Sizing a flyback protection bridge diode.

Thread Starter


Joined Jan 6, 2014
I am a teacher and I use an trembler type induction coil for demonstrations and experiments from time to time. Earlier this year I started using a bridge diode ( 400v 12A as suggested by coil supplier ) with a single diode across the DC output (with band toward + output terminal). The bridge diode goes between the induction coil and the the voltage source (12 volt battery or power supply). I works quit well. I get more reliable output from the coil and less sparking at the points. The single diode is there to channel the flyback voltage from the primary back through the primary circuit in the correct direction. I have both an ammeter and a voltmeter between the power source and the bridge diode. In general power is supplied to the coil a few seconds at a time and amperes are around two ( never have seen a reading much over four). Earlier this morning while using the coil the ammeter jump way up. I took my finger off the switch quickly at that point. Something had shorted. It turned out be the bridge diode. So, my question concerns sizing a flyback bridge. I know the secondary of the induction coil put out around 25,000 V but I do not know the value of the voltage or amperage in the primary circuit. It is too erratic to measure with any meter I have. Some colleagues have ruined meter trying this. I am wondering if anyone might any ideas what voltage and amperage rating would be appropriate in this application and if using too high of rating would cause any performance problems with the coil?


Joined Nov 7, 2013
A .5 to 10uf capacitor is often used to reduce arcing and increase output.
A smaller capacitor across diode may protect it.