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Tom made me look at this circuit again. I realized that the series diode is

not going to isolate noise all that much. The primary noise will be a

negative 14V spike.  At very high frequencies, when the switch is turned

off, stray capacitance around the wire will tend to hold the wire at +14V

for a very short time. The diode will be forward biased by the -14 volts

from the flyback circuit, so the major noise from the flyback circuit still

gets through to the wire. The diode will isolate any secondary ringing

noise, but that is very minor and probably not very significant. MOV's do

have a little capacitance, not enough to overload the switch contacts, but

enough to dampen the high frequency noise a little. As a result, the -14V

spike that the MOV allows is not going to have a very fast rise time (a

high frequency component), so, I don't think the MOV circuit is going to

need any extra components to help filter out the -14V spike. In other

words, drop the series diode idea, it is not going to help significantly.

Every extra component increases the probability of failure. Diodes usually

fail shorted, but they can on occasion fail open. Keep it simple, the

little noise this series diode will get rid of is not worth the added

complexity. We are stuck with either a low noise parallel diode, or a more

noisy but safer MOV. There is no way that I can think of to really help the

MOV, but it will not be that much more noisy than the diode. Tom suggests

putting all three, the diode MOV and Solenoid in parallel. The diode would

take the full flyback current, and limit the noise to the low 0.7 Volts.

The MOV would limit secondary ringing voltages to 14V. Secondary ringing

voltages would quickly short out the diode if they exceeded 50Volts. This

is a safer way to use the lower noise diode circuit. I think I will use

Tom's idea on my high current inductive loads.



Thanks Tom,

Regards,

Ed.