Choosing a relay - Page 1
Forum section: electronics / beginners

author: Mr1234kaki
author_rank: Contributor
timestamp: August 15, 2012, 12:15:27 am
content: Hi!Im building my own temperature monitor, i do not know how to choose a relay to be connected to a 1000wh to act just like a switch, not necessarily to run the heater itself, any suggestion?

author: Unknown
author_rank: Unknown
timestamp: August 15, 2012, 02:10:53 am
content: Quote from: Mr1234kaki on August 15, 2012, 12:15:27 amSure! I suggest you provide more detail!For a start:Is the load resistive or inductive? What is the load?What are you intending to use to drive the relay coil?What is the operating voltage and current of the load?The more you describe your problem, the more likely it will be for others to help with a solution.

author: Mr1234kaki
author_rank: Contributor
timestamp: August 15, 2012, 04:17:09 am
content: well im trying to use the relay to switch on a fan heater rating 2000W for 220V-240V operating voltage, im not sure the current draw..the relay coil probably will be drive by an arduino circuit, thus the output voltage and current is not much, around 5V and 80mA,is it possible?Thank you very much for your help sir, cheers

author: Unknown
author_rank: Unknown
timestamp: August 15, 2012, 04:36:51 am
content: Quote from: Mr1234kaki on August 15, 2012, 04:17:09 amYes it is possible. A fan heater will present a mostly resistive load. And given it is a plug in mains power device you would be wise to use a relay with contacts rated for at least 10A 240V continuous operation. Good design at this level would be to select one rate 16A or above. Arduino outputs wont drive larger relays without assistance so you could use a transistor driver or a solid state relay which your Arduino could drive directly.NEXT and IMPORTANTLY. you will be switching mains wiring which needs to be done safely so you don't upset the budgie with the smell of burning flesh. The relay needs to be mains rated as does it's enclosure and any wiring within that enclosure. Exposed metal needs to be securely earthed. In shortSeek assistance with mains wiring until proficient!. If you go the solid state relay route you will also need a heat-sink unless your relay is substantially over-sized.

author: SeanB
author_rank: Super Contributor
timestamp: August 15, 2012, 04:49:34 am
content: Not possible on 5V unless you use a solid state relay. Those will operate off 5V and then you need a unit rated at 250VAC and able to switch 10A minimum, preferably 20A. Choose one with a zero cross switch inside to reduce noise. They will work off 5V and draw typically 30mA when on.Otherwise you need to have a 12 or 24V rail available to drive a standard 20A mains rated relay, which needs a 12/24V drive ( depending on the coil selected) at a current of around 300mA to switch. This will need an external transistor to drive it.The opto relay has the disadvantage of some leakage current when off, but this is not likely to be a problem with a heater load, provided you do not connect the load to the relay via a plug and socket.

author: Unknown
author_rank: Unknown
timestamp: August 15, 2012, 04:56:28 am
content: Quote from: SeanB on August 15, 2012, 04:49:34 amHuh?

author: SeanB
author_rank: Super Contributor
timestamp: August 15, 2012, 05:08:38 am
content: It is a triac, and will show mains across the output if the mains is applied and there is no load. Normally the less than 1mA draw is otherwise not noticed.

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: August 15, 2012, 05:09:41 am
content: Quote from: SeanB on August 15, 2012, 04:49:34 amThe leakage current is usually less that 0.25mA so nothing I would worry about. But what does a plug/socket have to do with it?

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: August 15, 2012, 05:11:07 am
content: BTW, this is a good choice for a solid state relay:http://www.omega.com/pptst/SSRL240_660.html

author: Unknown
author_rank: Unknown
timestamp: August 15, 2012, 05:23:41 am
content: Quote from: SeanB on August 15, 2012, 05:08:38 amA standard GPO has KA of fault current at mains voltage behind it and a plug/socket combo pose negligible risk even at full load?

author: Mr1234kaki
author_rank: Contributor
timestamp: August 15, 2012, 03:13:12 pm
content: how about mechanical contact relay? is it gonna work?

author: SeanB
author_rank: Super Contributor
timestamp: August 15, 2012, 03:26:47 pm
content: Mechanical relay will work, and provided you use a double pole unit provides total load isolation when off.The tangent is on the opto units, which have a slight leakage current when off. This is only a problem if you have a very small load, like a neon indicator, and the heater is disconnected whereupon the neon will light even if the unit is switched off by the relay. Not much to worry about, and only if you have a remote plug and socket between the relay and the heater, which means there will be a hazard if the plug is disconnected and you are working on the socket, or plug something conductive in it, thinking it is off and getting a shock. If the relay is in the heater the opto unit is better, nothing to wear out like contacts, and no switching noise if you use a zero crossing unit.

author: Mr1234kaki
author_rank: Contributor
timestamp: August 15, 2012, 04:19:54 pm
content: so much information i get here, great! thanks for helping, really grateful.. cheers

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: August 15, 2012, 05:36:03 pm
content: A mechanical relay will certainly work, but you cant drive one directly from your Arduino. You'll need to add driver circuitry, and as mentioned before, need a decent supply of 12 or 24 VDC.That's the up-side of the solid state relay. You CAN drive that directly from your Arduino.

author: Unknown
author_rank: Unknown
timestamp: August 15, 2012, 11:44:20 pm
content: Quote from: SeanB on August 15, 2012, 03:26:47 pmWouldn't the correct advice be to remove all connections to mains wiring before service? It wouldn't be the first time a mechanical contact has welded closed, particularly one called upon to switch a large current many times in succession. With most SSR you are more likely to experience issues with insufficient load than with leakage current. But given the OP wanted to switch a substantial resistive load there was little reason to confuse the issue.Yes a mechanical relay can be driven but the circuitry to go with an Arduino will be much more complicated to do so. The input of many SSRs can be driven directly from the tiny Arduino signal. As mechanical relays go at 2KW your really require a contactor and you wont be driving that directly from an Arduino.Yes anything is possible but given we are in the beginners section, we'd be best to concentrate on simplicity and SAFETY with any suggestions offered.