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Please help me!

Discussion in 'Beginner Electronics' started by Alistair, Oct 17, 2003.

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  1. Alistair

    Alistair Guest

    Hello again,

    I still have a problem with this transistor setup. 4017 (5 Stage Decade
    Counter) output to an NPN (BFX34) to drive 2 21 watt 12 volt lamps. I have
    tried a suggestion of using a PNP FET (BUZ11A) whit a resisitor of 20 ohms
    onto the base. The transistor gets hot and so does the resistor. I have also
    tried a TIP42A, and found that with it mounted to the metal structure, it
    shorts out to earth - which the lamps are connected to and eventually the
    vehicle. So what I really need is a transistor that can be driven either
    straight from the 4017 or via the NPN transistor from the 4017, capable of
    sinking 5 amps without getting hot,so I dont have to mount it to the steel
    frame.Please dont suggest using relays as they are totally out of the

    Many thanks

  2. Dick

    Dick Guest

    There are many things to consider..
    1. 4017's can sink about 10mA with a 10v supply but have very little source
    2. Power transistors have very little DC gain so will need one or more
    stages to get enough current gain.
    3. If the FET or BiPolar output stage is not saturated the device will get
    hotter than necessary
    4. The output device and the driver stages will get hot as the device is not
    perfect and will have an "ON" resistance...
    by calculating the thermal resistance of the device, heatsink compound etc
    the size of the required heatsink can be calculated to maintain the junction
    temperature typically less than 125 degrees C. It is possible that the
    heatsink may have to force cooled with a fan. Driver stages will probably
    require heatsinking as well.
    5. All biasing must be such that all devices are fully saturated, and in the
    case of MOSFETS a voltage higher than the supply voltage may be needed to
    bias it.
    6. When driving incandescent lamps a limiting resistor is a good idea as the
    inrush current can be up to 10x the running current, as a cold filament has
    a much lower resistance than when hot.

    It may be easier to purchase a readily available solid state relay which are
    usually opto isolated, can probably be driven directly by the 4017 and have
    an isolated heatsink.........

    Hope this is of some help, Dick
  3. Jimmy

    Jimmy Guest

    I would use a darlington transistor.I think the TIP42A is one. cant see how
    you are going to get around not using a heat sink. Calculate the base
    resistor to give you about 1ma of base current.
  4. Luhan Monat

    Luhan Monat Guest


    You should use something like a TIP112 (NPN darlington transisitor).
    Use a 1k resistor to drive the base, ground the emitter, and connect the
    collector to one side of the lights. The other side of the lights goes
    to 12 volts.

    You must either insulate the transistor from its heatsink, or 'float'
    the whole heatsing on insulators to prevent the collector (case) from
    shorting to ground.

    Good luck,
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