Hi All,
I am so frustrated at this point since I have been at this problem for what seems like ages now.
I have a controller circuit which activates a FET board (contains 2 12N10L and 2 MBR40250G diodes. These diode/fet pairs activate 1) gas valves 2) ignition coil attached to spark plug.
Initially, I got it all worked out using a particular coil. Things work great but alas, the coils are no longer available. The specs were primary res 3.5ohm, secondary 14kohm. Not sure about the ratio.
Now I have moved to a Blaster SS 8207 coil. It's high performance. Primary res 0.355ohm, secondary 4.4Kohms and 70:1 ratio.
The first thing I notice is that I have to limit the input current. No problem there. Even when I dry fire (no gas applied), things seem to work ok but once gas is applied, at some point, my FET controlling the coil fails (STUCK ON) and this in turn sticks my gas on..... at which point I run screaming.
I understand why my spark FET failure causes a failure on the gas, even though I REALLY dont like it. My question is WHY does my spark FET fail.
I have attempted to raise my input resistance (via ballast resistor) all the way up to 8 ohms. I see a marked decrease in spark strength although it still fires well.... until it LOCKS.
I am kind of stuck looking at the secondary resistance. My new coil is so much lower, do I need to increase it? I use really short plug wires.
My theory is that for whatever reason, the kickback from the coil is overcoming the diode, and ravaging the FET. But why would it, the diode is rated for up to 250V blocking voltage and 40A.
Additionally, I noted that with no ballast my initial coil that works great has a current of 3.8A vs the new blaster that doesnt work at 8A.
On my FET board I have the diode in reverse polarity across the source and drain.
Any help would more than greatly appreciated.
Chris
I am so frustrated at this point since I have been at this problem for what seems like ages now.
I have a controller circuit which activates a FET board (contains 2 12N10L and 2 MBR40250G diodes. These diode/fet pairs activate 1) gas valves 2) ignition coil attached to spark plug.
Initially, I got it all worked out using a particular coil. Things work great but alas, the coils are no longer available. The specs were primary res 3.5ohm, secondary 14kohm. Not sure about the ratio.
Now I have moved to a Blaster SS 8207 coil. It's high performance. Primary res 0.355ohm, secondary 4.4Kohms and 70:1 ratio.
The first thing I notice is that I have to limit the input current. No problem there. Even when I dry fire (no gas applied), things seem to work ok but once gas is applied, at some point, my FET controlling the coil fails (STUCK ON) and this in turn sticks my gas on..... at which point I run screaming.
I understand why my spark FET failure causes a failure on the gas, even though I REALLY dont like it. My question is WHY does my spark FET fail.
I have attempted to raise my input resistance (via ballast resistor) all the way up to 8 ohms. I see a marked decrease in spark strength although it still fires well.... until it LOCKS.
I am kind of stuck looking at the secondary resistance. My new coil is so much lower, do I need to increase it? I use really short plug wires.
My theory is that for whatever reason, the kickback from the coil is overcoming the diode, and ravaging the FET. But why would it, the diode is rated for up to 250V blocking voltage and 40A.
Additionally, I noted that with no ballast my initial coil that works great has a current of 3.8A vs the new blaster that doesnt work at 8A.
On my FET board I have the diode in reverse polarity across the source and drain.
Any help would more than greatly appreciated.
Chris