Thanks Dave and Elvee. There is more to it than meets the eye... I can see that now. Its something I had never given much thought to before tbh.
A spirally wound heating element is likely to have an inductance in the 10's or 100's of uH range. As you can see, this is likely to be much larger than the wiring inductance.
Switch on can be more onerous for the contact due to bounce. Especially for loads with an in-rush characteristic, such as an amplifier. I don't think a water heater load would have the same high start current as a valve heater.
So the contact current could be quite high, due to in-rush, at the time the contact recoils and tries to break the circuit. I couldn't find any great examples quickly, and the timing would depend on the relay or contactor, but I think a bounce would not be longer than a few ms, so multiple contact times could occur during an ac half waveform.
Even with special contact metallurgy, it is not uncommon for the molten splotches on the contact surface to finally cause a welded closed contact, or a high resistance contact surface.
Allowing an alternate current path around the contact during bounce can alleviate the surface damage - such as a snubber, or a slow start path.
I guess many people would shy away from zero crossing solid state switching, as such a switch also needs to be rated for the in-rush so can end up with a high current rating itself - but that is a somewhat more expensive option.
So the contact current could be quite high, due to in-rush, at the time the contact recoils and tries to break the circuit. I couldn't find any great examples quickly, and the timing would depend on the relay or contactor, but I think a bounce would not be longer than a few ms, so multiple contact times could occur during an ac half waveform.
Even with special contact metallurgy, it is not uncommon for the molten splotches on the contact surface to finally cause a welded closed contact, or a high resistance contact surface.
Allowing an alternate current path around the contact during bounce can alleviate the surface damage - such as a snubber, or a slow start path.
I guess many people would shy away from zero crossing solid state switching, as such a switch also needs to be rated for the in-rush so can end up with a high current rating itself - but that is a somewhat more expensive option.
Hi,
Try a Varistor across the switch to prevent the arc when the switch it is open. I used them in all of my amplifiers or use an SSR relay to take the load.
Try a Varistor across the switch to prevent the arc when the switch it is open. I used them in all of my amplifiers or use an SSR relay to take the load.
A varistor across the load or across Live to Neutral is OK.
I don't know what safety implications would ensure in bypassing the ON/OFF switch with a varistor.
I don't know what safety implications would ensure in bypassing the ON/OFF switch with a varistor.
Not only safety, but it has no value, as the arc voltage has to get quite high. Contacts were RC snubbered for a reason.
Hi,
You ca use it in parallel with the transformer if you want. You must size it depending of the working voltage. Normally for a 120 volt AC I used a GMOV with a rating of 150 volts AC. Also used them in every amplifier I built for lightning protection. Belief me it works. You must use a fuse before the GMOV so when it activated it blow the fuses.
You ca use it in parallel with the transformer if you want. You must size it depending of the working voltage. Normally for a 120 volt AC I used a GMOV with a rating of 150 volts AC. Also used them in every amplifier I built for lightning protection. Belief me it works. You must use a fuse before the GMOV so when it activated it blow the fuses.
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