# 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
432
7
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

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2. ### #12 Expert

Nov 30, 2010
16,248
6,745
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.