VR trigger fan noise
This page is about a strong (600W) PWM fan in a car, located in close proximity of the crank VR trigger:
I've found the strange situation of VEMS starting fuel pumps and injectors with stopped and stand-still engine when the fan operates. Apparently the fans magnetic field is strong enough to trigger the crank VR signal-
Scope shot of this noise measured on VR signal cable:
It's in the range of +-0.5V, more than enough for LM1815 triggering
This is a normal scope trace of cranking the engine -without fan operating:
I'm looking for solution of this problem.
Since the signal/noise ratio is around 10/1, I've the idea to attenuate the VR signal with a series voltage divider at around 1:10 ratio.
[dnb] I too have seen this problem, but when headlamps are turned on or off with the engine not running. When the engine is running the LM1815 chip seems to be doing a good job suppressing noise so I am not too worried about the problem but a fix would be nice.
I am not convinced your attenuation idea will be completely successful - it might make the car hard to start when the VR signal is small at cranking speeds - but please keep us informed.
I have in the past used a 6dB "pi" attenuator so I can monitor crank triggers using a USB sound card (when trigger logging with VT was not available). It can be simply made with 3 resistors. I used the following to get impedances matching my requirements.
R1 = 13k1
R2 = 16k8
R3 = 3k6
Since the trigger is made of relatively low frequency sine waves, maybe an additional band pass filter could be built in to the front end? I suspect though that it all comes in at the same set of frequencies... It's difficult to tell without being able to look at an FFT of the data.