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Volt Readout to Ignitor

NJEngine

Registered
Hey guys,

I was wondering what my volt readout should be at the ignitor. I have a 12v 7ah battery with a 12 volt coil.

Thanks
 

Kirk Taylor

Registered
Hey guys,

I was wondering what my volt readout should be at the ignitor. I have a 12v 7ah battery with a 12 volt coil.

Thanks
Open-circuit voltage across the ignitor will be the same as battery voltage. A low tension coil is just a single continuous winding; no magic happens inside. When the points snap open, an instantaneous voltage spike of a couple hundred volts occurs across the ignitor due to "inductive kick". Because of the short duration, the spike isn't measurable with a standard multimeter. It may, however, wreck the meter.
 

NJEngine

Registered
Thanks for the reply. I should have been more clear. That is exactly what I was looking for, the voltage spike. I just didn’t know if there was range I should be looking for. Great info.

Thanks again
 

Thaumaturge

In Memory Of
Age
68
Last Subscription Date
07/12/2019
A back EMF (Electro Motive Force) spike from an inductive load may in theory be of infinite voltage. In practice usually a lot less than that. A reverse protection diode installed to conduct reverse voltage but not forward voltage to ground should protect your meter movement. For protection purposes a diode with a PIV rating of 1000V is customarily used. (1N4007, 1N5408 etc.)
Doc
 

Andrew Mackey

Moderator
Last Subscription Date
05/14/2017
As stated earlier, your volt meter reacts too slow to actually read the voltage spike. If you have access to a voltage reading oscillator, It may be able to capture the spike on a screen. They used to use them for checking TV sets high voltage output.
 

miro

Registered
It's not so much the voltage but how much current you are carrying in the circuit when the contacts are closed.
You should see 10's of Amps.
The voltage across the gap when the contacts open in the engine is depends on the inductance of the coil you have, and how fast the current goes from full current to zero current.
The overall power consumed isn't much although the current might be 10's of Amps - but only for a few milliseconds.

The coil is usually just several hundred turns ( 14 or 12 guage wire) on an iron core - no secondary winding.
Please be careful when measuring the current because you are ( sort of) applying a short circuit on the battery during the measurement. The time taken to make the measurement is way, way longer that the time taken when the engine is running.

Miro
 
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