Bestec ATX-250 12Z problems and solutions

[Before Work]

Hi,

Would you know anyone who has a schematic for the Bestec 250-12Z? The neighborhood had a power failure while the computer was on. When the power came back on, it wouldn't come back on. The LED in the back of it blinks at random. The caps look ok. I got my 5VSB. What does SB mean? Thanks

[everell]

Originally Posted by everell

First, when the computer is plugged into a wall outlet, the LED indicates that the 5vsb is working. The measurement from the LED indicator is actually taken from the PS-ON circuit which gets its power from the 5vsb.

Second, when you push the power on button on the computer, this signals the south bridge chip to take the PS-ON in the power supply from high state (5vsb) to low state (ground). This is like shorting the green wire to a black wire on the power supply plug. But you have to remember that the south bridge chip is what actually does this. So the LED indicator light goes off. This means that the south bridge chip is working.

Third, the LED indicator also monitors the PGO (power good out) signal from the power supply supervisor chip. When PGO is good, the signal is high (5 volts) and the LED comes on. When PGO is bad, the signal is low (0 volts) and the LED is off.

Power up sequence for the LED indicator is: when plugged into wall outlet, LED should come on. When you push the power on switch, the LED should blink off then on. This indicates that the south bridge chip turned the power supply on and that the power good signal came on quickly.

The 5vsb is a "stand-by" power supply used to start up the computer. It runs any time the computer is plugged into the wall outlet. The rest of the power supply does not turn on until you push the ON/OFF button.

If the green indicator light is blinking, it indicates that something is wrong with the 5vsb circuit. This circuit has a "crowbar" circuit which will turn off the 5vsb when it detects problems. Sometimes your problem can be resolved by unplugging the computer from the wall outlet for five to ten minutes, then plugging the cord back into the wall outlet. If this does not fix your problem, the 5vsb circuit will need some troubleshooting.

[SgtRock]

Dear Everell:
--You are definetly a top notch sleuth of power supplies, and so are awarded an honorary deerstalker hat and calabash pipe.
Cheers!
"Does not squirell crack nuts on bough of tree"
Lao Fu 1420 - 1580

[mauserman48]

I have a 250w-12z.. It was a donation. When I opened it, found 6 caps buldging. When powered up, had 11.90v,5.15v,and 3.5v.. Will replace the caps and see if the 12v rail comes up.

[mauserman48]

Like the person before me asked, where can I get the pcb schematics for the Bestec 250w 12z..All I've found so far was the pin outs for the connectors.

[mauserman48]

Hey fellas!! Just found the schematics for atx psu.s It's for a atx 300w. hope this works.

http://www.smpspowersupply.com/ATX_p..._schematic.pdf

[mauserman48]

After replacing 5 caps,the voltages were...11.77v 5.25, 3.35v. Did I do something wrong. I replaced the caps with the same uf values,only on one of them I put in a 25v/1000uf instead of a 16v/1000uf... Would this make a difference. I really only need the 12v rail for my project.

[mauserman48]

Question.I was reading where the atx needs a load resistor to keep it on..

[momaka]

Originally Posted by mauserman48

After replacing 5 caps,the voltages were...11.77v 5.25, 3.35v. Did I do something wrong. I replaced the caps with the same uf values,only on one of them I put in a 25v/1000uf instead of a 16v/1000uf... Would this make a difference. I really only need the 12v rail for my project.

You need to put a load on the output of the PSU if you want those voltages to look better. Many older PSUs will have the 12V rail run low if the 5V rail is not loaded enough. Try adding several 12V car light bulbs on the 5V rail of the PSU, one on the 3.3V rail, and a small one on the 12V rail, if needed.

Also, what capacitor brands and series did you use for the replacement caps and where did you buy them?

[mauserman48]

I've added a 1.8 ohm,10 watt power resistor on the 5v rail and the 12v rail came up to 12.35. I'm using this one only as a 12 volt psu to run the shop stereo. thanks for your help..

[momaka]

Originally Posted by mauserman48

I've added a 1.8 ohm,10 watt power resistor on the 5v rail and the 12v rail came up to 12.35.

Not a good choice of resistor values.

At 5V, that 1.8 Ohm resistor will be dissipating
W = (V^2)/R = (5^2)/1.8 = 25 / 1.8 = 13.8888 Watts - that's higher than what the resistor is rated for! As such, your resistor likely won't last very long and will surely burn wires if it comes in contact with them.

You need to get a resistor either with a higher resistance (but that will lower the load on the 5V rail) or a higher power rating.

