A small doubt about relays

Discussion in 'The Projects Forum' started by Roadrunner, Apr 9, 2013.

  1. Roadrunner

    Thread Starter New Member

    Feb 14, 2013
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    Hello all !!
    If my circuit requires a relay to be in the normally open (NO) state most of the time, will it shorten its life? Should I reconfigure the circuit ?
    I am referring to the 240 VAC range relays..
     
  2. edwardholmes91

    Member

    Feb 25, 2013
    186
    19
    It depends what type of relay you are refereeing to. A change-over relay or SPDT (single pole dual throw) relay would be fine. This will have 5 connections, two are the coil. Then three are the switch contacts, the common usually labeled C, the normally open (NO) and the normally closed (NC). This way you wouldn't need to keep the relay energized for long periods of time.
     
  3. Roadrunner

    Thread Starter New Member

    Feb 14, 2013
    14
    0
    But I am using a DPDT relay. Being a solar-based application , the coil remains energized all through the day from the mains. Will it get burnt ?
     
  4. edwardholmes91

    Member

    Feb 25, 2013
    186
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    Can I ask what it is you're actually trying to switch? If you're not trying to switch anything then a relay isn't needed. A DPDT relay will have 8 contacts because it has an additional set of switch contacts.
     
  5. Roadrunner

    Thread Starter New Member

    Feb 14, 2013
    14
    0
    I am switching the power input of an inverter between solar and mains :).
    The coil is energized from mains..
     
  6. edwardholmes91

    Member

    Feb 25, 2013
    186
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    So when the mains drops out you want to switch to solar power for the inverter? A quick google search reveals: first result ...that "Some relays are on for as long as the car is running. And the remote starter itself uses relays to keep the car running."

    Anyway, in answer to your question, it really depends on the relay itself. I am sure there are other solutions out there that don't require a relay, but other people on the forum may be able to help you a little more there.

    You know those electronic door stops that use an electromagnet, and release when you press a button or there is a fire? I know it's not the same thing... but they remain energized for long periods of time without burning out?
     
  7. DickCappels

    Moderator

    Aug 21, 2008
    2,662
    633
    Use a relay from a recognized quality manufacturer that is designed to handle the coil voltage you plan to apply, and observe the environmental conditions (primarily ambient temperature) and as long as the contacts are properly rated, you should as one manufacturer once said "A good long time".
     
  8. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Why would any relay or coil burn out if it is properly rated?

    Relay coils are rated for 100% duty cycle operation, i.e. they can be energized all the time.
    The coil can be damaged if the specified operating voltage is exceeded. The coil can also be damaged if the ambient temperature causes the coil to overheat.

    The current required to hold the relay once it is energized can be reduced after it has been activated. You can experiment with putting an incandescent light bulb in series with the relay coil. When no current is flowing, the resistance of the bulb is low. When the relay coil is initially energized, the low resistance of the bulb will allow sufficient current to flow. As current flows, the resistance of the bulb increases thereby reducing the current flowing but still keeping the relay coil energized.
     
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  9. edwardholmes91

    Member

    Feb 25, 2013
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    Thanks MrChips, I think this is what the original poster was looking for. I actually never knew this myself. There always just seems to be a bit of confusion as to whether relay coils will burn out if left energized constantly, even when the correct rating is used... although I have never read this anywhere.

    The bit about using an incandescent lamp is quite interesting. I seem to recall when I was younger me and my dad building a big Mechano twister ride and we had a car headlamp somehow wired in series with the motor and a selector to select three speeds. I think it worked something like: the brighter the lamp was then the slower the motor ran or something.
     
  10. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    I suppose you mean eight connections. A DPDT relay has six contacts: two wipers, two N/O, and two N/C.
     
  11. edwardholmes91

    Member

    Feb 25, 2013
    186
    19
    Yes, 8 if you count the coil.
     
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