Heater Fan Transistors, Which replacement....

Afternoon guys.

I was wondering, would the following be a suitable replacement for the blower motor transistors?

http://tinyurl.com/35yw7u

I can't find the original Motorola T1829-1s anywhere, and these Darlington MJ1105s looked a similar spec. I've heard of some people using TIP 147s, but have been reading reports of them having problems, and not working at the mid range speeds etc.

Any thoughts appreciated as always

Cheers

Ciarán

This post has been edited by Ciaran on September 10, 2007 03:44 pm

Hi Ciaran

I used ordinary NPN bipolar transistors and they worked OK. Rating needs to be 20A +

Hi Peter,

Excuse my ignorance, but are NPN and PNP interchangable? Some possible replacements I've seen touted seem to be PNP, but others are NPN, I presume it doesn't matter?

Cheers

Ciarán

PNPNPN?

This post has been edited by DerekW on September 10, 2007 05:33 pm

MJ 11015 (295-163) are favourite if I recall. Can be fitted with 6(?) mm screws and nuts rather than original rivets but don't forget the paste between the trannies and the heatsink.

Hi Ciaran - sorry to jump in on your question to Peter.

It does matter very much. If you imagine a transistor as a variable diode then using a PNP in place of a NPN It would be like putting in the diode the wrong way round.

This explains a bit -

http://www.markallen.com/teaching/ucsd/147...lecture3/10.php

Thanks wirdy - no problem, that's why its a 'forum' You are quite right of course,
if you fit NPN's they will have a very short and hot life.

Peter.N.

Evening Techman, Wirdy and Peter.

Well folks thank you very much for those informative replies, I now know which transistors to try, and whats more I'm that little bit more educated about how they work. I like your anology of them being like a variable diode Wirdy, that actually makes a lot of sense.
I did learn the basics of what a transistor does years ago (with one of those kids electronics kits no less ), but I never covered anything about NPN, PNP etc, so thanks for the eye opener, its been most informative!

Techman regarding the heat conductive paste betwen the trannys and heatsink, do you think the standard silicon stuff used for computer CPUs and their heatsinks would do the trick? I have a few sachets of 'high performance' stuff left over from when I last built a few computers....

Cheers

Ciarán

Hi Ciaran

Apologies for hijacking your post this time wirdy! Heat sink compound improves the thermal conductivity providing you only put on a very thin layer, otherwise it can actually become a thermal insulator!

As I grew up with valves I can only give you a valve/transistor analogy. The transistor 'collector' is equivalent to the valve 'anode', emitter = cathode and base = grid. The anode was always connected to a + supply rail and the cathode to earth. PNP transistors are the opposite way round, requiring a negative supply rail. This was OK with battery radios, probably the first application, but when they started being used in hybrid applications (mixed valves and transistors) it made things a bit awkard. Then the 'silicon' based transistor was developed (they had previously been 'germanium' they functioned in the NPN mode, the same as a valve. There are a few specific applications for PNP transistors but very few, hence the difficulty in obtaining them and relativly small range. The modern PNP transistors are also silicon based as they have better thermal stability than germainium, which used to 'runaway' and self destruct! They did have the advantage of requiring less voltage to 'switch them on' though, 0.3v as opposed to 0.6 for silicon, making them more senitive.

Hope you havn't gone to sleep now, just thought you might be interested.

Peter.N.

Just like with computers then. You wouldn't believe how many people I've had say 'MY CPUs overheating and I don't understand why, I did put loads of that thermal compound on it!'

Not at all, to the contrary in fact! I appreciate you taking the time to explain it, am always looking to broaden my understanding of things, and your informative post certainly did that.

Cheers

Ciarán

This post has been edited by Ciaran on September 13, 2007 11:39 am

Just a quick update on this...
Yesterday, after months of being run off my feet, I finally got a chance to fit the new transistors to the blower motor (which has been sitting in a corner of the living room for a couple of months ).
Anyway, pleased to report that the MJ11015s work very well, and the fan operated first time on all speeds when I plugged it back in.

Its amazing how hot the transistors get after just a few seconds operation, but there seemed to be very good transfer onto the heatsink, so the thermal compound is obviously working well.

I was feeling rather pleased with myself, so imagine my delight when I went out to a job late last night, and upon returning to the car I thought I'd turn on the heater.. to be greeted with.... nothing.
After thinking the trannys had failed, and inventing a few new swearwords, I leaned over and stuck my hand under the dash to generally smack the motor around a bit. I moved one of the cables and suddenly it sprang to life. Took my hand away, and it stopped.
I checked the tightness of all plugs etc (well, as much as you can by feel, sat in the driver's seat), and it seemed fine. I've now found that it can be made to work for a while by jiggling the wire into a certain position, so definitely an internal break or a looseness of the wire as it enters the motor plug.
Funny, it all tested out Ok with the multimeter.
I'm satisfied that the transistors were indeed faulty before, because I had the wiring in various positions while testing, and there wasn't a peep out of the motor.

Still, an interesting little gotcha that people may want to be aware of before they goto the hassle of dismantling the motor....


Ciarán

Hi Ciaran

I have had problems with the connector carrying the thick red and black wires, but even more, the 30a fuse at the front of the fuse box in front of the air filter box. Due to the high current carried it can get hot and oxidise the contacts and melt the plastic! If that is the problem, curving the fuse blade into a 'C' shape - looking at the end, will increase its effective width.

Peter.N.

Well, after a few weeks of this working on and off, its now back to square one sadly.

Peter, did you have to completely replace your connector?
The fan stopped working again on Friday night, so on Sunday afternoon I pulled the plugs off the motor and cleaned them with contact cleaner, and tightened up all the connectors by pushing them out with a screwdriver.

Theres a steady 12-14v at the plug with the engine running, but the motor works intermittently at best. It does this odd thing where it will refuse to work when you first start the car, then a few minutes down the road it will spring to life, and operate fine for as long as the car is running, hours on end sometimes. The next time the car is started, its back to not working for a while, then if lucky the blower will start eventually.

Interestingly, I noticed when I had my head down in the footwell, that the fan motor runs very slowly all the time when the engine is running, irrespective of the speed control on the dash. I believe this is a characteristic of those replacement MJ11015 transistors though, so not too worried about that, but it does raise the question why won't it work at normal speeds?

Does anyone know if the fan should run at full tilt when the little solitary control wire is disconnected?

Any thoughts appreciated!

Ciarán

Hi Ciaran

The heater fan in my car packed up last week, the garage let me have another one to try, said it might not work as it came from a non aircon car and didn't have the small control connector in the loom. When I plugged it in it only ran at slow speed no matter where the dial was set to. After I'd swapped the loom over and plugged the control wire in it worked fine. Noticed that it gives a little twitch when the ignition is switched on and doesn't start up until the engine is running or at least until the warning lights etc have sorted themselves out prior to starting.

Bob

Hi Ciaran

It sounds as though your problem could be due to a faulty alternator regulator. The fan circuit is designed to only come on with the engine running, what presumably tells it, is the rise to 14v when the alternator is charging. Check the battery voltage when you start the car to see if it is rising to 14v almost immediatly. It sounds as though the voltage is creeping up slowly hence the delay with the fan starting.

The fan won't normally run at all with the control wire disconnected as its the voltage from this that biases the transistors.

Peter.N.