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Registered Member #2553
Joined: Fri Dec 18 2009, 01:36PM
Location: St Cloud Minnesota
Posts: 97
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:
Registered Member #3282
Joined: Wed Oct 06 2010, 05:01PM
Location:
Posts: 224
r1= 38ohm-60ohm. Your snubber design should be a bit different. The diode should be in series with the coil and not in parallel with the fet. put a resistor in parallel with the capacitor. C1 should be 1nf and r2 should be about 100k ohm. there are many variations of Rc snubbers.
Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
lokeycmos wrote ...
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
Hi Joe, there is a good snubber calculator here
If you go to the bottom of the page there is also one for a non-dissipative snubber, which is quite useful as the dissipative types can consume quite a bit of power if your pushing more than 100 watts through your ignition coil.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Insert a fast 1A diode (e.g. BA159, Uf4007) or better a 3A fast diode (e.g. UF5406) between the drain-coil junction and the C-TVS junction, cathode/band to TVS side. C can now be anything from 10 nF to uF's. The peak drain voltage will be stored across C, it can even be measured with a dmm on dc volts range if C is fairly large, you can even use it as a dc output up to 400V. The TVS will clamp this dc voltage and the drain will be clamped to this voltage plus one diode drop. This kind of snubber draws NO power from the circuit until it operates .. very efficient, you do need the power rating of the TVS to be greater than or equal to the dc power input rating for times when there is no output spark etc. that may mean 10x 1.5ke400 in parallel. (I araldite/epoxy resin them to a small heatsink) if you have multiple coils they can share the snubber with one diode each. when you power off C will not self-discharge so a high value resistor across C is recommended, e.g. 1 MOhm It's worth the 10x tvs because it makes the ignition coil driver reliable when the load is not defined (random arcs/sparks ! ;<)
if you use an avalanche rated transistor then all you have to do is keep it cool, no snubber required at all, but I trust the above snubber under all conditions.
(can not protect the transistor from over-current if a continuous short-circuit is on the coil output)
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Help finding missing component values
