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.
[fphil] " Apparently the fans magnetic field " First I would search for the actual source of the noise. What happened if you shield your fans? Have you scope the relevant grounds?
Indeed the sensor shield can collect the electromagnetic field but then where is this shield ground? (if to the Vems, not a good idea)
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