Reusing pc ATX Switched Power Supply

Thread Starter

prspedro

Joined Nov 5, 2020
3
Hello, I'm trying to rebuild an ATX computer power supply. I'm using the scheme in the images.

I'm using 127V, when I turn on the power transistors. I thought it could be the 494 that was not being activated. I activated it from an external source and it still burned the power transistors. I removed the 330k resistors that connect the power transistor collectors to their bases and still burned the same transistors.

Can someone, with experience in switching power supply, could help?
Thanks.
 

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Dodgydave

Joined Jun 22, 2012
10,063
Hi what you have there is the typical TL494 circuit, that uses two self oscillator transistors to to start the 5V standby supply, then the TL494 takes over and synchronization takes place .

If your supply is 120V , then you need to feed it into the two Capacitors and one side of the bridge rectifier, this makes a Voltage Doubler around 350V.

If it's blowing the transistors then it's probably the starting transistors or the output diodes could be blown.

Here are several ATX circuit diagrams to compare..
http://danyk.cz/s_atx_en.html
 
Last edited:

Ramussons

Joined May 3, 2013
1,095
When testing these power supplies, if you use a 100 watt incandescent bulb in series with the main power input, you will save a whole lot of switching transistors from burning out. But as one of the members of this forum mentioned earlier, you cannot buy such bulbs in Developed Countries.
 

Thread Starter

prspedro

Joined Nov 5, 2020
3
Hi what you have there is the typical TL494 circuit, that uses two self oscillator transistors to to start the 5V standby supply, then the TL494 takes over and synchronization takes place .

If your supply is 120V , then you need to feed it into the two Capacitors and one side of the bridge rectifier, this makes a Voltage Doubler around 350V.

If it's blowing the transistors then it's probably the starting transistors or the output diodes could be blown.

Here are several ATX circuit diagrams to compare..
http://danyk.cz/s_atx_en.html
Hi Dodgydave,

thanks for the reply,

I think what I have is a simplification of this first one (http://danyk.cz/s_atx01a.png). Yes, when I connect between the two capacitors the voltage reaches 380. I tested the rectifier diodes and the starting transistors and they are normal. However, I think it might be a starting problem. Do you have any way of knowing the expected starting values? on the second transformer? just turning on the 494? The 494 is generating two 33KHz PWM signals.
 

Thread Starter

prspedro

Joined Nov 5, 2020
3
When testing these power supplies, if you use a 100 watt incandescent bulb in series with the main power input, you will save a whole lot of switching transistors from burning out. But as one of the members of this forum mentioned earlier, you cannot buy such bulbs in Developed Countries.
Hello Ramussons,

I will try to find an incandescent lamp, today they are difficult to find, but thanks for the tip.
 

Dodgydave

Joined Jun 22, 2012
10,063
The voltages on the primary are around 300V to 380v depending on input voltage, the primary winding is centred on one side at half supply, as pulsed Positive and Negative with the two transistors in a push pull synchronization, . If it's blowing up straight away then I would say it's the start up oscillator and the smaller Transformer not working properly, so both transistors are conducted at the same time.

These are a Pain to get going! As said use an incandescent bulb in the mains feed, .
 
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