I'm guessing the "feedback" John Sohl was talking about is unintentional
feedback as opposed to designed feedback. If that is not true, ignore
the rest of this message.
If you try to square-pulse a coil with a logic circuit, the inductance
can create voltage spikes that can fry or confuse the logic. You can
get the "tail wagging the dog." Another example of this is running a
stepper-motor with logic pulses. You need some components to shunt
induced emfs so they don't get back into the logic circuit.
Connect the two ends of the coil together with a varistor or with
back-to-back zener diodes. The breakdown voltage of the varistor, or
the zener voltage of the diodes, should be a couple volts higher than
the pulse voltage. For example, if the pulse voltage is 5 V going to
ground through the coil, put a couple 7-volt zeners in series with each
other, pointing opposite directions, across the coil (from the 5 V
signal to ground). The zeners will shunt any voltages higher than about
8 volts, but will not shunt the 5-V signal. A varistor would act about
the same way and can be purchased to shunt fairly high current. Surge
absorbers often use both; the varistor to take high current and the
zeners to act quicker. As mentioned, shunt diodes are necessary and
common in stepper-motor logic.
The already-designed circuit might work if these shunt diodes are added.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu
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