Safe, ready-built low voltage controlled mains socket - Page 1
Forum section: electronics / projects

author: cybergibbons
author_rank: Frequent Contributor
timestamp: November 25, 2011, 09:23:41 am
content: Morning all,I'm working on a project to allow me to control a plug-in 3kW electric heater using an external thermostat. The heater already has a thermostat built-in, but due to the proximity to the heating element itself, it just regulates the temperature of the heater rather than air around it.There areplug-in thermostatsavailable, but they don't work very well. The three big ones are:Heat is generated in the device itself, warming it above room temperature. This is both by the electronics themselves and that drawing 3kW on a 13A socket normally causes warming.The socket is still in close proximity to the heater and at socket height, so isn't representative of the room air temperature.Even though rated for 3kW, they are switched by relays with poor arc suppression and seem to burn out after 6-12m of operationSo what I want to do is control the heater using a remote (preferably wireless) thermostat placed in another area of the room. I'm planning on using aJeenodeandDS18B20to sense the temperature. Please don't Arduino bash - a small, ready made board with RF for €18.50 is great value and does what I need.Why am I doing this? Our new arrival seems to sleep significantly between with the room held at a constant 16-18degC overnight. Our hot-water central heating system heats the entire house, so would be expensive to run overnight. So we used an electric heater to keep the room at a more reasonable temperature. It's rented property so I can't do much else.But I need something to control the heater.Normally I would rig something up using a SSR or triac. But, because this is a baby's room, I want it to be as safe as possible and would prefer to use a ready-made solution that meets all appropriate standards. My current thinking is to use a remote socket and control it using a 433Mhz OOK transmitter on the Jeenode. I've been looking at these remote sockets - not many will switch 3kW, and the ones that will seem to use relays in a similar manner to the socket thermostats so will probably burn out. Also, there is no confirmation of operation. If I send OFF and it doesn't go off, the room could overheat (though I'd set the heater's own thermostat as a backup)The other alternative is to use another Jeenode as a receiver and connect it directly to something switching the mains. Something like thePowerSwitchTail. However, there is nothing like that I can find in the UK.So, are there any ready-made, ideally solid-state, cased products to switch 3kW of mains?

author: cybergibbons
author_rank: Frequent Contributor
timestamp: November 25, 2011, 01:05:46 pm
content: I totally missed this on thePowerSwitchTailsite...Looks better, even if not solid state.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 25, 2011, 02:43:19 pm
content: What about a heavy duty dimmer? Some designed for use in theaters can be controlled by a 0-10V signal and would have no problems switching 3kW of resistive load.

author: cybergibbons
author_rank: Frequent Contributor
timestamp: November 25, 2011, 06:47:27 pm
content: Good idea - only problem is anything affordable (<£100) and single channel seems to be limited to 2kW. PowerSwitchTail is looking like a good bet.

author: ivan747
author_rank: Super Contributor
timestamp: November 25, 2011, 07:02:01 pm
content: Have you considered using some X10 solution? It's a PLC system for home usage. Just a suggestion; I don't know if there is a module capable of switching 3kW but I think it meets your safety requirements.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 25, 2011, 08:03:42 pm
content: You might be able to find a used dimmer for much cheaper. Especially with the performing arts industry moving to digital control. Or what about use several smaller heaters (or reflector incandescent bulbs run well below rated voltage) with individual dimmers?