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PC SMPS issue

numberonekiwi

New Member
I have a SMPS that I am going to use for a desktop power supply it delivers around 40 Amps on the +12v and +5v rails powering up the unit is easy by grounding the ps on wire ( green )

The problem is however that the 12v rail is only 11.2 volts. The 5v is 5.02 v so this is ok.

Is there an easy way to pull the 12v rail back up to 12v or even 13.8v
 

kchriste

New Member
Forum Supporter
Once you load the 5V rail, you'll probably see the 12V rail rise. This is because the power supply only senses the voltage on one of the outputs and it is usually the 5V or 3.3V line.
 

numberonekiwi

New Member
thanks for the suggestion on the 5 volt rail I didn't really want to load it down as I would also get less amps on the 12v rail so I took a closer look at the circuit and found a track from the 5 v rail to KA7500B IC and some resistors etc which when researching the above part I found it to be the 5v ref ( heart of the circuit ) so I thought to myself maybe if I told that IC the voltage was less than it was it would bring both rails up so I cut the circuit track and inserted an 1N4001 which has 1/2 v drop this brought my 5v rail to 5.5v and the 11.2v (12v) to 12.2 not quite enough so I will now try with another diode. I know this will take this 5v rail to 6 volts but then I am not to interested in the 5 v side just heaps of amps on 12v.

Your comment though was a great help it did point me in the right direction.

I am also a little concerned about the 16V caps across the 12v rail taking it up to 13.x I may need to go 25v caps
 

kchriste

New Member
Forum Supporter
The caps should be OK, but if you change them be sure that they are low ESR types rated for high frequency ripple. Don't use a general purpose electrolytic.
EDIT: If you want better regulation of the 12V supply you could change the feedback path to the 12V rail. Disconnect the 5V feedback path and add a voltage divider to bring the 12V down to 5V and feed this into the voltage sense terminal (pin 1?) of the KA7500B instead.
 
Last edited:

glutnix_neo

New Member
To be more general, on power supplies, they usually monitor the output with the highest output power(in our case it is the 5V).

You can modify the feedback circuit(voltage divider with input taken from 5V) to add 12V output. This is called shared regulation, try research a little on this and do experimentations.

The idea for the output to be regulated is to make the output of the divider equal to the reference used when it enters the PWM.
 

albasith786

New Member
pl's say in detail about how to check the smps are in good condition or not. and how to check the voltage levels in every pin.....etc
 
If you increase the voltage on the 5v rail too much, the over voltage protect could kick in and disable the outputs. I think it's like an op-amp voltage sense circuit that controls it.
 

btcg

New Member
I have a SMPS that I am going to use for a desktop power supply it delivers around 40 Amps on the +12v and +5v rails powering up the unit is easy by grounding the ps on wire ( green )

The problem is however that the 12v rail is only 11.2 volts. The 5v is 5.02 v so this is ok.

Is there an easy way to pull the 12v rail back up to 12v or even 13.8v
40 amps... WOW

I can't imagine the desktop device that could load that supply down.
 

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