Help finding missing component values

Discussion in 'General Electronics Chat' started by lokeycmos, Feb 24, 2013.

  1. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    on the attached pic of an ignition coil driver R and C need to be determined so that the inductive kick will be below the maximum safe voltage of about 400V for a IRF740 MOSFET. The frequency wont be fixed at 300hz as in the example. my question is, what would be some good starting values for R and C. how would i go about calculating these? i know it says to experiment, but im looking for a ball park value to start with. Thanks

    i actually have 3 ignition coils here are primary specs if you need it:

    1) 6.7mH 1.5Ohm
    2)5.9mH .8Ohm
    3)6.8mH 1.5Ohm
  2. #12


    Nov 30, 2010
    First, I'd lower that TVS to a hundred volts.
    Second, are you going to let these coils go to 70V/ DC resistance?
    1/2 CV^2 = 1/2 LI^2
    for 5.9mh, 87.5 amps, 45.172 = CV^2
    to set the capacitor voltage limit at 100 volts, 4517uf at 100V

    You can mess around with a 200V TVS or a 300V TVS and alter the capacitor charge that way. You should add a resistance between the catch capacitor and the drain of the transistor to avoid di/dt = infinity.

    I lost my formula for inductance and time. The short "on" time might be enough to reduce the current from maximum.

    You can also look here for ideas.
    Last edited: Feb 24, 2013