Mains Relay

[Bill]

Son's house is wired so he can, via his iPhone and home wifi, control his central heating etc. etc. from the train. Not sure what system, but it has a controller for the heating and a separate controller for the immersion. The heating is OK, but he didn't read the spec and the immersion took so much current that the controller died. He has now, with some drawings from me, wired in a 30A rated relay with

230v ac coil. I bought him a box to mount it in, but it apparently isn't decorative enough for his walk-in airing cupboard, so as a very temporary measure he has wired it all up al fresco via the old immersion isolator switch. He reports that it all works well except for the buzzing from the relay coil, which is bound to get louder when it's mounted in a box on the wall.

I have said I'll ask around to see if anyone knows whether all this type of relay is noisy, or whether anyone knows how to select a silent type.

[Theo]

A triac would be a silent switch:

(heatsink may be required)

Use the relay to switch the gate of the triac, which will take basically no current so less likely to rattle. You can also use a small optocoupled-triac to drive the gate of the big triac for a fully solid-state solution.

Theo

[tabbypurr]

Buzzing is not sue to current flowing in the switch, it's down to relay design. Mounting it on bits of rubber would help - its box can also be rubber washer mounted.

NT

[Tim Watts]

I decent relay should not be obviously noisy. Try an Omron brand from RS/Farnell.

His immersion needs a relay capable of handling 13A resistive load AC at mains voltage with a 230-250VAC coil.

[Brian Gaff]

Mains relays are noisy, but does it have to be a mains relay? Could he just mot add a stage to his controller that could sink more current into a dc one fed from a another supply. Sounds like he is using the weedy switched supply in the controller to just switch a mains relay. That is always going to buzz.

Brian

[Brian Gaff]

Ofr if you don't want that just rectify the mains and have a capacitor across it and a different relay. There are lots of ways to do this in a less clunky way. Brian

[Dave Plowman (News)]

AC coil relays are more likely to make noise than DC types for obvious reasons. But not all do. Where did you buy it? I'd expect one from RS Components etc to be good.

[Dave Liquorice]

The Omron mains coil relays (MY21) I have are silent, well apart from the click as they operate/release. B-)

Not sure that they are rated at 13A resistive though nope 10A.

[Andrew Gabriel]

This really needs a contactor rather than a plain relay, and contactors can hum (so it might be that's what you already have). As someone suggested, mounting it on some sort of soft rubber mounts (and using loose flex wiring to the contactor) will help prevent coupling to vibration to a sound board. Trying a different mounting orientation might also reduce noise.

[Andrew Gabriel]

Feeding an AC coil with DC will mean current is limited by the resistance alone, and not by the coil impedance. This means the coil will run hotter, hot enough to burn out in some cases. (With small mains relays, the resistance is a significant portion of the impedance, but resistance becomes less significant as the relay coil size increases.)

The capacitor will also raise the RMS voltage from 240V up to

340V, and exactly double the coil power dissipation over the 240VDC case (which will already be higher than designed).

[newshound]

Solid state relay, even easier?

[Dave Plowman (News)]

Had a quick look at RS and they list several suitable - this one is about a fiver:-

[newshound]

Standard 40 Amp solid state relay, available with low voltage or mains switching. Been using them for years to control immersion heaters at my mate's brewery. May need a small heatsink, depending on how and where they are mounted. RS/Farnell for conservatives, eBay for cheapskates like me.

[tabbypurr]

The immersion will draw much more than 13A when the element is cold.

NT

[Adrian Brentnall]

Do you find that the ebay ones are reliable? There's a heck of a price difference between the ones from HongKong and the ones from RS - £3 vs £40 (need 40A (probably) to switch 7kw load @ 240v.

I'm currently using a mains-driven contactor, that comes on with a satisfying 'clunk' (and, sometimes, a whine/whistle) - but, if it (or the ssr) were to fail 'closed' then that might get interesting rather quickly... as it's feeding a resistive heater in a medium-sized glass kiln.

What's the failure mode of the ssr's - in your experience? Thanks Adrian

[tony sayer]

In article , Dave Plowman (News) scribeth thus

Got some in use here Dave, all quiet:)..

[Dave Plowman (News)]

Got one here that came from Maplin many many years ago. Open frame type. Used to switch the power to my AV setup via the switch on the main amp. Only 10 amps, but more than enough. You can hear it make, but no buzz etc after that. Must be getting on for 40 years old. ;-)

[newshound]

I used RS originally and, I think, had one fail after a couple of years. Did a rebuild with eBay ones (not necessarily the cheapest), one channel is currently down (there is redundancy) but I havn't confirmed if it is the relay or some other fault.

I hadn't worried about protection. The whole system is powered dowm before draining. Overheating caused by one failed channel would be picked up by the brewer's manual check on water temperature. (This is a hot liquor tank, for anyone interested). I suspect a single heater (3 kW) would not be able to get the tank to boiling point, but if it did there is about eight feet of water to boil off before exposing elements!

[Theo]

As usual, they're a bit optimistic of the spec and the construction. I don't think I'd want to put 30A through one. Big Clive's teardown:

Theo

[Andrew Gabriel]

Nichrome resistance doesn't change anything like that of tungsten, and in any case, the temperature change is only a small fraction of that of a tungsten filament.