A question regarding relays

Discussion in 'General Electronics Chat' started by Rickvan, Aug 28, 2014.

  1. Rickvan

    Thread Starter New Member

    Aug 28, 2014
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    Hi. I hope someone can help me with use of relays. I'm very much an amateur but I do understand the basics. My question is as follows.

    I want to use a relay to switch on a fan when a certain temperature is reached. I have a temperature controller with a probe and I know how to wire it up. What I want to do is to also connect a relay to the controller so that when the temperature is below a set point the relay is held in the NO position. When the temperature is exceeded the controller will cut off the power and the relay will drop to the NC position and that is when the fan will be switched on. So the fan is connected on its own circuit to the NC pole but the reply is held in the NO position for prolonged periods.

    Question...can a relay be used like this or would the coil be prone to overheating if expected to hold in the NO pole position for days at a time?

    Thanks for the help.

    PS....sorry if my terminology isn't quite right...hopefully I got the point across all the same.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Yes, you can do what you describe, but it is very inefficient. It is much easier to just get a relay that has a Form C set of contacts (3 terminals, No, C, Nc), and invert the logic by wiring the load to the Normally closed set of contacts.

    To invert the logic at the coil, you would have to get a relay that operates on half the normal supply voltage, then put a resistor in series with the coil, and then short out the coil with the controller output...
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    That depends on your intentions/the device..

    What the OP does while yes it uses energy it is actually a more "failsafe" way to do it and is required/insisted on for many "high reliability" devices.
    By energizing the relay when the fan isn't on ensures that if the relay or the controller does not energize/is damaged that the fan will be turned on thus preventing a possible safety issue/fire.
    I'd gladly have more reliability and a fan be on in a temperature sensitive device than worry about a few mA from a relay coil.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I dispute that. Which of the following two ways of inverting the current through the relay (compared to the circuit on the left) do you think is more reliable????
     
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  5. eetech00

    Active Member

    Jun 8, 2013
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    Hi

    Hmmm..

    I guess I see mcgyvr's point....you can have a perfect driver, but the relay itself can still fail.
    However, my question to the OP is "what is at risk if the FAN doesn't come on?"

    eT
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    My response is simply in a circuit with ONLY a form C relay with a coil rated to the operating voltage. No "inverting" required at the coil side.. You simply swap the leads on the contact side..
    I simply want the fan ON when the relay is OFF/broken. nothing more.. nothing less.
     
  7. Rickvan

    Thread Starter New Member

    Aug 28, 2014
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    Hi. Thanks everyone for your comments. I'm afraid all the talk about 'inverting the coil' is above me. I have decided that since it seems there should be no issue with energizing the coil for prolonged periods, and that seems the simplest and easiest to me, I'll go with that. For those who are interested, this is the relay I've bought and will use. http://www.maplin.co.uk/p/30a-spdt-power-relay-240vac-n24aw

    Just to elaborate a little on my requirements, I am setting up the temperature control on a marine aquarium. A pretty big setup of a bit over 1000 litres. I've decided I'll use the relay in "energized" mode (power ON from the temperature controller) to give power to the 3 heaters. When the temperature goes too high and the heater power is cut by the controller then the relay will go to NC and start up the fan. Should work.....I think!

    Once again....thanks all.
     
  8. MikeML

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    What is the fan supposed to cool?
     
  9. Rickvan

    Thread Starter New Member

    Aug 28, 2014
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    The fan will cool the actual tank water. Blowing air across the surface of the water is a very effective way of cooling by evaporation. Yes, the water will need to be topped up as the level will drop over time.

    The 3 heaters each have built in thermostats so they are set up to maintain 25 degrees C. They are connected to the controller that is set to switch off at 25.5 degrees C. This is a fail-safe so if one of the heaters fails and stays on above 25 the controller will kill the power. The power will be supplied to the 3 heaters through the NO of the relay. This will be powered as long as the controller is supplying power (ie below 25.5 degrees C). If the temperature exceeds 25.5 then the controller switches off, the power is cut to the relay and the NC gate closes and activates the fan to cool the tank.
     
  10. faley

    Member

    Aug 30, 2014
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    I take it the controller has dry contacts? Or is the output voltage supplied by the controller? In the latter case, the current draw of the fan might be a consideration. Given the conditions, you're right Rickvan. However, you may wish to drive a larger relay with a smaller one (driver). In this case the driver contacts would be wired NC and the driven relay's contacts NO. You could also look to SSR's (solid state relays) which are programmable and would consume less power during the desired off state (no fan). Then again, this gets complicated, pricey and can be equated to pole vaulting over mouse dung (to say it politely). I say use a driver- if your fan draws more than 2 amps- for two reasons: 1) larger relays can become noisy over time if they remain on for long periods. 2) A smaller driver would use less power and reduce the load on the controller's power supply and related circuitry.

    You may also consider the use of a free-wheeling diode on the relay coil (if the controller output is solid-state DC).

    However, if you'd like to depart an off-the-shelf solution, I'm sure there are many folks on this forum who could guide you through building your own solid-state intermediate board capable of reversing logic. Then again, we might be back to pole vaulting. Whatever you decide, have fun with it.

    All the best.
     
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