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Dual transistors

Discussion in 'Electronic Basics' started by Ed, Jun 3, 2007.

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  1. Ed

    Ed Guest

    The circuit described at is to improve the
    dash temperature gauge performance on certain Jaguars. I'd like to
    try the circuit, but it calls for
    a dual transistor and suggests 2N4014 and that is not a dual. What
    about something like NTE 45
    Also, I get a bit worried when a project like this doesn't give
    correct part numbers.

    Is this circuit likely to perform as described in the article?


  2. Meat Plow

    Meat Plow Guest

    Use two singles and tie their bases together.
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  3. Ed

    Ed Guest

    According to the designer, two singles won't work well because it
    would be
    difficult to keep the base temperatures precisely the same. But he
    does suggest
    that as an alternative, using power transistors clamped together with
    thermal paste.

    BTW, I have sent e-mail to him, but no reply. He may have lost
    interest, or
    doesn't check e-mail very often.


  4. Phil Allison

    Phil Allison Guest


    ** The 2N4044 is a dual - however an NPN one !

    ** No, the emitters are commoned - so no good for you.

    ........ Phil
  5. Gareth

    Gareth Guest

    What country are you in? it may help us to recommend suppliers.

    Do you know how much current the transistor needs to handle?

    There are quite a few dual PNP transistors in very small surface mount
    packages, but I expect they would be quite fiddly to solder?

    see and select "PNP (Dual)" under "transistor polarity" on the page below:

    The only dual PNP I can think of right now in a leaded package is the
    MAT-03. This is a precision audio part (that means expensive). I don't
    know if it is suitable though.,,773_865_MAT03,00.html

    Given the errors you have discovered so far with this design, I'm not
    sure I would want to fit it to my car.

  6. Eeyore

    Eeyore Guest

    The emitters are commoned. Useless.

    You know what I'd do ?

    I'd get maybe 6 - 10 low cost, general purpose, low-to-medium power pnp
    transistors but with decent current gain, such as the type you'd get as the
    driver in a modest audio amp.

    I'd find the 2 devices with the best match for Vbe and then glue them together
    back-to-back (ensuring that the collector tabs are insulated) . That'll ensure
    good thermal tracking.

    Job done.

  7. Eeyore

    Eeyore Guest

    It's the *die* temperatire that matters.

    Thermal paste is messy and not required here. Since there's no power involved
    ordinary epoxy glue would do the job just fine.

    Duals are rare as hen's teeth btw and *expensive*.

    Use the 2 power transistor method.

  8. Ed

    Ed Guest

    I'm in California
    Very low. I've measured the current through the sensor to be less than
    100 mA.

    Thanks, Gareth.

  9. I guess the definition of "very low" depends on who is
    asking. 100 mA from a 12 volt source can produce enough
    heat to put a blister on your finger (1.2 watts). But that
    is pretty low, compared to the current a cranking motor draws.

    I think you can make the matching a lot less important if
    you add a couple ohms in series with each emitter. I think
    that mounting two TO-220 power tab transistors on the same
    small heat sink (with insulated mounting) would make this
    work just fine. Any PNP jellybean power tab should work,
    like TIP30 or TIP32.
  10. Eeyore

    Eeyore Guest

    Beware the low beta for those devices. I forget how beta dependent the circuit
    was or wasn't.

  11. I think the circuit is a crude current mirror that boosts
    the current that passes through the sensor by an adjustable
    factor. Nothing precise about it. It is just a simple way
    to expand the central part of the meter scale.
  12. Phil Allison

    Phil Allison Guest

    "Eeysore Idiot "

    ** Bollocks.

    A typical TIP31 /32 has a beta of * 150 *

    with Vce = 2 volts & Ic =100mA.

    ......... Phil
  13. Ed

    Ed Guest

    Yes, you're right. Now I notice that the gauge has a 2W 50 ohm
    resistor at the
    sender input. Also, my calcs show the sender current can be up to
    about 0.2A.
    OK, once I settle on the transistors I'll do that.

    When I search for TIP30 at (easily available locally) I
    am led
    to NTE153, a PNP power transistor in the TO-220 package. However, I
    that h_fe1 can range from 40 to 200. It might be hard to get a pair
    with "matched"
    h_fe, if that is what is called for, ideally. I also notice that the
    NPN version, NTE152,
    is available as a matched pair, NTE152MP. A quick Google tells
    me there are current mirror circuits based on NPN transistors. So, I'm
    if I could use NPN in my application. Is that possible?

  14. Guest


    Actually, what needs to be matched is the relationship between base to
    emitter voltage and collector current. But adding 2 ohms in series
    with each emitter adds an additional resistove drop to each effective
    base to emitter (plus) resistor drop, that swamps out any mismatch in
    this current range, so unmatched transistors can be used. These
    resistors do not reduce the current gain of the transistors, but
    increase the voltage required to turn them on (by 0.2 ams * 2 ohms =
    0.4 volts, which totally reduces any millivolt difference in the ~.6
    volt base emitter drop to insignificance, even if the two transistors
    are not at the same temperature. You might be able to use 1 ohm
    resistors (0.2 volts drop, max) and do almost as well, since this is
    really not a high precision application.

    John Popelish
  15. Marra

    Marra Guest

    So we have amateurs fixing Jag electronics ?
    Well I wont be buying a Jag !
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