<VV> Tachometer

[James Davis]

This discussion is on how to adjust a working tach; not how to repair 
a tach.  A signal generator would be ideal but Tony, Lew, Fred, and 
Rad are probably the only Corvair people that have one.  Most of us 
have AC electricity near when working on Corvairs.  Since power 
companies spend millions keeping the frequency of AC perfectly stable 
at 60 hz (cycles per second to you old ee's), why not use that 
standard to calibrate the tach.  The tach wire in the coil is a handy 
place to insert the frequency standard.  The problem is the tach is 
looking for a 6 to 15 volt pulsed DC signal and the house current is 
117 volts, sine wave, AC.  Fortunately the tach has a zener diode in 
the input which can change a sine wave AC to pulsed DC, so we just 
need to drop the household voltage to the tach voltage.  A door bell 
transformer changes 117 AC to 14 volts AC so it will work.  The 
proper sized resistor in the probe will also work.  As for why 1,200 
rpm?  Household voltage is 60 hz or cycles/sec.  That equates to 
3,600 cycles/min.  Since the there are three sparks per crank 
revolution, divide 3 into 3,600 sparks to get (tada) 1,200 rpm ;-).
Jim Davis





At 06:13 PM 3/1/2007, Mike Demeter wrote:
>I do have a tachometer that does not work.
>
>Explain the doorbell transformer?
>Explain the 60 hertz?
>
>Am I confused???
>
>Mike