
 #5

Sorry to bring up this old topic but the resistance is 10 ohms.

They are 10W 10 ohm ceramic resistors.

The PSU is a Corsair AX1200 with 30A on the 5V line as far as I can see.

What is the amount of resistors needed and the wiring ( series or parallel ) ?

Thank you !
P4man
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December 31, 2011, 03:33:58 PM
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You only (potentially) need this  if you are using 2 PSUs on one machine. The PSU thats only powering GPUs is only loaded on the 12v rail, which can be a problem. I assume you are using only 1 PSU, so you are powering the motherboard with it; in that case, there is no point in wiring light bulbs or resistors.

sadpandatech
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December 31, 2011, 04:55:10 PM
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You only (potentially) need this  if you are using 2 PSUs on one machine. The PSU thats only powering GPUs is only loaded on the 12v rail, which can be a problem. I assume you are using only 1 PSU, so you are powering the motherboard with it; in that case, there is no point in wiring light bulbs or resistors.

I get around this by powering a 5v enclosure fan(or two) with secondary PSUs

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bulanula
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December 31, 2011, 06:29:21 PM
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You only (potentially) need this  if you are using 2 PSUs on one machine. The PSU thats only powering GPUs is only loaded on the 12v rail, which can be a problem. I assume you are using only 1 PSU, so you are powering the motherboard with it; in that case, there is no point in wiring light bulbs or resistors.

I am using 2 PSUs on one mobo and I need to load up the 5V line to put some stress on the PSU that is only delivering through the 12V line etc.
P4man
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December 31, 2011, 07:22:59 PM
 #9

A 1200W PSU ought to be enough for any rig. Why are using 2 PSUs?

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December 31, 2011, 07:31:02 PM
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A 1200W PSU ought to be enough for any rig. Why are using 2 PSUs?
4 6990's?
DeathAndTaxes
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December 31, 2011, 10:38:57 PM
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A 1200W PSU ought to be enough for any rig. Why are using 2 PSUs?
4 6990's?

A good 1350W PSU (I use Enermax MAXREVO) can power 4x 5970.  Not sure about 6990 since you can't bring their memclock down as low.  Silverstone makes a 1500W PSU.  Smiley That has got to be enough. 
bulanula
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January 01, 2012, 04:30:24 PM
 #12

Think I have solved it now. Those two 10W 10 ohm resistors in parallel are pulling about 20W of heat etc.

Since the PSU is rated for max of 150W on the 5V line at 5A that 20W should be putting about 13% load on the 5V line so that should not stress the PSU etc.

Can anyone confirm this is the right procedure ?

Thanks !
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January 02, 2012, 11:09:49 AM
 #13

What about running a fan on 7v ? (12 & 5v) and make use of it ?

I think I have the fan on 5v, I must give it a spin for it to start, might not happen on 7v,

It fixed my PSU problem (it's a 0.6 amp fan)
DeathAndTaxes
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Gerald Davis


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January 02, 2012, 03:35:11 PM
 #14

What about running a fan on 7v ? (12 & 5v) and make use of it ?

I think I have the fan on 5v, I must give it a spin for it to start, might not happen on 7v,

It fixed my PSU problem (it's a 0.6 amp fan)

Thats a good idea.  Instead of producing heat do something useful w/ the load.  There are some fans which can start on 5V check reviews in silent PC forums/sites.  I don't think running on 7V will help.  The load is going to be on the 12V rail right? 
bulanula
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January 02, 2012, 08:10:31 PM
 #15

I don't have any extra fans ATM Grin. Hopefully my room will handle the cooling requirements.

Is the maths with the 10W 10 ohm resistors in parallel on the 5V line correct for a PSU like the Corsair AX1200 with a maximum of 30A ( or 150W ) on the 5V line ?

Thanks !
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