Root cause insight into the common BMW blower motor resistor failures

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On Tue, 26 Mar 2013 14:23:08 -0400, Michael A. Terrell wrote:

Here's what the heatsink looks like when I cut into it today:

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Ha, keep going!
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On Tue, 26 Mar 2013 14:46:25 -0400, tm wrote:

Well, I only had lunchtime, and it got uglier & uglier as I went!

Unless there is some chemical way to remove that black/gray rubbery (like a tough pencil eraser) gunk, there will be no way to read the numbers on the two transistors.
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On Tue, 26 Mar 2013 19:24:08 +0000, Bimmer Owner wrote:

I only found 1 transistor.

This is what is suggested in this quote below:
[QUOTET0iman]There is only ONE mosfet in the FSU and I would forget about trying to replace it even if you had one. Also, we have no reason to believe the mosfet has caused any FSUs to fail. I have yet to see the resistors in any of the picture shown anywhere in this thread. Can someone circle the resistors for me? I would think they would be fairly decent wattage so they would be very easy to see, but I don't see any resistors.[/QUOTE]
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I think he is incorrectly calling the SMT IC a mosfet.
What went in the other three holes on the PC board? What was under the two spring clips on the heat sink?
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On Tue, 26 Mar 2013 19:41:47 +0000, Bimmer Owner wrote:

I 'think' (but I'm not sure) that these are the resistors in series:

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irly

If they are resistors, I've never seen any that look like that. Also, given that you want to thermally bond any components that generate major heat, why are they not heat sinked? With any power design I've seen, the key components, eg the transistors are directly bonded to the heat sink.
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" snipped-for-privacy@optonline.net" wrote:

You've never seen strips of nichrome in a space heater?
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Bosch has done the nichrome insert power resistor for many years; I know that they used them in the turn signal flasher in the late seventies when they first went electronic. They aren't really very good resistors but they are very cheap. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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snipped-for-privacy@optonline.net wrote:

THey make power resistors in T0-220 style cases, for heat sink mounting however, I doubt those are that.
Jamie
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Bimmer Owner wrote:

Yup, and I see the legs of the transistors (two). So those R's are in line with the emitters where they joint.
Most likely thermo stress cracks due to the potting restraint.
Jamie
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On Tue, 26 Mar 2013 19:52:26 -0500, Jamie wrote:

Other people have suggested this also.
Some say the potting is what is causing the stress cracks.
Re-insert without potting, is the "said to be" solution.
One question: If the FSU works without potting, what was the purpose of the potting?
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On 03/28/2013 06:13 AM, Bimmer Owner wrote:

to keep the elements out. and the inquisitive.
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I'm still trying to figure out if there was only one or two:

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Bimmer Owner wrote:

Two.
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Bimmer Owner wrote:

I was never crazy about that style of heatsink.
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Nothing wrong with linear motor control, it's just inefficient and produces a lot of heat. I used to work in a place with a 1.2 MW DC motor whose field coil voltage was controlled by a couple rooms full of cast-iron resistors. The resistance array lasted nearly 80 years before the whole facility was taken down.
As long as you keep within the safe operating area of the semiconductors, you're fine. If you exceed them, bad things happen. But we don't know if the semiconductors are failing on these things, or if it's just ordinary RoHS solder failures; the RoHS crap doesn't like thermal cycling so well. --scott
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On 03/21/2013 08:08 PM, Scott Dorsey wrote:

right, but what's the point in using 140W to run a 60W motor? and /certainly/ not when the devices to do so are so cheap and abundant. i can see doing it back in the day when there weren't any other options, but today there are, and there have been for 20+ years.

right. but again, we're dealing with mba's here. as an engineer, you're going to design with reliability and a safety margin built in. as an mba, you're going to cut and keep cutting until it meets "business objectives".

very true. but at the end of the day, that's still heat. and a linear semiconductor controller is just stooopid when a wire coil will do the same job more reliably and at a fraction of the cost.
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On 03/21/2013 08:08 PM, Scott Dorsey wrote:

that must have been interesting - what did that motor do?

that must have been a sight to see too.

shipped to china? or replaced by pwm controllers?

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I've got ask- what was this motor used for?
pumping station? mining equipment steel mill?
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