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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


    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


    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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