Best bet is to use one or two 12V incandescent light bulbs on the 5V rail. Typically, a 20-40 Watt-rated bulb should do, as it will draw around 5-6 Watts @ 5V (about 1 Amp). With two bulbs, you will get pretty much the same load as with the 1.8 Ohm resistor, but the bulbs won't run anywhere near as hot as the resistor. And they may be cheaper and easier to find than a new resistor.

[mauserman48]

thanks, will do that. I have a lot of auto 2 element bulbs from work.

[msb83]

Hello everyone
My question: is it possible to transform a Bestec ATX-250 12Z to obtain an adjustable voltage from 2v to 6V on the 5v rail as it is possible to do on PC power supplies using KA7500 controllers?
Using the information found in "Bestec 5v and 12v regulator circuit.pdf" and "Bestec ATX-250 12Z 5 & 12 volt regulator circuit.pdf" published by Everell, I tried to replace the R59 and R60 with a potentiometer for modifying the voltage but the protection of the A6351 puts the system in safety (information found in "Bestec ATX-250 12Z crowbar.pdf"), therefore impossible to obtain a variable current. Is there an easy way to prevent the A6351 from cutting off the power to get a variable voltage from 2 to 6 volts (to make a salt water chlorinator for water treatment for a swimming pool? )? Thank you in advance for your answer...

[sam_sam_sam]

Just use a LM317 regulator which would be the easiest thing to do or buy a adjustable power supply to do this

It not easily done with an ATX power supply because of protection of each power supply rail

[msb83]

Thanks for your quick reply, but I need 15A and LM317 can only supply 1.5A and even LM150 and LM138 would be insufficient (LM150 (3A) and LM138 (5A)). I still think that it must be possible to use a Bestec 250 power supply ...

[everell]

Hello msb83,
Changing R59 and R60 to a potentiometer works well when trying to center the 5 volt output voltage. I don't think the power supply will allow you to get a range of 2 to 6 volts due to its design. The Bestec power supplies have some really good protection circuits (except for 5VSB on 12E model!) compared with the bargain basement cheapie power supplies.

I see that you are wanting to use this Bestec in an application involving a swimming pool. Personally I think this is an extremely dangerous situation. This power supply is encased in a metal box which is then encased in a metal computer case. The ground pin on the AC plug also gives some added protection. But if you use it in a swimming pool environment, if something goes wrong you could easily end up hot wiring the water in the swimming pool, and possibly killing someone. To me, this is a BIG safety issue, and I strongly advise you to not use a computer power supply for this type of application. Hopefully some others on this forum will add their thoughts to this situation.

[PeteS in CA]

1. You probably could, hypothetically, change the feedback divider resistors, making at least one a potentiometer, to get an adjustment range of 2V-6V. However ...

2. The supervisor circuit, if the PSU has one, may shut down the PSU if the 5V (and other voltages, which will be affected), is out of the ATX regulation range. That would definitely happen with modern server-grade PSUs.

3. The transformer ratios are designed for a certain pulse width when the line voltage is the nominal value. Changing the output voltage will change that pulse width. At some output voltage above 5V the pulse width will be at the maximum, and the output voltage could not be increased, and the output voltage could change with line voltage fluctuations. At some output voltage below 5V the pulse width will reach some practical controllable minimum, with unpredictable effects on the output voltage.

4. Changing the feedback resistors will affect control loop compensation, and therefore could affect loop stability.

5. Increasing the 5V by 20% (to 6V) will similarly increase the 12V output voltage. If the 12V output capacitors are rated for 16V, the output voltage at no load may be very close to that rating.

[msb83]

Everell is right, this idea is probably not the best I have had and the potential danger for use in swimming pools is too risky. However, now that my Bestec ATX-250 is open and already modified, I will continue my investigations to carefully attempt experiments on it despite PetesInCA's pessimistic forecasts. Realizing a variable voltage power supply could be useful for my assembly-discoveries in electronics. Hence my new question: to prevent the A6351 from cutting the power supply, wouldn't the simplest solution be to connect S and D on the A6351, which would supply the transformer with 330v permanently? What could be wrong with it? It being understood that my tests would be carried out at a reasonable distance without taking any risks!

[msb83]

In the absence of an answer and in doubt, given my incompetence in electronics, I will therefore refrain from trying the experiment and put an end to my project by contenting myself with the constant 5 volts of my good Bestec. Hi.

[sam_sam_sam]

There is a way to do what you want to do but and but you have to very careful about doing this project

This is a disclaimer notification

What can be done is where the switching transformer is and please note that not all switching ATX power supply can you do this on please be aware of this fact

I have done this and can be done however you have some limitations on how much current you draw also be aware of this as well

Here is the post where I did this and I would pay close attention to the comments that I made in this post

https://www.badcaps.net/forum/showth...ht=sam_sam_sam