Blower Motor MOSFET

[Dylan W]

The fan speed control in my Acura TL stopped working. I traced the problem to the "resistor" unit, which is actually a heatsinked 2SK2500 MOSFET with a couple of resistors, a cap, and a fuse. The drain and source were shorted, so figure it's a bad FET. (Apparently a common problem with this car.)

Rather than pay $50 for a new unit, I figure I'll replace the MOSFET. No 2SK2500 at Mouser, only on Ebay, and I'd rather get a new, better-rated FET anyway.

Can't find a datasheet for the 2SK2500, but I read somewhere that the blower motor has a DCR of about 0.6R, meaning that with a 14V full battery charge it's going to pass close to 25A. (I'll go out and double check the DCR.)

So guessing the 2SK2500 is a 25A part running close to its limit?

This guy is a 50A part and has the same pinout. Any issue with using it as a replacement?

http://www.mouser.com/ProductDetail/Fairchild-Semiconductor/FQA46N15/?qs=sGAEpiMZZMshyDBzk1%2fWi6u8DDnmpUwtW1I%2fhQa5ZHY%3d

 

[PRR]

> the blower motor has a DCR of about 0.6R, meaning that with a 14V full battery charge it's going to pass close to 25A.

No. Or Yes, for the first instant. But as the motor comes up to speed, it also acts as a generator, back-EMF, bucks the supply voltage, and the current drops off.

For a truly UN-loaded motor the current would fall to zero. We need current to cover the friction and the load. A blower is typically well-loaded at MAX.

I predict the true running current is closer to 6 Amps than to 25 Amps. (I just went through this with another heater: 5.7 Amps on HI.) This means roughly a 2 Ohm load.

But yes the first-instant current can be in the 25A range.

The speed controller "must" have resistance well below 2 Ohms. I remember 1 Ohm MOSFETs. You want under 0.2 Ohms. I know they now come with a point-oh in the Ohms rating.

But take a 0.2 Ohm MOSFET. Say 6 Amps. 6A*0.2r= 1.2 Watts. Is it heat-sunk that much? A TO-220 will do a Watt easy, on the bench, for days. I think this part needs significant heat-sinking or a very-low on resistance.

Personally I'd rather have a dumb resistor, like a 1968 Chevy. Those worked for years; that's what I put in my tractor. However they have changed. My parent's NEW car only works on HI. The dealer knows and has a fix. But it *seems* that after some duct-fire event, new blower resistors have thermal fuses. And they haven't got the parameters right yet. So the fuses blow, the low speeds quit.

Anyway I *assume* your fancied-up Honda has a MOSFET so the fan is "continuously variable", instead of L-M-H like a switched resistor.

OTOH, my neighbor knows where to find old-fashioned rheostats for heater controls. "Continuously variable" with no silicon. They even have a clamp to mount to the lower lip of your dash panel. (I doubt there's anywhere they'd grab on modern plastic 13-part dash systems.)
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EDIT -- there's material about this car-part on-line, and it woulda saved some speculation if you had linked to it instead of hoping we'd stumble over it.

There's a good hunk of heatsink there. (But did Acura nail the FET solidly to the sink?)

The '500 part seems to be 0.007 Ohm on-resistance, the part you picked is higher.

And I suppose you proofed the thermal cut-out?