OT pc power supply regulation...

[bigandy]

Evening all.

Just a quick query regarding the regulation of the 12volt line in PC power supplies under load. I've bought a couple of new power supplies for use in the garage, and I have got them working fine. The 12v line is rated to 14amps on each of them. I've tested the output and at 0amps it is 12.03v. with a 7amp load it is hovering around 11.45volts. At 10 amps it has dropped down to 11.3volts.

This is all with a 2.25 amp load on the 5volt line (2 12v/21Watt car bulbs in parallel). Is it normal for the 12volt line to drop this much? Or should I need more of a load on the 5volt line to ensure the 12volt line is better regulated?

Cheers

Andy

[reflex]

Loading the 5V line more should help.

Consider in PC P.S.U's the 5V line is the most critical
and is monitored by the feedback ccts NOT the 12V line.

[bigandy]

That's what I was wondering. The 5v line seems to hover around 5.09v regardless of the load on it. I tried a massive 20amp load on it this morning (it's rated to 35 Amps!!!) and the 12v line still fluctuated with a varying load (12.1v at 1amp to 11.46v at 10amps).

I guess I'll just have to live with it! It's a good job my charger works down to 10v before it gives up and dies...

Cheers!
Andy

[mdare]

Hi Reflex

"Consider in PC P.S.U's the 5V line is the most critical
and is monitored by the feedback ccts NOT the 12V line. "

This is simply untrue. Like with most things the situation is more complex. The feedback to the switch mode regulator is taken from both the 5 and 12 volt lines. A resister from each of the outputs is connected to the sense terminal on the chip. I have modified 4 different types to act as 12 volt only power supplies by changing the resister network so that feedback is from the 12V supply only. The regulation is superb. You can also destroy them quite easily this way but at only a few pounds each it doesn't really matter (4.50 at EBuyer).

I wouldn't recommend doing alterations inside the box unless you know what your doing - those 400 volt capaciters really do have nearly 400v and that can make you a tad nervous once you've been bitten [img]/ubbthreads/images/graemlins/blush.gif[/img]

Best Regards

Martin R Dare

[ QUOTE ]
"Consider in PC P.S.U's the 5V line is the most critical
and is monitored by the feedback ccts NOT the 12V line. "

[/ QUOTE ]

it is true that it is the most critical. 12v powers the drives and some other IO devices (pci cards...), but the 5v lines run the logic. most mobo's also have on board regulation for the 5v lines, the power supply is fairly noisy for pc usage.

[bigandy]

Well, I have tested my pc powersupply with my Orbit battery charger.

I used it charging a big old car battery (12v) at 6 amps. On the powersupply I had 3 car brake light bulbs (rated at 12v 21watts) connected in parallel on the 5v line.

5.03volts and 3.64amps on the 5v line
11.76volts and 7.9 amps on the 12v line.

So it works okay for me! The fan on the pc powersupply was running at the same speed when cold (it speeds up when the tmep increases in the psu) after 4 hours, so it must be well within it's capabilities. Whic is nice!

Now, I need to find some circuit diagrams for a 12v lead acid battery charger that works off 12volts (or 11.76ish!)so i can keep the field battery in good shape and free up the orbit...

cheers!
Andy

[reflex]

Hi Martin

Andy's voltage readings prove the 5V line is
the primary regulated supply.

5.03volts and 3.64amps on the 5v line
11.76volts and 7.9 amps on the 12v line.

[ QUOTE ]
The 5v line seems to hover around 5.09v regardless of the load on it. I tried a massive 20amp load on it this morning (it's rated to 35 Amps!!!) and the 12v line still fluctuated with a varying load (12.1v at 1amp to 11.46v at 10amps).

[/ QUOTE ]

So between 3.64A and 20A the 5volt line varies by only o.o6V over a 16Amp range

The 12V supply however is another matter
It's voltage varies by 0.64V over a 9amp range


Regards
Paul

[mdare]

Hi Reflex
I never said the 5 volt supply was not dominant, only that the feedback circuit monitors both 5 and 12 volt lines. If you take one apart and trace the circuit you can see that this is so. The sharing of voltage feedback is also incorporated in the IBM power supply specification. It is done in order to reduce costs.

By making the feedback only from the 12v supply, no 5v load is needed .. . but its not for the faint hearted, and you need to know what your doing. I originally used two of the supplies for charging multiple battery packs used in car racing sessions which my son had at weekends.

Best Regards

Martin R Dare