AC relay using light bulb to switch on/off ! will it work >

[paulwalldall]

Hello. first post. hope in right section.
Farmer friend has a hen brooding pen. (large enclosed box with 240watt heat lamp) with one lamp. when lamp fails the chicks get cold and soon die which is sad but also costly in birds. thought there must be a failsafe device somewhere to turn on an alternate bulb but he says not.
Thought that wiring his one bulb in series with the coil of a 240v 16amp relay would work and when bulb fails, relay de-energised and goes to normally closed and switches a second reserve bulb on would do the trick. wired this up , switching reserve bulb on/off by removing and replacing first bulb works fine but first bulb will not light at all. anybody know why. current must be flowing through first bulb to energise relay but bulb wont light. tested all components individually and all ok. only thought is the coil has a resistance of 7kila ohms and the bulb only 260 ohms. measured over each bulb and coil and all read 240v when they should. am i being a bit rubbish or am i missing something fundamental. this was just a mock up at home so no dead birds yet but has anybody any ideas. or suggestions to achieve the ' one light fails and another comes on goal'

thanks Paul.

 

[paulwalldall]

thanks for the replies. dont know anything about 'current sensing relays' and price searches make them a little dearer than normal ones, though it seems they draw little current when active so seems a good solution. as for making sure light comes on first before relay, does that actually work ? cant get a hen with a big enough bottom as hes looking at getting 150 day olds soon. stick with 2 lights in parallel for now but will be looking at a photocell or bimetal switch as a cheprer alternative if i can get them switching on/off reliably.

 

[D Skelton]

as for making sure light comes on first before relay, does that actually work ?

Unfortunately not, I was trying to be funny, seems like it failed miserably leccy doesn't work that way I'm afraid.

IMHO, the best option here is to stick with two lights although I would make sure they are supplied from seperate RCBO's to ensure one light going down can't cause the circuit supplying the other one to fail.

Alternatively you could use Tonys suggestion of a current sensing relay, however I can't see the point really. The simpler you can make the solution to a problem, the less there is to go wrong.