Motorcycle ignition coil tester help

Thread Starter


Joined Mar 20, 2007
I have the task of constructing a Motorcycle Ignition H.T.Coil Tester.

Now i have been reading some of the posts on here and quite frankly your all very very clever as apposed to my Electronics skill level of -100 as m y user id suggests.

You guys may be able to help me :confused:
Does anyone here understand the principle of a Motorcycle coil and how it produces a spark as i need to construct a tester that will power the coil with a 12v feed about 6 ish Amps and make and break the earth side of the connection exceedingly fast.

I have an idea that i need in the circuit a 555 Timmer IC and some sort of transformer to step down from 240v to 12v as i wish to power from the domestic mains supply.

Does any of this make sense :rolleyes:

A circuit diagram wouuld be Kin marvelous me thinks:D


Joined Apr 20, 2004
An ignition coil works on the principle of self-induction. When current flows in the primary, it creates a magnetic field. Interrupting the current flow causes the field to collapse. That is the driving force that induces the high voltage in the secondary.

Lots of FET's will handle 6 amps of current and are cheap. IRF520's should cost very little. Use your 12 volts to run the 555 which can drive the gate of the FET. Put a nice big diode like a 1N4006 across the FET to protect against the inductive kick.

You tenasformer will be the pricey part. Coils are pretty undemanding about voltage, so something that will output 8 volts at 8 amps should do. You'll need to rectify the output. You will also want about 10,000 microfarads for the filter capacitor. Get something rated at at least 25 volts.

If nobody can point at something close, I'll see if I can post up a schematic in a bit (life is busy, so please be patient).


Joined Apr 20, 2004
I'm going to be a weenie and suggest you look at the site here-

You can eliminate the lower FET and coil for your purposes. The moderate frequency of operation is probably better for testing a scooter's coil.

The transformer you found is not big enough for the supply. Something like 18 volt center tapped with an output of 8 amps would be good. That would be called a power transformer. Another dodge is to use a motorcycle battery.