You would need a steady 12v supply i.e. from the battery to power the
electronics, then you can use the rectified AC signal from most chargers as
the signal as these are not usually smoothed i.e. a steady 12v but pulse
from 0v up to something above 12v. For a voltage sensing tach (73 on)
connect the -ve from the charger to the tach ground and the charger +ve to
the tach sense terminal, ideally via a load like a coil. For a current
sensing tach (72 and earlier) you *must* use a 12v coil in series with the
charger terminals and the tach pickup. Experiment with the current
direction, only one way will work.
----- Original Message -----
> Can one use the battery charger to calibrate the tach?
> How would it be done?