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Power transistor overheating

Discussion in 'Electronic Basics' started by talisman, Dec 22, 2005.

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  1. talisman

    talisman Guest

    I am building an electric fence using a 12 volt battery to energise
    NE555 timer to drive a TIP41C transistor into a car ignition coil .
    Coil resistance is 1.7 ohms as measured
    Drive pulses to TIP41 are about 30ms duration at one pulse pe
    Problem is that TIP41 gets hot enough to destroy itself after about 3
    Difficult to measure current as spark upsets multimeter
    Using a lamp as load I measure 1.6 amps and TIP41 still gets ho
    enough to burn
    What am I doing wrong !!
    Should anyone volunteer to help me I can email the circuit which I go
    from the net but with no contact address
  2. Guest

    recheck your wiring....and heatsink the transistor....
  3. A 30 ms pulse is way too long. The coil has reached a steady state
    maximum current, long before that. Think of a coil driving the sparks
    for an 8 cylinder engine turning 3000 RPM. That is 3000/2 * 8 sparks
    per minute or 200 per second. Even if the charge time was the whole
    time between sparks (and it is actually more like half that), that
    would mean that the coil had 12 volts applied about 1/200 s = 5 ms.
    And it still makes sparks just fine.

    The second thing to be sure of is that the transistor has enough base
    drive current that it is well saturated at 12 volts / 1.7 ohms = 7.1
    collector amperes. That may be hard to accomplish with a TIP41.

    According to its data sheet:
    the gain falls off dramatically for collector current above 3 or 4
    amperes, especially if you expect it to stay in saturation.

    Another potential (pardon the pun) problem, especially if you are
    using a TIP41 (not suffix B or C) is that it may stand only a little
    more 40 volts before breaking down right after turn off, when the coil
    puts a large voltage stress on it. This puts a lot of power into the die.

    You may email me the schematic, if you wish, and I will critique it
    and make suggestions for better performance.
  4. Pooh Bear

    Pooh Bear Guest

    The TIP41 is a power transistor. It can handle the volts and current but
    it needs the help of a *heatsink* to get the resulting heat away.

    Used in free air with no heatsink it'll overheat if much over 3 watts is
    dissipated in it typically.

    If you don't understand what I'm talking about you need to go learn some.

  5. Rich Grise

    Rich Grise Guest

    I'm with John Popelish on this one. The 555 isn't providing enough
    base drive. I'd try to find a darlington that's rated for a much higher
    voltage, and put a transzorb that's rated at the transistor's BVCEO across
    it, to snub the spike at whatever the breakdown voltage of the transistor

    Or, you could do it completely different - lose the transistor, and
    buy or build a little DC-DC converter to give you a few hundred
    volts, charge a cap with it, and periodically fire an SCR to discharge
    the cap through the coil.

    Good Luck!
  6. ehsjr

    ehsjr Guest

    Use a BU941ZT on a nice heatsink. It's a darlington made for driving
    ignition coils. The datasheet is here:

  7. Sjouke Burry

    Sjouke Burry Guest

    If you have the on/off the wrong way around,it gets very hot,
    else if the transistor is not saturated(enough base current),
    it dissipates more ,and last ,30 msec might be a bit long,and
    if so, the high voltage might become dangerous.(And drain your
    battery faster then you would like).So make the pulse time as
    short as possible,for the output voltage to shock in stead of
    kill.It also saves the battery.
  8. ehsjr

    ehsjr Guest

    All excellent points. With the transistor I mentioned, he'll
    have 1.8 V C-E (from the datasheet) at 1.6 amps (from his
    measurement), for 2.88 watts. He needs ~100 mA base drive
    (from the datasheet), and the 555 can source or sink 200 mA
    (from the datasheet), so the BU941ZT will saturate easily.
    It can handle 150 watts (from the datasheet), so with a nice
    heatsink 2.88 watts won't be a problem. So it should be able
    to handle the op's requirements, easily. I wouldn't want
    to run the base drive the wrong way round either! However,
    more to the point, the 30 mS figure is not necessarily the best
    choice. If it drives the ignition coil into saturation
    or if the voltage produces is too high that needs to be
    reduced. I think 30 mS is too long - I'd think 1 mS would be
    a good target figure.

  9. ehsjr

    ehsjr Guest

    Grumble. Left off a sentence:
    He can work up from there, increasing the pulse length until
    he gets the desired voltage, without overheating or saturating
    the coil.

  10. talisman

    talisman Guest

    Gentlemen, Thankyou for your comments and suggestions. I have trie
    some with modest success
    However I have triumphed. My circuit is now fully functional an
    scaring the hell out of the possums which attack my vegetabl
    The problem was that the TIP41 was conducting for the 970 ms and of
    for 30ms. I did not know how to find this out. The LED flashed, tha
    is all
    I invested in a PC sound card oscilloscope and while difficult t
    interpret because of the AC coupling, it enabled me to see the pulse
    and set the pulse length.
    It now conducts for 2ms and rests for 998. It does not overheat, i
    fact it does not even heat at all!
    2ms is the optimum, shorter weakens the spark, longer does not improv
    Now to more exciting projects for which I will no doubt again solici
    your advice !!
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