Eliminating Ignition Induced EMF Spikes

Thread Starter

RDC1955

Joined Feb 16, 2019
6
I have a situation where an EMF induced voltage from the ignition coils is causing issues with a new electronic flasher relay on a vintage motorcycle (12V system) and was wondering what the most effective way is to eliminate these spikes. The control line to the flasher relay is picking up the EMF and appears in the neighborhood of a couple hundred volts at the ignition frequency – the spikes are very narrow but I don’t have an actual measurement. I have tried installing a .1 uf ceramic 100v capacitor to the flasher control line and it eliminates the noise. So the question would be would this be acceptable protocol, and if so how would the voltage rating for the capacitor be determined?
Thanks in advance for any response.
 

crutschow

Joined Mar 14, 2008
25,116
I have tried installing a .1 uf ceramic 100v capacitor to the flasher control line and it eliminates the noise. So the question would be would this be acceptable protocol, and if so how would the voltage rating for the capacitor be determined?
That is a common method to minimize high frequency EMF interference.
Since the capacitor absorbs the high voltage spikes, its voltage rating only needs to be 150% of the normal DC voltage across it so 100V is more than adequate.
 

Thread Starter

RDC1955

Joined Feb 16, 2019
6
Thank you for the response. I had almost convinced myself that as you stated the capacitor was absorbing the voltage and therefore could be rated for the DC voltage, but then I would imagine a scenario where the number of spikes (higher rpm) or possibly the duration of the spikes could change and somehow overwhelm the capacitor and cause it to breakdown.
 
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