[R-390] MOV's and snubbers

[Tisha Hayes]

One of the ways to overcome the long term reliability of an MOV is to use
one that is substantially higher in current handling capacity than the
aspirin sized MOV's that are available. You need the larger quarter sized
or bigger MOV's as they are capable of tolerating much more.

I gave Perry a few  V150PA10A MOV's that you could install by screwing them
onto the chassis. This helps in heat dissipation. Still, you want to
protect the circuit with a fuse so install the MOV after the line fuse.

The switch contacts weld due to high inrush current when the power is
applied when the current is at peak levels on the AC sine wave. In
industrial applications this was remedied by "zero crossing" switching
where current is at an absolute minimum on the sine wave.

How we can address it is with either a device lilke a PTC (positive
temperature coefficient) resistor like the CL-80 or CL-90, or with a
soft-start setup with a relay, series load resistor and a small RC timing
circuit on the coil side of the relay.

A soft-start can add tens of milliseconds or up to more than a second of
current limited startup to the power supply so capacitors can start to
charge up and the inductive slap of the transformer is minimized. It all
depends on how you size the RC time delay and the series load resistor.

An MOV is just providing protection from incoming voltage transients above
the triggering voltage. MOV's are not exceedingly fast devices. They occupy
the middle ground between avalanche diodes and gas tubes. A more
appropriate transient protection system would be a hybrid with a fuse in
series,  gas tube in parallel, inductors in series, MOV in parallel, PTC in
series, avalanche diode in parallel and Y safety capacitors in parallel.

The idea behind a hybrid protector is to put the more robust protective
devices in line to first catch the transient. They are not very fast
devices, fuses and gas tubes are millisecond-acting types of devices. The
MOV and inductors are middle-ground and delay and stretch out the time of
the transient to the ~1 millisecond response time. Next is the silicon
avalanche diode (SAD) that has a response time that is really limited by
the speed of light through wire (lead length) but can respond in micro and
nanoseconds. SAD's are very fast but fragile devices so you want the gas
tubes, inductors and MOV's to eat up the device-killing part of the
transient.

You may see hybrid protectors designed into high end electronic devices or
some of the more sophisticated transient protection systems.

-- 
Ms. Tisha Hayes. AA4HA