To protect AC contacts, it is common to add an RC across wither the
load or the contact. You get slightly better protection if it is
across the contact, but more off state leakage current.
I'm using a relay with 240VAC coil and 240VAC contacts. From what I've
been advised before with relays (not 240VAC), I understand it's a good
idea to protect the contacts with a capacitor and possibly a MOV like
Is this required for AC and is it a good idea to have a MOV?
Also, I assume I need do nothing on the coil side, seeing as it's AC?
Normally, I'd expect a reverse biased diode in there somewhere.
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The resistor should limit the peak instantaneous current at contact
closure to something like half of the contacts current rating. The
size of the capacitor depends on the inductive energy the load will
dump when its current is interrupted by the contact opening. If you
under size it, it might be damaged by over voltage, unless it and the
resistor are paralleled by an MOV, as a voltage clamp. However, if
the MOV fails shorted by line over voltage (and the network is across
the load), it will probably destroy the contacts. However, if the
network is across the contacts, not only is it much harder to blow the
MOV (because the load impedance limits peak current during a line
voltage spike) but failure shorted just turns the load on.
You cannot put an MOV in series with the load, as you have shown,
because it has a high resistance, except during over voltage
situations. So you would get very little voltage across the load.
I have often used an MOV across AC relay coils, to limit the rate of
rise of voltage (by virtue of the MOV's inherent capacitance) and a
voltage limit by virtue of its voltage dependent resistance. This
will make contacts driving the coil last longer and make less