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Driving DC relays from AC

Discussion in 'Hobby Electronics' started by jrobbo, Nov 8, 2003.

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  1. jrobbo

    jrobbo Guest

    Hi all,

    I have a handful of relays with a coil rated at 24V DC. Only problem
    is that the source that I want to drive them from is 24V AC....

    The coil current on the relay is 15mA. I was thinking of just using a
    half-wave rectifier (single diode) and a 100uF cap to drive it. Will
    that work, or should I use a full-wave (4 diode) rectifier instead?

    Thanks in advance for any help

    Regards

    John
     
  2. Bushy

    Bushy Guest

    Depends, if you don't mind a small delay in operating the relay then use
    half wave and accept up to a half wave delay in operating the relay.
    Peter
     
  3. Phil Allison

    Phil Allison Guest


    ** One diode ( half wave) rectification is fine - but use the diodes
    connected alternately to the AC so the DC component in the transformer does
    not build up to any large amount.



    .......... Phil
     
  4. KLR

    KLR Guest


    experiment with both setups and see what works best. Only takes a
    minute to do.

    Other thing to keep in mind is that rectified 24vac is going to
    produce about 34v dc (multiply the ac x 1.414 to get the DC voltage)
    which might be too much voltage for your relay's coil - specially if
    its left on for any length of time. An appropriate dropping resistor
    would be a good idea too;.
     
  5. Phil Allison

    Phil Allison Guest



    ** This sort of relay is typically rated for 150 % over the nominal DC
    voltage - see Farnell cat. The average voltage can be reduced buy
    selecting the filter electro value - using a 22uF will allow about 10 volts
    p-p ripple at 15mA and 50 Hz hence 5 volts average drop.



    ............ Phil
     
  6. The best approach is to use a diode in series with the coil to the AC, and a
    diode in parralel with the coil, cathode to positive. This will keep some
    current in the coil during the "off" half cycle and control any voltage
    transients that might destroy the series diode, especially when switching
    it. A DC component in the supply may not be an issue if the relay current is
    small compared with the supply capacity.

    All the best

    Ian Macmillan
     
  7. Arpit

    Arpit Guest

    WOuldn't the capacitor do the same as the parallel diode? my
    experience using relays on 50 hz half wave rectified without a cap is
    that they buzz, so a cap is needed.
     
  8. Phil Allison

    Phil Allison Guest



    ** Correct - the extra diode in parallel is not enough to stop severe
    relay oscillation at 50 Hz.

    A 10 uF or greater cap will however do both jobs.



    .............. Phil
     
  9. D.Castles

    D.Castles Guest

    DC component in a transformer? UH?
    Dave
     
  10. D.Castles

    D.Castles Guest

     
  11. Phil Allison

    Phil Allison Guest

    Phil Allison wrote:


    Hi all,

    I have a handful of relays with a coil rated at 24V DC. Only problem
    is that the source that I want to drive them from is 24V AC....

    The coil current on the relay is 15mA. I was thinking of just using a
    half-wave rectifier (single diode) and a 100uF cap to drive it. Will
    that work, or should I use a full-wave (4 diode) rectifier instead?



    ** One diode ( half wave) rectification is fine - but use the diodes
    connected alternately to the AC so the DC component in the transformer does
    not build up to any large amount.



    .......... Phil

    DC component in a transformer? UH?


    ** Transformers have always been a big problem for Dave's dinosaur brain.





    ............ Phil
     
  12. D.Castles

    D.Castles Guest

    What about the average voltage one Phil? (see below)
    I can't wait for your explanation and next insult. I love them, they
    make me stronger.
    Dave
     
  13. Michael

    Michael Guest

    I'm not sure what you had in mind here Phil, were you thinking of a
    CT with a diode to each end?
     
  14. Phil Allison

    Phil Allison Guest


    ** No.

    My post is VERY clear - do not have all the diodes rectifying the
    same. Alternate them.



    ......... Phil
     
  15. Michael

    Michael Guest

    Dave, by selecting a lower capacitance, the voltage across the cap
    discharges more between cycles. Hence the average voltage across the
    relay coil is lower. What is your argument?
     
  16. Phil Allison

    Phil Allison Guest


    ** Dave is a highly qualified mental cripple - see:

    http://www.latrobe.edu.au/ee/people/profiles/Castles.html


    He gets his jollies by baiting naive Christians on their newsgroups -
    and pulling the wings off flies.




    ............ Phil
     
  17. Albm&ctd

    Albm&ctd Guest

    Dunno, I'm braindead but I'm considerate enough to others who have
    text based readers to NOT post in html.
    As for running relays from AC, I'd use a bridge rectifier with a
    decent sized electro taking note of the pull in and hold in voltages,
    maybe even use a variable supply to test the relays I want to power so
    I could choose a suitable voltage transformer but I'm just a mechanic,
    not a farkin electronic genius. Ask Phil.

    Al

    2003 insult page awaits your contribution
    http://kwakakid.cjb.net/insult.html
     
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