Rating of PC power supplies?

[Terry Pinnell]

Not necessarily. The most common PWM controller IC is the TL494 or its
equivalent, KA2500B/C. All you need to do is to confirm the voltages
at the inputs to the IC's error amp. One input will usually be 2.5V (=
internal 5.0V reference divided by 2), the other input will be derived
from the +12V rail and/or the +5V rail via a resistive potential
divider. All you need to do is to recompute the resistor values in the
feedback network so that the target voltage, eg 13.8V, produces 2.5V
at the input to the error amp.


The two that I modified were both AT types. I had the schematics for
neither. Only very minor reverse engineering was required.



- Franc Zabkar

Mine has no manufacturer's name on the case, only 'Model PTP-2006'. It
is 200W and has +5V @20A (red), +12V @ 8A (yellow); -5V @ 300mA
(white); -12V @ 300mA (blue); 0V (black); and an orange wire marked
'PG', the purpose of which I'm unsure about.

 

[Pooh Bear]

Mine has no manufacturer's name on the case,

Nothing new there !

only 'Model PTP-2006'. It
is 200W and has +5V @20A (red), +12V @ 8A (yellow); -5V @ 300mA
(white); -12V @ 300mA (blue); 0V (black); and an orange wire marked
'PG', the purpose of which I'm unsure about.

PG *may* be protective ground. Orange seems a odd colour to use though !

Graham

 

[SioL]

Thanks, very useful.

I've repeated my tests after connecting all but one of the 5V (red)
outputs, and all but one of the 0V (black) outputs. This time I was
also more careful where I measured my voltage; I used the unloaded
5V/0V wires. Happy to say that I now get pretty solid 5.1 V right up
the range. And, like you, I did not have to load the 12V output, only
the 5V.

Cool. If you ever get tempted to raise voltage on the 5V output,
make sure all elco caps and rectifiers can handle higher voltage.
Those fast diodes have pretty low maximum voltage.

 

[Franc Zabkar]

Terry Pinnell wrote:

PG *may* be protective ground. Orange seems a odd colour to use though !

PG = Power Good. See the schematic diagram at the URL I quoted.

Here is Intel's ATX PSU spec (versions 2.2 and 2.0):
http://www.formfactors.org/developer/specs/ATX12V_PSDG_2_2_public_br2.pdf
http://www.formfactors.org/developer/specs/ATX12V PSDG2.01.pdf


- Franc Zabkar

 

[Franc Zabkar]

As per my reply to SioL, I did not have to load the 12V output, only
the 5V.

I have seen only one AT PSU (made by Skynet) that regulates the +5V
rail. In fact I'm using it in this machine. All the others have used
the same technique as in the example quoted elsewhere. My other
machine uses an Antec 300W PSU which also appears to regulate the +12V
and +5V rails.


- Franc Zabkar

 

[SioL]

I have seen only one AT PSU (made by Skynet) that regulates the +5V
rail. In fact I'm using it in this machine. All the others have used
the same technique as in the example quoted elsewhere. My other
machine uses an Antec 300W PSU which also appears to regulate the +12V
and +5V rails.

Just how do they do that? There is just one "switcher" in the box, with a couple
taps at the output. I'd say they probably "stabilise" just one of the outputs (5V) and
check the +12V for the sake of protection, making sure it does not go too high.

In a PC 5V probably runs much of the logic directly without any voltage regulators and is
as such probably far more critical as far as stability goes than 12V.

 

[Pooh Bear]

Just how do they do that? There is just one "switcher" in the box, with a couple
taps at the output. I'd say they probably "stabilise" just one of the outputs (5V) and
check the +12V for the sake of protection, making sure it does not go too high.

In a PC 5V probably runs much of the logic directly without any voltage regulators and is
as such probably far more critical as far as stability goes than 12V.

It's possible to take 'weighted' feedback fom both rails. Basically you sense both and scale the Rs accordingly.
It'll provide a compromise 'cross-regulation'.

Graham

 

[Pooh Bear]

PG = Power Good. See the schematic diagram at the URL I quoted.

Silly me - I forgot that one.

Not sure if anyone actually uses it !

Graham

 

[Franc Zabkar]

Silly me - I forgot that one.

Not sure if anyone actually uses it !

Graham

I think you'll find that most (all?) motherboards use it. If PG is
false, then the motherboard remains in the reset state. Some
motherboards have the option of generating an on-board power-good
signal.

