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Do I need a snubber/rc for high-frequency ignition coil driver?

Discussion in 'General Electronics Discussion' started by plat, Sep 26, 2017.

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  1. plat

    plat

    8
    0
    Sep 26, 2017
    I have constructed a square-wave multivibrater that is switching a 2N3055. I am running this at high frequency, anywhere from hundreds of Hz to dozens of kHz.

    I am currently using this to drive an ignition coil but will also try some high-frequency transformers and other inductive loads. Do I really need a snubber/rc to protect the 2N3055? I have ran the device for a long time without one and haven't seen any signs of trouble, but want to be sure.


    There are two reasons that I would rather not use a snubber if I can get away with it long-term:

    1. A snubber diode just doesn't work. I can't explain why but when I attach a rectifier diode between the 2N3055 collector and +(parallel to the inductive load), the ignition coil simply won't fire. I have made sure the diode polarity is correct. I have tried several different diodes and each produces the same result. The oscilloscope shows that I am not getting the full voltage through the transistor with the diode in place.
    2. Using an RC(0.1μF + 10kOhm) isn't good either because it introduces lots of noise to the output signal. Enough that I can easily hear it over the base frequency when driving a speaker or arc from the ignition coil. I would like the cleanest, most pure output signal that I can get, for testing/research purposes.

    I appreciate your thoughts on all this. I would really like it if the diode would work, since I assume that wouldn't introduce noise.

    Maybe I could also ask for your thoughts on using a power MOSFET instead of the 2N3055? I have never used MOSFETs before but for this high(ish)-speed switching application, they seem like a good fit. My goal is to have a variable-frequency square-wave drive with lots of power behind it.
     
  2. duke37

    duke37

    5,277
    734
    Jan 9, 2011
    The flywheel diode restricts the high voltage on the 3055, there will need to be restrictions on a fet also but fets are often specified to take more abuse.

    There are many ways of restricting the collector voltage.
    1. Place a resistor across the coil, if this is the same value as the coil resistance, then the voltage will be twice the power supply voltage. Obviously this wastes power.
    2. Place a capacitor across the coil, the energy in the coil = I*I*L/2, this is transfered to the capacitor = V*V*C/2. This is similar to the R C snubber but without the resistor so that some ringing could occur.
    3. Use a zener diode in series with flywheel diode to allow the collector voltage to rise.
     
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