PWM revealed...

[wpoll]

Finally found our Fluke ScopeMeter so I was able to shoot out and grab some screen shots of what goes on with the PWM tail lights on an LCI...

Firstly, the PWM frequency is 80Hz. I thought it was much higher, and of course being a square wave, there ARE harmonics at higher frequencies (160Hz, 320Hz, 1280Hz etc.) - I guess it's these higher harmonics that can sometime produce the faint "singing" you can hear in high current PWM circuits on the LCI (ususally when there's a short circuit!).

Secondly, as I had the probes backwards, the shots below are upside down. The centre of the screen zero volts and the waveform makes more sense if a positive voltage is above zero but in this case postive is down.

First images are the tail light PWM. 80Hz, 11.4v peak to peak but about 27%duty cycle, resulting in 5.6v average D.C. (My guesses hade been 30% and 5v so not far off!).



Second images are the same lamp but with the brake light PWM. 80Hz, 11.3v peak to peak, duty cyle about 95% and average D.C. voltage is 11.1v.



So there ya go - it's about what we expected and knew was happening but it's good to see it for real...

[jcp240z]

Thank You