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Last week, my A/C blower kept kicking on and off for no apparent reason. Had the 'blinking lights of death' on the ECC control panel. Since the blower powerstage was the problem, I removed it and checked what kind of device it was.
So, after analysing this fairly simple circuit, you can see that the blower power stage is simply a current chopper using a high power MOSFET transistor. This MOSFET transistor is the problem when the unit goes bad.
What does it do? It is fairly simple. The transistor acts like a high frequency switch. Without you noticing it, it turns on and off the blower motor very fast. The amount of time it turns on the motor versus the time it is turned off is directly proportional to the blower speed. This is what they call Pulse Width Modulation (PWM) control.
The problem is, turning on and off a high power motor like the blower motor generates heat. This is why the MOSFET transistor is attached to a HUGE heat exchanger (the 5 big aluminium fins). Why it needs this? It is because MOSFET are really heat sensitive. More your transistor will heat up, shorter will be its service life. So, finally, I think the guys at the development facility chose the best MOSFET they could find back in 1992. But its power rating was not high enough and the huge heat dissipator kept it working until now, making the power stage break after some 10 years of use.
So, what to do to repair it at low cost? Unsolder the MOSFET (It is the only part that is screwed to the aluminium dissipator), find a new one with more that 150W power rating and resolder the new one to the circuit board.
I ordered 3 types of transistors to see wihitch one will work (i assume all will work since they are all rated at 200W and more)
STB100NF04T4 N-Channel 40V - 0.0043Ohm - 120A - D2PAK StripFET II MOSFET
STP140NF55 N-CHANNEL 55V 0.0065 OHM 80A TO-220 STRIPFET II MOSFET
STP60NF06L N-Channel 60V - 0.012 Ohm - 60A - TO-220 StripFET(TM) II MOS
You can obtain those transistors on the internet for less than 3$ or order samples directly from the manufacturer for free (what I did)
I will keep you informed when I recieve them
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