Attach Relay to Freezer Warning Light

Discussion in 'The Projects Forum' started by Contax, May 2, 2008.

  1. Contax

    Thread Starter New Member

    May 2, 2008
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    When my freezer warning light comes on I want it to trigger a relay to open a pair of alarm contacts on wireless alarm, if I get a relay with 240v AC Coil can I wire it onto contacts at rear of warning light so when warning light comes on it will trigger relay & open volt free contacts to activate alarm, do I need any type of circuit on power to relay or just feed 240v AC striaght into coil terminals, I am hoping the warning light is 240v will have to get panel off & test it (Vestfrost Chest Freezer) bungalow 60ft from garage with freezer & I have a small wireless alarm to do job. I have used relays on 12v DC for split charging & on alarms to trigger flood lights with no extra circuits, it's only way I can think to do it, someone should make one to trigger contacts from a digital thermometer but can only find ones with small buzzers.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You will need to check the alarm circuit. It will probably be off one leg of the 240, so expect 120 VAC to the chassis. The awakward part is figuring how much current it can handle. A neon lamp runs in the milliamp region, and a relay will need more current than that to pull in. If you have some clip leads, try putting a night light across the neon indicator. The bulb runs about 7 1/2 watts, so it it light up, there is probably enough juice to run a relay. Do make connections with the power plug pulled, though.
     
  3. Contax

    Thread Starter New Member

    May 2, 2008
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    I know the circuit can run a small buzzer together with the warning light in parallel as it is an option on circuit diagram and I hope to be getting one from Vestfrost as it should have been fitted but they are now penny pinching as they have removed the digital thermometer but then sent me one to fit myself, I read a small relay uses about 1 to 2 watts for coil but I was worried if the extra drain could damage the thermostat. I am intending putting the relay in a small plastic project box behind the compressor vent for safety, I will have to remove panel and test the voltage, did not realise it could be 120v. Thanks for commenting.
    I will be carefull to make sure it is unplugged before I touch anything inside.
     
  4. Contax

    Thread Starter New Member

    May 2, 2008
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    I have checked circuits with a meter and get 242v on warning light connections. Warning light has negative linked from Power On Light and Fast Freeze Light, Positive is fed by a red lead from Signal Contact on Thermostat (Manual Type).
    Vestfrost shows a warning buzzer option tagged onto same circuit as Warning light so would a maximum of 2 watts for relay affect the circuit or thermostat?
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    282
    It won't bother the thermostat at all as long as the power limit is observed. The 2 watt limit means that any relay tied to the alarm circuit needs to be limited to about 8 ma current in the coil. That would be a 30,000 ohm coil which isn't reasonable. A circuit with a voltage comparator and a driver for an external alarm relay will probably be required.

    Want to pursue this?
     
  6. Contax

    Thread Starter New Member

    May 2, 2008
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    0
    I estimated 2 watts as relays I have looked at use between 0.75 watts and 1.5 watts (do these have a surge to pull on), I am looking for a small PCB with relay as it seems they use least current, I have ordered a spare thermostat to try useing that but the I still think a relay is best option as I only want to be warned red warning light has come on. A sensor like on the digital freezer thermometer/alarm (like I use on freezer in house) would be best but I am not into electronics and none of the manufactured ones are loud enough so the mini wireless pager/alarm is best option, the transmitter is only similar to a wireless door bell push/transmitter, runs off an A23 12v battery just like some car alarm remotes so can't be much current to switch so only need very low current relay, small 240v relays are harder to source most are 12v, it seems a minefield of technicalities.
     
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