COMPUSA 250 watt ATX power supply

[PCBONEZ]

Re: COMPUSA 250 watt ATX power supply

Pretty cool.
-
On you next mod consider this.
If you double the silicon [not by capacity, but by component count] you reduce the TOTAL heat loss [dissipation] by something like 2/3-3/4.
What I mean is:
If you have some diode w/10 amps going through it at it's disipating 20 watts....
Two of them in parallel passing the 5 amps each [still 10 amps total] will dissipate something like 4 watts each [8 watts total].
Works similar for rectifiers and mosfets.
That little trick is how PSUs like the FSP Epsilon 700FX [I think you've seen one] can get away with such small heatsinks.
.

[370forlife]

Re: COMPUSA 250 watt ATX power supply

The only problem is that most of these old designs are only silkscreened for 1 schottky on the 3.3v and 12v, while the 5v is silkscreened for two. I wanted to use two of the old 30A, 45V to-247 schottky's (bigger, dissipate more heat) I scavenged, but it was only silkscreened for to-220's on the 5v, so I had to delve into my ONsemi stock and use the 30A, 60v to-220's. The transformer in this is only a 33, so I don't think I will super-overbuild this like I usually do.

[PCBONEZ]

Re: COMPUSA 250 watt ATX power supply

Mind you I can't see what you are looking at..
.
If the TO-220 has nothing on the other side of the heat sink from it then using a bit of creative engineering you should be able to mount another TO-220 using a longer bolt and then connect via wire to the other TO-220.
You won't need a bigger heatsink.
In fact you could probably get by with 1/2 as much heatsink.
.

[370forlife]

Re: COMPUSA 250 watt ATX power supply

I was thinking about this then I remembered something from the JG bargain basement roundup. The cheifmax 650W, a leadman, used a single (fake mind you) 30A, 40V part for the 5v, while the RealPC Silence power blue or something like that (also a leadman) used two 30A, 40V parts for the 5v, but they cut a leg off of each of them, halving the capacity. I wonder if heat control was the reason?

[PCBONEZ]

Re: COMPUSA 250 watt ATX power supply

I've seen that before and couldn't figure out why they did that.
- I think you nailed it. - That makes total sense.
.

[everell]

Re: COMPUSA 250 watt ATX power supply

Decided I needed some excitement for a Saturday night so....decided to play with this CompUSA power supply and see what it will do.

I have a new unused 18N50 FET....so I removed the P7NB60FP switcher and replaced it with the 18N50. That is going from a 7 amp switcher to a 18 amp switcher.

First, I didn't expect the UC3843 pwm chip to have enough power to drive the 18 amp FET. If it couldn't, I had a two transistor driver I was going to use to add some driving power, but as things turned out, the pwm chip did drive it OK.

I put some loading resistors on it as follows:
5vsb 4.87 volts 2.5 ohm load 1.95 amps 9.49 watts
5v 5.24 volts 1.65 ohms load 3.176 amps 16.64 watts
12v 11.74 volts 3 ohms 3.89 amps 45.4 watts
Total power 71.5 watts

After running for about ten minutes and turning power off, both heatsinks were cool. The main switching transformer was cool. The 5vsb transformer was very hot.

To my surprise, it will handle 72 watts without blowing up! Do you think I should be daring and add another 60 watts worth of loading???

[momaka]

Re: COMPUSA 250 watt ATX power supply

I'd say you could pull ~200 watts even with the old FET (provided that the rectifying diodes are good enough). Not sure what's going to happen with the new FET you put in, though. Maybe the PWM controller doesn't have a problem driving it now, but what about under a higher load?
On my LP6100-D, the main FET is a W8NA60 (8A continuous, 30 pulsed). As a comparison, the Thermaltake TR2-430W (XP550 NP) has a STP10NK60Z FET (rated at 10A continuous and 36A pulsed) and can put out about 350 watts. Heatsinks in both are about the same thickness.
If you decide to add that extra 60 watt load (or even more), put it on the 5v rail.

Also, I am quite surprised that you were able to pull almost 2A from the 5vsb. Honestly I would have expected it to blow at around 1.5A (if even that), considering how small that 5vsb transformer is.

The crappy 5vsb design aside, I still think this PSU is actually not that bad.

[everell]

Re: COMPUSA 250 watt ATX power supply

Snap, crackle, and pop. It did not survive the additional load. FET blasted into two pieces, pwm chip has small hole in top, some burnt resistors, and some nice black spots on the bottom of the pc board.

Funeral arrangements are pending!

[370forlife]

Re: COMPUSA 250 watt ATX power supply

gotta post pictures of that. Even a LP-8860 can make it to 291W before puking it's guts up. This psu is just pathetic.

