I have an AC circuit which has suffered from a welded relay contact and want to use an MOV to protect it.

I have two 240VAC inputs (redundancy) feeding a 50VA toroidal transformer via a (two contact) relay. When AC1 is present the relay is energised and AC1 feeds the toroid. When AC1 fails the relay de-energises and AC2 then feeds the toroid. One relay contact switchs Live (hot) and the other Neutral. The secondary circuit is 24VAC. However during this transistion one of the relay contacts has welded and I suspect this is caused by arcing (high voltage). I'd like therefore to put an MOV in parallel with the toroid and have got this far:

Working voltage: 240V + 10%= 234V
Peak current: 2.08A (50VA/24VAC)

But I do not know how to work out the transient energy, power dissipation or clamping voltage.

Help!!

 

Hello, first you have a calculation error, 240V +10% = 264V (not 234V).

I have never used a MOV for this type of transient, they are very good at clamping high voltage transients, for example, transients from induced voltages caused by a near lighting strike or motor back-emf. Even then the MOV must be protected by a fuse and I would normally be interested in protecting the relay coil with a MOV.

Are the relay contacts specified to handle the peak worse case mains voltage ( 264V x \(\sqrt{2}\) ) and the max load current?

Here is a link that discusses Relay Contact Life it may help with your choice of relay.

 

It sounds like your torroid wanted to keep the current flowing when you broke the circuit, probably when the current was at its peak. When you neglected to provide a discharge path for the inductor, it made a path (across the relay contacts.) The simplest (< $ 2.00) solution for this problem is probably a RC snubber across the contacts. There are commercially made devices called quencharcs made specifically for this problem. Suggest 100/0.1mf, 600 VAC unit (Newark). An MOV across the torroid too will help to keep HV spikes in check.

Cheers, DPW [ Spent years making heaters out of op-amps.]

 

Thanks guys but I want to use an MOV...how do I calculate the transient energy?

I do have a line fuse.

 

This paper on transient suppression by ON semiconductor is quite informative:

http://www.industrologic.com/transient.pdf

The GE booklet cited at the end of the paper is also good but I don't know where you will find it. Mine is away in storage.

awright