The original IBM AT had no reset switch. However, it could be reset by
shorting the PG pin to ground.


- Franc Zabkar

 

[Franc Zabkar]

Just how do they do that? There is just one "switcher" in the box, with a couple
taps at the output.

Sorry, bad choice of terminology. As I pointed out elsewhere, and as
the example PSU schematic shows, the one "switcher" gets weighted
feedback from both the +5V and +12V rails. I should have written that
the Antec PSU "appears to regulate by sensing both the +12V and +5V
rails".

Having said the above, notice that in the example ATX PSU the +3.3V
rail *is* independently regulated. The technique involves "bleeding
off" excess charging current by making one of the rectifier diodes
appear leaky, or at least that's how it looks to me.


- Franc Zabkar

 

[SioL]

Sorry, bad choice of terminology. As I pointed out elsewhere, and as
the example PSU schematic shows, the one "switcher" gets weighted
feedback from both the +5V and +12V rails. I should have written that
the Antec PSU "appears to regulate by sensing both the +12V and +5V
rails".

A kind of a compromise than, probably a feature of the new units.

Having said the above, notice that in the example ATX PSU the +3.3V
rail *is* independently regulated. The technique involves "bleeding
off" excess charging current by making one of the rectifier diodes
appear leaky, or at least that's how it looks to me.

Interesting technique.

3.3V is loaded heavily by the CPU, with power consumption
around 100W max. Now that takes some serious current at 3.3V.

 

[legg]

Sorry, bad choice of terminology. As I pointed out elsewhere, and as
the example PSU schematic shows, the one "switcher" gets weighted
feedback from both the +5V and +12V rails. I should have written that
the Antec PSU "appears to regulate by sensing both the +12V and +5V
rails".

Having said the above, notice that in the example ATX PSU the +3.3V
rail *is* independently regulated. The technique involves "bleeding
off" excess charging current by making one of the rectifier diodes
appear leaky, or at least that's how it looks to me.

If you see a small inductor placed between the main transformer and
the 3V forward rectifier, it likely is a set reactor regulator
controlling the 3V output ( input will be the 5V transformer winding).

This was a common method of post-regulating 3V on commercial ATX
supplies originating ca '98 (Delta in Taiwan etc).

RL

 

[Rich Grise]

That works fine if you have the schematic to calculate the values. ;-)

I suspect Terry's old PSU is likely an AT type btw.

And I really only meant the three or four black leads that terminate in
the two MB connectors.

I have found, however, that a 10 ohm, 10 watt resistor on the +12 on one
of the drive connectors was a suitable dummy load to power a bare MB.
(old-style, of course.)

Thanks!
Rich

 

[Rich Grise]

Silly me - I forgot that one.

Not sure if anyone actually uses it !

On the old-style supplies and MBs, it was used as a system-wide reset.
It's an open collector, that's held low until the supply decides it's
regulating properly, then released.

Nowadays, with these newfangled PCs that don't even have a power switch,
I'm pretty much out of the game. )-;

Thanks,
Rich

 

[Rich Grise]

Silly me - I forgot that one.

Not sure if anyone actually uses it !

I think you'll find that most (all?) motherboards use it. If PG is false,
then the motherboard remains in the reset state. Some motherboards have
the option of generating an on-board power-good signal.

The original IBM AT had no reset switch. However, it could be reset by
shorting the PG pin to ground.[/QUOTE]

Come on! Don't you remember that big red rectangular button, that we'd
tell users was the "Any" key? ;-)

Cheers!
Rich

 

[Roger Johansson]

On the old-style supplies and MBs, it was used as a system-wide reset.

Power good is a signal sent from the power supply, when the voltages
are good, so the computer knows when it is okay to start reading bios
and writing to RAM.
If it started as soon as the cpu gets enough voltage to run we risk
reading and writing faulty values in memory.

It's an open collector, that's held low until the supply decides it's
regulating properly, then released.

I built my own PC power supplies when building my first PC's.
Old , heavy analog power supplies with modern motherboards.

I discovered that some motherboards needed a fast positive flank on the
power good line, it was not enough to let the voltage rise and use the
5V line as power good, as I did for my first PC motherboards.

So I had to build a little circuit which sensed the 5V line, and
tripped when it was close to 5V, and sent a positive flank on the PG
line.