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Need Help!!

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tj107us

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I'm making a 12vdc power supply out of a ATX computer supply for running a 4 amp exhaust fan for my car hauler. My question is that the power supply has two 12V+ circuits and i was just wondering if the two circuits could be hook together with out any harm? Each circuit can supply 28 amps at 12vdc. thanks in advance for any help!!
 

Willbe

New Member
if the two circuits could be hook together with out any harm? Each circuit can supply 28 amps at 12vdc.
You could use two steering diodes; each +12v output connects to a diode anode, and both diode cathodes are tied together, and this is your 11 V output.
If you use Shottky diodes you'd get almost 12 V.
JC Whitney might make such a gadget.
 
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rezer

New Member
You could use two steering diodes; each +12v output connects to a diode anode, and both diode cathodes are tied together, and this is your 11 V output.
If you use Shottky diodes you'd get almost 12 V.
JC Whitney might make such a gadget.

Yes, the steering diodes would prevent current from feeding into one of the power supplies due to an imbalance? But would not having the diodes really have any effect on the power supplies? What of batteries that are connected in parallel (non-rechareable)? I've never seen steering diodes used there. But I guess it would make sense to do so.
 
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Willbe

New Member
Steering diodes are regular diodes that steer the current away from places you don't want it to flow.
These diodes need to handle the current (4 A?) and they will drop about 1 V.
4 A x 1 V = 4 W each, so they probably need a heatsink.

I don't understand why you need a 12 V supply for a car.
 
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Willbe

New Member
Yes, the steering diodes would prevent current from feeding into one of the power supplies due to an imbalance? But would not having the diodes really have any effect on the power supplies? What of batteries that are connected in parallel (non-rechareable)? I've never seen steering diodes used there. But I guess it would make sense to do so.

I think the main reason for using diodes is the possibly low internal impedance of the sources.
If Rsource = 1/2 Ω, a 1 V difference in battery voltage can cause 2 A to flow. This may not be a problem.
If it's 1/20 Ω then very small differences in source voltages can cause big and probably undesirable current flows.
 

tj107us

New Member
I did a voltage measurement across the two different 12vdc supplies and they don't have any resistance so i dont think thier going to a problem, but on the safe side i contacted the manufacturer about this issue, so i hope they say its ok to connect them together.
 

rezer

New Member
I did a voltage measurement across the two different 12vdc supplies and they don't have any resistance so i dont think thier going to a problem, but on the safe side i contacted the manufacturer about this issue, so i hope they say its ok to connect them together.

You can't measure the internal resistance of power supplies by just measuring the voltage. Did you vary the load?
You can refer to the spec sheet, if you have one.
If you don't try this:
Measure the voltage at no load, than apply a load that will draw 50-80% of the max I rating and note the voltage of the power supply. Take the difference between the voltage meas. and divide by the current of the loaded state. This will be the internal resistance of your power supply.

(Vno-load - Vload) / Iload = Rs



Where Rs is the internal resistance.
 
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kchriste

New Member
Forum Supporter
I'm making a 12vdc power supply out of a ATX computer supply for running a 4 amp exhaust fan for my car hauler. My question is that the power supply has two 12V+ circuits and i was just wondering if the two circuits could be hook together with out any harm? Each circuit can supply 28 amps at 12vdc. thanks in advance for any help!!
Why would you need to connect them together, when the fan can run easily on just one of them? Unless there is a typo in there somewhere? The issue here will be the minimum load on the unused outputs. What is the minimum load spec on each of the outputs? (3.3V, 5V, 12V, etc)
 
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Boncuk

New Member
Why would you need to connect them together, when the fan can run easily on just one of them? Unless there is a typo in there somewhere? The issue here will be the minimum load on the unused outputs. What is the minimum load spec on each of the outputs? (3.3V, 5V, 12V, etc)

PC power supply required a minimum load some until some 10 years ago. Today's power supplies run without any load. Makes it easy to test the no load voltages.
 

kchriste

New Member
Forum Supporter
Don't know about that. Check out the specs on this power supply:
**broken link removed**
 

rezer

New Member
Don't know about that. Check out the specs on this power supply:
**broken link removed**

Yes, I've worked with power supplies as well that had min. load requirement to operate. But Bonkuk is referring to PC power supplies, which probably have no load requirements.
 
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tj107us

New Member
Hey thanks for all the info, but i guess im just going to run only one circuit of +12vdc for now. I just knew i had 56 amps worth, and being a man i got wanted all of it!!
 

rezer

New Member
Hey thanks for all the info, but i guess im just going to run only one circuit of +12vdc for now. I just knew i had 56 amps worth, and being a man i got wanted all of it!!

So what your saying is you had a "Tim the Tool Man Taylor" moment?:D
 

Hero999

Banned
Try posting on topic for a start.
 
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