Heavy duty relay control - Page 1
Forum section: electronics / beginners

author: MadsMP
author_rank: Newbie
timestamp: June 05, 2015, 12:12:10 pm
content: HiI have started making a small system so that I can turn on some electric radiators in my summer house.I have bought a device with four relays that are controlled via SMS. Since the relays can only cope with 12v, I need to use a heavy duty relay that can be controlled with the little ones.I have found many relays, but I can not decipher the diagrams that come with them. So I can not see if they can what I need.Would you be able to send me in the right direction, with respect to the type of relay and how it should be put together. Perhaps a tutorial video would be helpful.I live in Denmark where we have 230V outlet.Thanks in advance.Mads Pedersen

author: PSR B1257
author_rank: Frequent Contributor
timestamp: June 05, 2015, 03:24:45 pm
content: QuoteHmm...electric radiator with remote control...Anyway, therefor a contactor is more suited than a relay. Since a radiator is a pure ohmic load driven with AC voltage, you have only make sure the nominal switching current of the contactor is greater than the nominal current of the radiator. Then you take two main contacts of the contractor to disconnect the radiator from the grid.

author: Pillager
author_rank: Unknown
timestamp: June 05, 2015, 07:02:35 pm
content: 12 Volts seems a bit low for the contacts, maybe it's the coil voltage?I'm asking, because contactors (which would be your best option) which have 12V coils are rare.Maybe you could link in the datasheet to your relay-board, that would help.

author: tautech
author_rank: Super Contributor
timestamp: June 05, 2015, 07:40:00 pm
content: Cascading relays, 2 or 3, each with a higher coil voltage and the last with sufficent contact rating to switch your load.Yes you might find a 12V coil contactor, it will be the best solution but expensive.Have a look for Din rail relays or contactors.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: June 06, 2015, 05:30:48 am
content: Cascading relays, 2 or 3, each with a higher coil voltageAre you seriousQuoteThat's what I think also. I'm not aware of any relay which is limited to switch only 12V (would be a rather useless relay card in this case....)

author: Ian.M
author_rank: Super Contributor
timestamp: June 06, 2015, 08:19:53 am
content: OTOH relay cards with inadequate clearances for mains voltages are horribly common - to the point where it is difficult to find one with proper layout.If this is going to be permanently installed (wired in place), DIN rail mounted contactors are the way to go. They wont be cheap but they are available with a 12V AC coil. You can also get a 12V AC DIN rail bell transformer to power the contacters and even DIN rail mount PSUs in a variety of voltages to power the SMS relay board. You'll probably need some way of manually overriding the SMS control board which if it doesn't already have an override feature could be some SPDT-CO rocker switches wired to bypass or disconnect the relay contacts on the SMS board. If mounted in an appropriate DIN rail enclosure, with a main isolating switch and each load protected by an appropriate circuit breaker, you shouldn't have any problems getting the installation approved by an electrical inspector.If its intended to be plug-in, it gets a lot trickier because a Schuko socket is only good for an absolute maximum of 16A and connecting more than 3KW of heaters to one is unwise. Also running extension cables from your controller to your heaters is undesirable. Your best bet would be commercially available remote control plug in appliance modules, and either hack the handheld controller to interface to your SMS board (which may need an extra circuit to get momentary contacts you can wire across the ON and OFF buttons that pulse when the relay changes state), or simply get a SMS controller for the same signalling system as the appliance modules, which may have the benefit that you can interrogate it remotely to check the status.In all cases, the applience module or contactor *MUST* be rated for the full load current with an adequate safety margin and it would be extremely unwise to use the system with anything except thermostactically controlled convector heaters (e.g. storage heaters or oil filled electric radiators). Specifically, *DO* *NOT* use it with radiant or fan assisted heaters. (Unless you want to meet the fire truck coming back from your summer house when you are going to it!)

author: cs.dk
author_rank: Unknown
timestamp: June 06, 2015, 08:22:41 am
content: If your relay output from the controller really are 12V DC, there are a few options - Something like this.http://dk.rs-online.com/web/p/kontaktorer/8453301/(Sorry, Danish page)It's a 3-phase contactor, they are rare as 1 phase with 12V coils.

author: Pillager
author_rank: Unknown
timestamp: June 06, 2015, 09:37:46 am
content: Just a side note on contactors: most of them have coils for 24VDC, which is the industry standard voltage. There are also some with 230VAC, but then you would need the relay card to be properly designed.And, as Ian wrote, please no schuko plugs on the supply side, unless you use one of the red 3-phase CEE-plugs. With 3 phases you can also distribute your load nicely.Here for 16 ampshttp://be.farnell.com/walther/210/plug-free-400v-16a-3p-n-e-way/dp/843520and for 32 ampshttp://be.farnell.com/walther/230/plug-free-400v-32a-3p-n-e-way/dp/7161785

author: Ian.M
author_rank: Super Contributor
timestamp: June 06, 2015, 09:52:25 am
content: Yes. Its worth taking a careful look at the SMS relay board. If it uses real mechanical relays, unless the relays don't have an AC rating at all, or the clearances are under a milimeter it should be good for 24V AC which will make the contactors much easier and cheaper to source.If it uses solid state 'relays' or transistor outputs it may actually have a real 12V limit and be DC only, which will make your life much harder.

author: Pillager
author_rank: Unknown
timestamp: June 06, 2015, 11:54:40 am
content: Or, you might use a solid state relay, like this:http://www.digikey.at/product-detail/de/G3NA-210B-DC5-24/Z918-ND/206389Then you wouldn't really need your relay board...

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: June 06, 2015, 02:22:38 pm
content: QuoteI'm certain he is. Remember the OP actually suggested cascading two in his initial post.Three is probably a bit much though.

author: Bloch
author_rank: Unknown
timestamp: June 06, 2015, 03:10:13 pm
content: That are ratings for you radiators ?If they are 2300watt or under I will suggest this relay per radiatorhttp://dk.rs-online.com/web/p/relaeer/3415130/+http://dk.rs-online.com/web/p/relaefatninger/3416874

author: MadsMP
author_rank: Newbie
timestamp: June 07, 2015, 07:54:45 pm
content: WAV Thanks for all your answers, it's great to readI could not find anything schematic of the box. Unfortunately.I will start and read up on the relays you have sent.Again, thank you./MadsPS. Sorry for the late reply, I was on weekend without a computer