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I was planning to install this Metal-Oxide Varistor (MOV) across a relay switch. But after I read about the use of MOVs for switches here, it seems to me that MOVs are useful only if the load is inductive.

I will turn on/off an SMPS with this 5RL-1A-E-HR-5DC DC input high inrush relay as shown in the diagram below:

schematic

simulate this circuit – Schematic created using CircuitLab

I know that the SMPS is a highly capacitive load with 45A inrush current. Does that mean the relay does not require a MOV across A and B?

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The other answers here are giving mixed answers, because an SMPS has a complex input circuit.

During startup, there is an inrush transient. This is capacitive in the sense that it's due to the main energy storage / filter capacitor, but it will stretch over several cycles (while the inrush limiter device -- usually a NTC resistor -- is active). So, on a cycle-to-cycle basis, the waveform won't have much phase shift during this event, and it's more that the envelope of current flow is initially large, then decreasing.

Inrush also takes place somewhat later than the instant of turn-on. In the first nano to microseconds after contact closure, current flows into the mains wiring: this can occur very quickly, as the nearby wire segments get discharged from anything up to mains peak voltage, down to ~zero difference across the contacts. This occurs very quickly, so expect transient emissions up to about this many volts, and with fast risetime (similar to any other EFT transient). The current associated with this event corresponds to transmission line impedance: in the ballpark of 50 or 100 ohms. So, it's low energy, but certainly enough you can pick it up on a 'scope.

Over a somewhat longer time scale, current flows into the SMPS input filter elements, usually a CLC ("pi") filter. How much, depends on line impedance or inductance, and filter component values. For a typical supply with say 0.47µF at the input, and for a mains network similar to a LISN (50R || 50uH), this will give some ringing around 30kHz, decaying over a handful of cycles. There's also a common-mode choke which has some leakage inductance, and another capacitor behind that (and usually another one after the rectifier too), which will put some finer ringing on top the first waveform.

Conversely, at turn-off, when the contact opens while current is flowing (particularly near peak), the energy stored in all these inductances, small though it is -- gets discharged across the contacts. Typically a small spark develops, generating some hundreds of volts with a risetime in the nanosecond domain. This can spark multiple times: the voltage rings down after the transient, giving time for the contacts to open a little further, until the voltage rebounds to breakdown (at the now slightly higher voltage due to the contacts having opened further), and so on until the energy has dissipated. This describes the phenomenon of EFT (electrical fast transients), but mostly requires much higher inductances to occur -- coils and motors in the 10s of mH. So there should just be one or two sparks in this case.

In summary: the phase angle depends on frequency and operating condition. Operating condition ranges from inrush, to normal operation, to turn-off (with operation being a range, since the rectifier and PFC stage impedances vary with current). Frequency is driven by the LC components at the front end of the supply, as well as the wiring up to it.

For a relay on an SMPS, inside a machine, especially if the relay is supplied from an inlet mains filter -- it's only a concern if the few hundred volts peak of emission may upset other things inside the machine. Which, mind -- things should be well enough wired that this doesn't happen -- but if your wiring/layout isn't great, well, that's not great to begin with, but it takes two to malfunction, and you might manage to solve the problem by quieting the contacts instead of hardening the other one.

Without an inlet filter, just switching plain old mains, at whatever wiring length it may have -- not likely to be much worse. Again, there can always be something, but it's unlikely to upset anything meeting basic levels.

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I know that the SMPS is a hihgly capacitive load

No, it's not. Your SMPS is capacitive only for a very limited time. It's a high PF device i.e. there's an active PFC circuit inside:

schematic

simulate this circuit – Schematic created using CircuitLab

Before the active PFC block starts, the grid will see the output cap through bridge, L1 and D1. This is where the inrush current comes from. But after a few milliseconds (or even less) the grid will see the PSU as a resistor, not a capacitive load.


MOVs are required to dissipate the energy caused by a surge. For inductive loads, due to the nature of an inductor, the current cannot be stopped immediately (e.g. by only opening the contacts of a relay), and this will create a higher voltage which is called back EMF. Capacitive loads do not have this behaviour i.e. they don't generate a surge, so you don't have to place an MOV across the relay contacts.

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Does that mean the relay does not require a MOV across A and B?

A capacitive load will not create any significant back-emf when the relay contact opens. Therefore, providing the relay contact is rated at the correct AC voltage and inrush current it won't require a snubber circuit (MOV or otherwise).

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The Meanwell product is well designed and the MOV to protect the relay contacts is not required.

Inrush Current (typ.) COLD START 45A/230VAC

It doesn't say maximum. Typical is the mean of all surges due to the random phase of Vac(t). A Zero-crossing triggered Triac might be better rated for >= 20A continuous.

Inrush is caused by DC resistance of Cap ESR and inductance DCR and peak AC voltage on closure being a broad spectrum step function.

If you plan on switching power daily, I would use a mechanical switch or just an AC cordset and use the remote ON/OFF control instead. This will reduce the inrush stressed on the input caps and eliminate the need for this relay. MTBF accelerates downward on caps and relay contacts with this level of surge currents repeated daily. It might work but maybe not 10 yrs. More verification is required on assumptions.

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