MOVs and NTCs (was Re: Using MicroSim)

Hi Albert, Daniel,

> Original Poster: "Albert Race" <race-at-dgms-dot-com>
> Well, Daniel... Maybe I can help here a little bit...
> >>I'm not sure I know what all of these
> parts are. Can someone explain to me what a MOV is and what it does?

> MOV stands for metal oxide varistor.. This device is used to limit the
> inrush of current that allows large voltage spikes to form. (To an
>extent) the extra energy that is accumulated in this device is wasted
>in the form of  heat.

This isnīt quite correct. A MOV does NOT limit the current in
any way (otherwise it would have to be in series with the
"to be powered" circuit). Look at a MOV as a kind of safety
spark gap (at lower voltages) or even a Zener diode. It has
a very high resistance at or below itīs rated voltage. Above
a certain (firing) voltage, itīs resistance decreases rapidly
and it acts as a short (although it doesnīt quite go down to
zero ohms). That way, it protects the circuitry, which it was
placed across.

Usually they are composed of zinc oxide, which has a nice
"determinable" voltage vs. resistance curve. The problem with
MOVs is that they have a limited lifetime in accordance with
the energy they have to dissipate. Letīs suppose you have a
MOV with 28J and 2000A for 8/20ĩsec (normal S20K size)
surge capacity. If the circuit, you use it in, actually can supply
this much power, it might well be possible that the MOV will
only withstand a single shot. If, on the other hand, you really
oversize your MOV, so that it only has to dissipate a small
amount of itīs rated surge capacity, it will last "indefinitely".
I donīt remember the derating curve from memory, but I do
recall it doesnīt change in a linear fashion.

I think you confused MOV with NTC. A NTC (Negative Temperature
Coefficient resistor) is used to limit the high inrush of current.
These devices typically have less than 1 ohm in cold state and
decrease their resistance at turn on (as they grow warm) within
a few seconds. They are used in most SMPSU to prevent the
high initial current flow from blowing the rectifier. Another use
is for devices that are running on the edge of what your circuit
breaker can take (like the large 2.2kW angle grinders, etc), to
prevent the high start up current from taking out your circuit
breaker as you switch on the device. Opposite in action are
the PTC (Positive Temperature Coefficient resistor). These
are used in temperature control or measuring devices. The
resistance gets higher as the temperature increases.

Coiler greets from Germany,