How to calculate a VDR resistor

[Sharky]

I have made a small control panel using a 12F629.
I'm controlling a relay that has to switch a pump or a solenoid valve.

Since these inductive loads can couse peaks when switched I would like to protect my construction with a VDR-resistor.

I think that the VDR must be placed between Live and neutral on the mains-supply.

The mains supply it 230V AC so I suppose that a 250V VDR is what I should select, but how do I calculate if it shold be a 0.1W or 1W or?

Kind regards
Gert

[Melanie]

250v is too low for the UK but OK for Europe...

275v 0.25W (20 J) is more than adequate... and remember it's fitted on the Circuit side of the input Fuse.

[Sharky]

Waaauu, that's what I call a quick reply.

Okay, I will use a 275V VDR then, because it's for use in all 230V AC countries.

Is there any rule of thumb that I can use, to learn how you get to the 0.25W?

Thanks Melanie, I really appreciate your answers in these forums and have learned a lot from them.

[Melanie]

Difficult answer to a simple question. How noisy are your solenoids and motors? Not easy to determine. As a rule of thumb, I generally allow 5J suppression per kW. I specify 20J devices as a starting value on the 230/240v supply mains, because at only a few cents l find they're actually cheaper than the lower values. Bizzare economics!

[Darrel Taylor]

Don't you need some Verne's to go with those Joules,

Just thought I'd add that if you use a Solid State Relay that uses a Zero-Crossing detector, it makes those spikes from inductive loads almost imperseptable. Making MOV's etc unnecessary, or at least smaller.

[Luciano]

If you use a Solid State Relay that uses a Zero-Crossing detector,
it makes those spikes from inductive loads almost imperseptable.

The Zero-Crossing detector in the Solid State Relay helps when
the circuit with inductive loads is switched on. When the same
circuit is switched off, you will need transient suppressors.

If you use a SSR equipped with internal transient protection,
there is no need for adding additional MOV's or other transient
suppressors. (See SSR datasheet).

You will need additional MOV's or other transient suppressors if
you use a SSR which is not equipped with internal transient protection.
(See SSR datasheet).

Luciano