[ARC5] Troubleshooting Tips from the Late Nick Broline, #3

[David Stinson]

Choosing MOVs to protect your power supplies:

This is a long treatment of what you are asking for in your MOV selection.
Depending on how your MOV manufacturer specifies the device, it may not be
what you want.

Your 122V power system has a peak voltage of 1.4 x the rms value (122V)l, or
173V.  This peak voltage should be the voltage used in selecting the MOV.
One with a lower rating would mean it would be attempting to conduct on the
peak of every half cycle.

Looking at an offering from the Mouser catalog for the Bourns MOV products I
see they have provided application assistance by specifying the RMS voltage
design center, the DC hold-off voltage, then more data that gives us insight
as to how they turn on, or start conducting, and how hefty it is.

Their line for part number
MOV-20D201K is called a 130V rms MOV that can operate across a 170V dc
source without drawing significant current.
Continuing down the listing, they specify the voltage at which it will draw
1 ma: 185V min, 200V nominal, and 225V max. Now we can see these are not
precision voltage devices, so we need to select one with some "elbow room"
at the top voltage. They do no go to full conduction at the threshold
voltage, either. They turn on gradually Increasing the voltage just causes
them to draw incrementally larger currents, and the voltage across them also
rises. Note that the lowest threshold voltage at 1 ma. is comfortably below
your peak line voltage.

MOV's are not anything like back-to-back Zener diodes, which will have sharp
knees at turn on. They would certainly try to clamp a transient at their
rated voltage.....once......unless one uses very large honkin' diodes. For a
transient like a lightning strike somewhere distant from the protected
product they would clamp the transient once, then possibly be destroyed by
the peak energy they are subjected to. Most likely they would short, and
that would blow the fuse.

MOV's are designed to take such transient "on the chin" and continue
protecting without destroying themselves.....but they are limited in the
number of times they can do that, and that is normally not a problem. A good
way to think of them is their ability to absorb a relatively large amount of
energy instantaneously and will draw a very high peak current (6500 A in
this part number) for a few microseconds in the process. This makes them a
good and cheap circuit protector that knocks off the sharp edges of
transient waveforms.

The product I used as an example is about 1" in diameter and is a fairly
typical size. It looks like it would be a suitable selection for your needs.
73
Nick Broline W5FUA

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