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 below;
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.
Regards, Mark
240VAC(in) o o | | |L N| | | | | | | )| o | RY )| \\ | _)| o | | ___ || | |-|___|--||-| | || | | LCR | .-. | | |MOV | | | | '-' | | | o o
240VAC(out) (created by AACircuit v1.28.5 beta 02/06/05
If the relay is just replacement of manual switch for equipment designed for line supply then don't improve on it. If the load is sensitive then add whatever is needed.
Right.
What is "normally"? On DC the diode allows (shorts) the magnetic field for faster collapse and faster opening of the contacts to eliminate "welder" spark between them. On AC the spark can at max be 10msec.(50Hz) till next zero voltage crossing of the line.
Too modern for me. What "driver"? ~1960 AD working on telephone exchanges, relay technology, diodes(?), were called rectifiers then, selenium plates, and were used (rarely!) to improve responses by allowing faster collapse of the magnetic field. Anything changed since?
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.
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 electrical noise.
** General term for whatever solid state device switches the coil current on and off.
** The laws of magnetics must have been suspended in that clunky old exchange.
Solid state transistor drivers are/can be destroyed by the voltage spike caused by the collapsing magnetic field. Back in the selenium rectifier era, they used toobs which were unaffected by the spikes.
The diode acts to short the reverse emf developed by the coil and this supports the magnetic field for a short period of time, slowing the action of the contacts opening. - similar to a the shading pole they use on AC relays to prevent them from chattering.
Slower collapse - the diode causes a current to flow in the coil, that current produces a magnetic field.
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That should work if both the capacitor and MOV is rated for 240 volts AC operation. If the capacitor is not AC rated, it should have something like a 1000 volt DC rating (or at least as high as the MOV clamping voltage specification).
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