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Teardown: SAAB electronic heater fan speed controller.

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Refrigerator:
While pulling my old crashed SAAB apart i found out that the heater fan speed was controlled electronically instead of a resistor pack.
I thought it was quite modern for a mid 90's car and so i decided to take the module apart and reverse engineer it. But as i traced the circuit out i noticed something strange, that being that there was no freewheeling diode. Weird i thought this module worked with PWM, efficiency and all that right?
Nope it's actually just a huge linear constant current regulator, shame.
There's only one IC inside which is an LM2904 dual op-amp. The muscle is an IRFP054 MOSFET, 60V and 64A continuous at 100oC is quite nice.
At first i thought that i could use this to control the motor on my homemade electric scooter but since it's linear that's not really possible.
But i could definitely use the MOSFET to make a speed controller.  :-+

Refrigerator:
And a couple more pics.

Dabbot:
Interesting! The values for "R3" and "R3?" are a strange choice. The same could be achieved with 8K2 and 10K.
Maybe double-check that bit?

Edit: On second thought, the 16K5 resistor looks to be hand soldered in. Manually trimmed?

Refrigerator:

--- Quote from: Dabbot on August 18, 2020, 01:20:33 pm ---Interesting! The values for "R3" and "R3?" are a strange choice. The same could be achieved with 8K2 and 10K.
Maybe double-check that bit?

Edit: On second thought, the 16K5 resistor looks to be hand soldered in. Manually trimmed?

--- End quote ---

I might have read the colors incorrectly, as the resistors are super small and telling the difference between blue and purple was hard.
But i also think it's manually trimmed, kind of weird that they didn't use a trim pot. Perhaps trim pots aren't good enough for this application?  :-//

GlennSprigg:

--- Quote from: Refrigerator on August 18, 2020, 12:57:39 pm ---While pulling my old crashed SAAB apart i found out that the heater fan speed was controlled electronically instead of a resistor pack.

--- End quote ---

Yea, I just had flash-backs of 'old' (older) cars...  To keep the 'resistors' cool, the wiring went to a Block with the resistors, mounted
inside the main fan/conditioner unit body, so that the air-flow cooled it!  Was a locating trap for the uninitiated.  :)

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