I still haven't mastered posting circuit diagrams but my circuit is simple and easy to describe.
The purpose is to limit the current through a 1.58mH ignition coil to 4 amps. The coil is driven by a Fairchild IGBT specifically used for ignition coils so all the spike suppression is already on the IC and it's a 5 volt logic level drive.
To limit the current I first tried limiting the gate voltage. The IGBT got very very hot, and yes, the magic smoke escaped. Then I thought of using a high power NPN in series, and a low or high side current sense resistor through an op-amp which sort of worked, but now the NPN was getting very hot.
So I thought of a completely different approach.
When a normally closed switch is opened it goes through a series of hex Schmitt triggers with a resistor/cap delay to give a low pulse of 100 microseconds duration at the output. This goes to the trigger pin of a 555 set to monostable mode making the output pin go high for 0.75 milliseconds (adjustable) which will, via the IGBT charge the coil to about 4 amps, not precise but good enough for me. After the 0.75 milliseconds the IGBT is turned off and the coil sparks.
I haven't built the circuit yet but the circuit simulator said it will work. Have I overlooked anything, it just seems too simple a solution? I'm asking before I fry my last IGBT.
The purpose is to limit the current through a 1.58mH ignition coil to 4 amps. The coil is driven by a Fairchild IGBT specifically used for ignition coils so all the spike suppression is already on the IC and it's a 5 volt logic level drive.
To limit the current I first tried limiting the gate voltage. The IGBT got very very hot, and yes, the magic smoke escaped. Then I thought of using a high power NPN in series, and a low or high side current sense resistor through an op-amp which sort of worked, but now the NPN was getting very hot.
So I thought of a completely different approach.
When a normally closed switch is opened it goes through a series of hex Schmitt triggers with a resistor/cap delay to give a low pulse of 100 microseconds duration at the output. This goes to the trigger pin of a 555 set to monostable mode making the output pin go high for 0.75 milliseconds (adjustable) which will, via the IGBT charge the coil to about 4 amps, not precise but good enough for me. After the 0.75 milliseconds the IGBT is turned off and the coil sparks.
I haven't built the circuit yet but the circuit simulator said it will work. Have I overlooked anything, it just seems too simple a solution? I'm asking before I fry my last IGBT.
