Your story demonstrates one of the keys to trouble shooting many problems: Being able to duplicate the failure. So many times there is a problem that appears randomly and until I can find out why, I am never totally satisfied with the fix. Even if it is my own error, just correcting it without knowing the thought process that caused the error to begin with drives me crazy.
It's called surge impedance Zo, which is defined as √(L/C), where L is the inductance in Henrys, and C is the capacitance, which for a coil is the interwinding stray capacitance.
The back EMF V= ZO(∂I/∂T), and it will have an oscillation frequency 1/(2π√LC). When you open up contacts in an inductive load, ∂I/∂T goes to ∞: You see this as an arc when you unplug an iron; and also when relay & motor starter contacts are switching off an inductive load.
This is also why contactors have serious current deratings when switching off DC: Once the arc is established and current flows through the ionized channel, there is no zero crossing to extinguish the arc, as occurs with AC,
When dealing with AC, you design using the peak (not RMS) value of the load current when calculating, because you don't know where in the AC cycle the contacts will open.
Note: Those of you who are RF jocks will quickly recognize ZO = √(L/C) as the equation for the characteristic impedance of a transmission line: Yes, it's the same thing.
We had changed the output driver from a completely analog design, to one with a digitally controlled output driver. The new driver would reset occasionally with the noise or the communications to it would get interrupted.
On the old module the noise would pass through to the output, but the frequencies were well above the system response time.
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