EEE question on power transistors

Well, you CAN put transistors electrically in parallel to 'divide the load' - but it's risky. First off
both transistors must be very precisely matched (especially current gain - H21) - spec sheets
are not good enough. You'd need to actually measure the current gain of both to be as exactly
matched as possible. But even then you'll need to put 'equalizing resistors' in series with the
two transistor emitters - this is to equalize the current flowing through the transistors. Otherwise
one or the other can 'run away' and self-destruct from current overload. But none of this is really
done much these days - paralleling transistors (at least bipolar silicons) is pretty much a thing of
the past. You'd be far better off just finding a heavier duty transistor with the same specs but
greater current capacity (not gain). You could also just massively beef the heat sink, using good heat sink
silicon heat-transfer goo, and lots of surface area. But be careful - not knowing which transistor
you're talking about, the large metal variety (TO-3 is the most likely package - large-ish oval
metal) is usually the collector (electrically) and if the transistor is a PNP, the case has full V+ on it and if you
ground it, you'll get a nice short and probably destroy the transistor and blow fuses.

Again, I'm not familiar with the circuit you're working on but I still don't recommend paralleling transistors.
I would stick to getting more efficient heat dissipation (heat sink) and maybe add a small 12vdc CPU style
fan to move more air over the heat sink.

Dave

 

It's really just a matter of increasing the surface area of the metal of the transistor. Check out
www.digikey.com for commercial sinks. But you can just fashion your own out of aluminum. Fins
are good because they increase the surface area. But you MUST use 'heat sink compound' - it's
a special blend of silicone that has a high heat transfer rate. You cannot use glue! Normally a
heavy heat sink will use the mounting screws of the transistor to hold it tightly.

But I must caution you - the case of the transistor is probably HOT. Not temperature, but meaning it
has B+ voltage on it! You cannot allow the transistor or the heat sink to contact metal. Now,
depending on the circuit it's possible it's not hot. But don't take chances - don't allow the sink to
contact ground. If you remove the transistor from it's socket, you'll find a thin fiber wafer underneath
that's there to insulate the case from the chassis it mounts to. Make certain you replace the wafer,
preferably with a new one (cheap).

I wouldn't recommend changing transistors without having a qualified engineer involved to make sure
all the other specs are still compatible.

Good luck,

Dave

 

Vince - Assuming that the 1986 Testarossa used the same transistor, I can attest to failure being uncommon. Mine never did. Have others experienced failure? I have to admit that I do not recall seeing a power transistor when removing my dash to clean the air conditioning motor. I thought that the variable speed was controlled by a simple rheostat in the console. I assume that you are, indeed, referring to a power transistor and not the thermister found in the plastic housing.

Might a motor failure lead to power transistor failure? Technically yes. If there is a short in the motor windings the motor could continue to rotate while excessive current is drawn. This would lead to excessive collector current in the transistor.

Dave is correct in leading you away from parallel transistors. Better to choose a transistor with larger collector current and power dissipation specifications. However, the original engineers who designed this circuit knew what they were doing. I doubt that a larger power transistor is required. Something else is going on.

If the bearings in the motor or the fan cage are dry, this would lead to larger current requirements and ultimate transistor failure.

If, in fact, the transistor has failed, I would replace it with an equivalent device. The part number should cross reference to a more recently manufactured power transistor.

Good luck.

Jim S.

 

I'm only EE (what would the third E be?) and totally agree on all points with Dave zommbee. Will add that heat sink grease is too often applied too generously. It is only meant to fill in molecular sized gaps between mating machined surfaces...so sparingly thin is appropriate.

Once upon a day, 0.1 ohm (depending) resistors were used for paralleling but now days, larger devices are available, some with dual dies and onboard matching.

Be curious to learn details of the project. huh huh? Audio maybe?

 

I went over to our cabinet here in the Engineering Department and dug out some old semiconductor reference books. Assuming that you are trying to replace a 2N5886, the 1978 Howard W. Sams replacement cross reference indicates the following parts numbers as replacements for the 2N5886 transistor.

General Electric GE-75
Mallory PTC175
Motorola HEPS7007
Raytheon RE37
RCA SK3036
Sylvania ECG181
Workman WEP222

 

Vincenzo,

Doubling up transistors is really not the best way to do it, and probably not a good idea in general. The better solution is to use a higher rated transistor. Also, I was not able to see from the picture if there is a heat sink on there, but if not then adding one will make a HUGE difference, and if there is then changing to a better one will make a HUGE difference.

I'm guessing it uses a potentiometer as the speed controller that's just a voltage divider, through a resistor which then uses the transistor as an amplifier to control the blower motor. If so, then the beta (Hfe) of the transistor is going to matter. I'm pretty sure that's the setup they will use and if so you could switch to any other transistor provided it had a similar beta curve and you matched it with a resistor - for example

http://www.onsemi.com/pub_link/Collateral/MJ11028-D.PDF

Can handle a continuous current of 50A (vs. 25A for the OEM part) and will dissipate 300 watts vs. 200 watts for the stock unit. But the Hfe of this unit is around 400 vs the stock unit around 20, so this one would almost instantly go to full power (the knob would be very sensitive - a tiny adjustment would have a huge effect on blower speed - unless you matched it with a resistor).

But that's all sort of academic. I would suggest just using a heat sink on the stock part. Is there one on there now? I'm guessing not. That is a TO-3 package. You can go to digikey and type in "heat sink" and go down to the heatsink menu item. Choose one for a TO-3 package then take your pick. They are rated in thermal resistance (degrees C per W @ a given air flow). This basically means how much does the sink heat up per watt of power it's dissipating. A lower number is better. But a lower number will mean a larger sink usually. So if you can fit it, go with this bad boy

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=345-1050-ND

If there is no sink on there now, this puppy will have that transistor barely working at all. If you can't fit that on there, just use the best rated one that will fit, and your problem will be solved.

Honestly, if you were thinking of doubling up the transistors anyway, you would be way better off just switching to a higher rated transistor and matching it's beta curve with a resistor. At least that way you're just replacing whats there now and putting a single resistor inline. If you try to double up transistors, you'll be modifying the car to try to fit them in there.

So in summary, heat sink is the best option by far

Second best would be changing transistors

Last resort would be doubling up transistors

Hope this helps!

 

Most are factory installed very poorly/dangerously. If the heat sink contacts ground fuse is gone--and possibly wiring burned.

This pic is with an added heat sink and I also added an isolated transistor socket which involved enlarging the holes in both heat sinks.

Note: Heat sink fins cut so they don't touch the AC coils in the dash (Mondial). Liberal use of heat sink compound even between heat sinks.

!!!! ONLY Do this work with the NEGATIVE car battery DISCONNECTED !!!!

Even when running properly these transistors run very hot.

The whole system is ridiculous (imho).
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I don't think Gain, HFE, etc. is an important consideration. It's a bipolar NPN running in saturation mode. IIRC, the controller sends a PWM signal to the base (all or or all off). It's simply being used as a switch, not in linear mode, in other words. A power trans with a bigger surface area, larger heat sink and/or a transistor with a lower forward drop will probably do the trick. I replaced one in a TR (not mine) years ago with a generic one from Radio Shack and the owner hasn't had trouble with it since, AFAIK.