Have a little patience man.. Yes, I just took out the transformer part, as well as the electronics on the AC side, and replaced it the my own creation.Quote:
Originally posted by Charles Enwesi:
<STRONG>damnit if u guys dont post a schematic or something i will close this post. I have being waiting and it seems like Jeff and PreeLab has this thing under control now. PressLab, If i am not mistaken, reading ur post. You simply took an ATX psu, ripped out the AC transformer part and reduced/increase another transfer to drop the voltage to a 12v......ok i am lost. Someone explain this electronic stuff in simpler form for me. Sounds like i can do this stuff...i hope :)</STRONG>
Okay a transformer works on turns ratio. Let's say you have 120VAC and you want 12VAC. Your turns ratio would be primary:secondary, or in this case, 120:12 = 10:1. So as you see, the number of turns, as well as the input voltage determine the output voltage. Example web page, not mine
Let's say you wanted both 12VAC and 24VAC on the output. You would then have two secondary windings, with the 24VAC one having twice the number of turns as the 12VAC one. The voltage output is not related to current (actually it is a little bit because of resistance in the wire), so with the two windings, you will have 12VAC on one, and 24VAC on the other, regardless on how much current each one is using.
The turns ratio is a little more complicated with a switching supply, but you get the idea.
So, why do they use this "switching" power supply? It basically ups the frequency of the wall outlet, 60Hz, to somethen from 20kHz to 500kHz. This allows the use of much smaller transformers because it reduces the chance of core saturations at high loads. This happens with the core can't hold any more magnetic energy, and your windings basically turn them self into little heaters instead of energy converters (inefficiency).
So what happens if our input voltage changes, but we want a constant output voltage? With the turns ratio, our output will be ratiometric with the input, right? That's where the PWM comes in. This changes the duty cycle, or the on vs off time of the primary windings. So with a 8V input your duty cycle would be 40%, and with 16V input your duty cycle would be 20%.
This maintains the constant output voltage by monitoring the output voltage, and comparing it to a fixed reference voltage. These two voltages are fed into an error amplifier. The output of the error amplifier then controls the PWM modulator, which will increase the duty cycle if the output voltage is too low, and decrease the duty cycle if the output is too high. This maintains the correct output voltage, if the duty cycle is not too large, > 50%, or too small, < 5% or so. If your duty cycle is too small, your transistors don't fully turn on and turn off, so they get hot, or it does't work at all.
Anyone correct me if I'm wrong.