Strip Light for DIY Timing Strobe?

[emsworthy]

Fitted a Varia tachometer to our Lombardini 502 engine yesterday and obviously needed to calibrate it somehow. I'd appreciate any comments on the accuracy or otherwise of what I did.

The tacho feed is from a "W" output from an internal alternator direct to the flywheel so is effectively 1:1 ratio. From the instructions with the tacho I needed to know how many poles there were in the alternator, which I didn't but guessed at 12 (which turned out to be right!) and adjusted the dial accordingly. Then there was the matter of fine adjustment.

What I did was put a small piece of white tape on the pulley at the front of the engine, blacked out the windows on the boat as best I could and then viewed the running engine under just the light from a shore powered fluorescent strip light. I figured that as this would be "flickering" at 50 times a second and at 3000 rpm the engine would be rotating at 50 times a second too, when the white mark appeared stationary I was sure to be at 3000 rpm. Set the tacho accordingly and Bob's your Uncle! Or so it seems to me.

It all seems logical to me and seems to be about right but is it really that simple??

 

[peterb]

I suspect that your strip light will be flickering at 100 Hz; it will be bright at both the positive and the negative peaks, but dark at the zeros. Not to worry, you'll just get two stationary marks instead of one.

 

[emsworthy]

Thanks Peter,

You're right of course about the 100Hz flicker. It's a very, very long time since my last electrical principles lecture.

If I'm reading your response correctly though, the fact that I could see at least one stationary white "blur" that wasn't slowly rotating clockwise or anticlockwise meant that engine was rotating at 3000 rpm? (if there was a 2nd "blur" I obviously couldn't see it as I had put the mark on the upper face of the pulley.)