ChadMan said:
I have a PIC counting pulses. I know the software is working. I need to
knock a 15kV spike down to 5 volts to be counted by my PIC. The spark is
20hz for a count of 200 pulses. I need to monitor it and make sure it is
sparking every time. I tried a MPSA350 with a 5.1 volt Zener in parrallel to
no avail. Too much noise. The PIC gets flakey. Can anyone point me to a
circuit or give me a hint on how to tame this spark?
TIA
First, the PIC and any other 5 volt circuitry needs to be isolated and
shielded from the spark, spark leads, spark coil, capacitors, charging
resistors, SCR's or any other circuitry associated with the discharge. All
of this stuff radiates EMR when the spark fires. Secondly, the spark may not
be a single pulse but may oscillate many times as the discharge progresses.
Depending on how the PIC is programmed, it may multiply trigger on each
discharge. Be sure the code can't interrupt or poll the port in quick
succession causing erratic behavior. In other words run time delays to
insure single pulse triggering.
A number of ways have been mentioned to pick up the pulse including resistor
dividers, optical, etc. Any of these can be made to work if they are done
properly. One method you may try is a current transformer to sense the spark
current made by running one of spark leads through a small ferrite torroid
core. On this core wind a secondary of 100 turns more or less of fine wire.
Load this winding with a resistor, capacitor and zener and use it to feed
the PIC port. The advantage of this method is that it does not load the
spark as a divider scheme might plus its low impedance makes it less
vulnerable to interference and EMR, furthermore, it can easily and safely
isolate the HV from the PIC.
Bob