Lamp and relay coil in series

smartalek · Mar 14, 2011

I have a 100 w heat lamp (basically a normal filament lamp) on a stat and timer and need to make a locking fault indicator in case the filament burns out. I've drawn a design I'm sure will work apart from it involves putting the lamp in series with the coil of a 230 v relay. I'm just not sure if the coil will still pull in with the volt drop.what do yous reckon?

Dayo · Mar 14, 2011

no i dont think it will i will have a think / look around

Dayo · Mar 14, 2011

i had a quick look but couldnt find anything, i remember i did what you are trying and the relay just chattered and wouldnt stay in, i think the resistance of the lamp must be equal to the resistance of the relay. not sure about the volt drop accross the two tho. my mind thinks they should both be the same voltage and resistance i.e. both 115 volt as its a 230 volt supply

smartalek · Mar 14, 2011

Yeah thanks for looking, I did some research myself but was struggling to find anything useful, what you say makes sense tho, with a 230 supply using 110 lamp and coil should work. don't suppose I'll get them off the shelf in england tho.doesn't america use 110 mains? Might have to order off the internet. that or go back to the drawing board

Dayo · Mar 15, 2011

havent looked but what about a triac? well that sort of component it has three legs lets say top bottom & side. the bulb is connected and the neutral connection goes to the side leg, this goes down on to the bottom leg which is connected to the neutral connection of the supply, the bulb lights. this voltage coming into the side of the triac? makes a path between the top leg and the bottom leg to neutral this path can switch on a relay, if the bulb is turned off or blows the flow between the side to bottom legs is gone and the path between top and bottom legs opens and relay drops out.

i`m not sure if the component is a triac i will have a look in the maplin cat. there is a component that works on this principle i`m sure, it may be a power transistor?

Knobhead · Mar 15, 2011

You can’t put a standard relay in series with the lamp, the resistance of the coil will be to great to allow the lamp to light. You need a current operated relay which has a low resistance coil operated by the current flow to the lamp. With a 100W lamp a ½A relay should do.
Have a look on Google

smartalek · Mar 15, 2011

I'll look into both of these, and perhaps a trip into maplins or rs will be of help, the lads usually know their stuff in there. Thanks for the help.

ian.settle1 · Mar 15, 2011

smartalek said:
I have a 100 w heat lamp (basically a normal filament lamp) on a stat and timer and need to make a locking fault indicator in case the filament burns out. I've drawn a design I'm sure will work apart from it involves putting the lamp in series with the coil of a 230 v relay. I'm just not sure if the coil will still pull in with the volt drop.what do yous reckon?

It can be done as a similar system is used on ships navigation lights so if the lamp burns out the relay will drop out and sound a audible alarm so the nav lights can be switched over to the backup light.

Try googling navigation lights and see what you come up with.

smartalek · Mar 15, 2011

The diagrams I've found for ships navigation lights all have relay coils in series with the lights with the contacts either switching to emergency power or sounding a buzzer and are pretty much the same as what I've already come up with, looks like its just a case of choosing the right combination of components. Cheers all.