[everell]

Re: COMPUSA 250 watt ATX power supply

The "viewing"

[everell]

Re: COMPUSA 250 watt ATX power supply

One more picture.....the pwm chip. Looks like someone shot at the lower circle in the 8 on the part number AP3843. From the small hole there is a small crack extending outward. It was all too small to see in the picture in the last post, so I looked at it under a microscope.

[momaka]

Re: COMPUSA 250 watt ATX power supply

I guess that means no DM311 mod for this one
Or does it ? Let me know if you need part numbers/resistor color codes on any blown parts . Just noticed my LP6100-D not only has the same revision number but also the same manufacturing date (April 13, 2001). Weird...

gotta post pictures of that. Even a LP-8860 can make it to 291W before puking it's guts up. This psu is just pathetic.

here's the link too...
http://www.jonnyguru.com/modules.php...Story4&reid=71
It's a slightly newer model than what we have, but I doubt that matters much. The transistor for that LP-8860 looks tiny (last page of review). Too bad JG didn't mention part numbers.

[Khron]

Re: COMPUSA 250 watt ATX power supply

I think that's one of the first times i've seen a (SINGLE) TO-220 switcher used instead of a TO-247 Sheesh, how cheap can you get?

In case you need spares, i see you can get (up to) 5 free samples from Texas Instruments (UC3843)

[370forlife]

Re: COMPUSA 250 watt ATX power supply

I guess that means no DM311 mod for this one
Or does it ? Let me know if you need part numbers/resistor color codes on any blown parts . Just noticed my LP6100-D not only has the same revision number but also the same manufacturing date (April 13, 2001). Weird...


here's the link too...
http://www.jonnyguru.com/modules.php...Story4&reid=71
It's a slightly newer model than what we have, but I doubt that matters much. The transistor for that LP-8860 looks tiny (last page of review). Too bad JG didn't mention part numbers.

I use my lp-8860 as a power supply for powering motors and stuff, but the primary switcher is so off brand I couldn't find a data sheet for it.

[everell]

Re: COMPUSA 250 watt ATX power supply

I guess that means no DM311 mod for this one

The 5vsb transformer is TOO SMALL to consider improvement with a 5vsb mod. To do the mod would also require a different, BIGGER 5vsb transformer.

The other question in my mind is the size of the main switching transformer. Is the transformer on your psu the same size? Is your power supply still operating in a computer?

[everell]

Re: COMPUSA 250 watt ATX power supply

The crappy 5vsb design aside, I still think this PSU is actually not that bad.

The "crappy 5vsb design" is what made the Bestec ATX-250 12E famous!

The crappy 5vsb design is also what makes many Antec power supplies bad.

BUT..........Bestec and Antec did have a decent sized 5vsb transformer.......and are easy to modify with DM311 chip.

[momaka]

Re: COMPUSA 250 watt ATX power supply

The 5vsb transformer is TOO SMALL to consider improvement with a 5vsb mod.

Yeah I know, that's why I put all the smileys there .

The other question in my mind is the size of the main switching transformer. Is the transformer on your psu the same size? Is your power supply still operating in a computer?

- No, main transformer is actually bigger (size 35 like most other normal PSUs)
- Currently not used in any computer since quite a few caps went bad (even the 200v ones on input) but it is operational and I might actually recap it. After all, it did last 5 years. Computer was turned off when not used (but I don't think this helped the 5vsb though - in fact, maybe even the opposite).

[mur]

Re: COMPUSA 250 watt ATX power supply

I use my lp-8860 as a power supply for powering motors and stuff, but the primary switcher is so off brand I couldn't find a data sheet for it.

What is the switcher's part number?

[370forlife]

Re: COMPUSA 250 watt ATX power supply

I can't remember. I couldn't find it on any search engine. I can tell you it's tiny and probably not rated very high at all.

[everell]

Re: COMPUSA 250 watt ATX power supply

CompUSA 250 watt psu has been repaired. Blown switcher, blown UC3843 pwm chip, shorted transistor, two blown resistors, shorted snubber diode (unexpected!), and found four more small RULYCON capacitors I had neglected to change. ALL Rulycons are now gone!!!

The psu again powers up with a HDD and CDROM loading it. Step one.....I put the 71.5 watt load on it. Worked fine. Step two......again added another 62 watts of loading. This time it took the load and worked fine. So put the HDD and CDROM load back on it, and it is running as I am typing this post.

So why did this psu blow up????? So why does any psu go bad????? One thing that surprised me was that for a switcher to be blown apart as it was, the fuse did not blow and none of the four diodes in the bridge diode circuit blew. I would have expected the fuse to be blown and at least one of the diodes to be shorted.