EEE question on power transistors

Discussion in 'Technical Q&A' started by vincenzo, Mar 30, 2008.

  1. vincenzo

    vincenzo F1 Rookie

    Nov 2, 2003
    Any triple EEE's out there that can help with this TR thread?
    Wondering if (and how) these power transistors might be mounted in parallel to reduce heat and subsequent failure rates.

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  3. Jedi

    Jedi Two Time F1 World Champ
    Lifetime Rossa Owner

    Mar 18, 2008
    Seattle Area
    Full Name:
    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.

  4. vincenzo

    vincenzo F1 Rookie

    Nov 2, 2003
    Thanks Dave - sounds like you are very familiar with this type of black magic.

    The transistor is question allows the H&V fan speed in the TR to be adjusted. They frequently overheat & self destruct on these cars. It is a 2n5886 as shown on this datasheet:

    The entire TR discusion is shown here:

    Since it is difficult to replace (buried in the H&V ductwork) it would be great if we could find a better solution.

    Are the heat sinks designed for a specific transistor & shape, or are we talking about a generic 'glue it on' set of aluminum fins?
    How does a person select and then source an appropriate heat sink and/ or bigger transistor?

    Thanks for taking some of the mystery away...

    Vince (NOT a EEE)
  5. Jedi

    Jedi Two Time F1 World Champ
    Lifetime Rossa Owner

    Mar 18, 2008
    Seattle Area
    Full Name:
    It's really just a matter of increasing the surface area of the metal of the transistor. Check out 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,

  6. jselevan

    jselevan Formula 3

    Nov 2, 2003
    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.
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  8. Paul_308

    Paul_308 Formula 3

    Mar 12, 2004
    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 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?
  9. Horsefly

    Horsefly F1 Veteran

    May 14, 2002
    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
  10. vincenzo

    vincenzo F1 Rookie

    Nov 2, 2003
    Great stuff from all of you - many thanks

    EEE = Electrical & Electronic Engineering => The little stuff - radios/ circuit boards, PCs etc
    EE = Electrical Engineering => the big stuff, electric motors, power lines, generators etc etc
    Some schools do not differentiate.

    The project is just a repair on the TR H&V fan control. The TR folks here on FChat seem to see these fail fairly regulary - mine needs replacement as well.

    Thanks especially for the PN cross reference - I have yet to pull mine from the dash, but having these numbers in hand should help confirm the correct replacement.

    The TR is 18 yrs old now - the potential high loads due to a lack of fresh lubricant is a good issue to double check.

    Many thanks again,
  11. vincenzo

    vincenzo F1 Rookie

    Nov 2, 2003
    If I understood 91TR correctly, Fcar did a change from the rheostat to a system which included an air duct mounted power transister. I cannot confirm and am already in over my head in the EEE stuff. You can see photos here:

    Thanks for the input - when I replace it, I'll check for a root cause of the failure as well.

    many thanks,
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  13. SRT Mike

    SRT Mike Two Time F1 World Champ

    Oct 31, 2003
    Full Name:
    Raymond Luxury Yacht

    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

    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

    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!
  14. jdog

    jdog Rookie

    Jul 13, 2009
    #11 jdog, Jul 13, 2009
    Last edited by a moderator: Sep 7, 2017
    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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  15. ricrain

    ricrain Karting

    Nov 1, 2003
    Dallas Area
    Full Name:
    #12 ricrain, Jul 13, 2009
    Last edited: Jul 13, 2009
    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.

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