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Solid State Relays for use on a 24V DC motor.

Discussion in 'Hobby Electronics' started by John Crighton, May 8, 2004.

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  1. Hello All,
    a friend of mine wishes to replace four heavy duty relays
    that control a 24 V DC electric motor.
    The motor is either off, runs forward or runs in reverse.
    My friend tells me the normal running current is 10 amps
    but at start up he thinks the current is around 50 amps.

    He cant get the correct relays anymore, the equipment
    is old. Suitable electrical replacement types are expensive
    and physically don't fit. If modifications have to be made
    he thought SSRs would be good.
    Something like this.
    http://www.crydom.com/pdf/crydom_D06D.pdf

    Those particular SSRs look great.
    The only problem is the price. 150 dollars locally
    here in Australia. Too expensive for my friend.

    As a hobbyist, I am even more of a cheapskate
    than my friend is, so I am interested to know what
    is in these things to make them so expensive.

    We want to make something similar to the Crydom
    relay, specification wise, with locally easy to obtain
    parts and a lot cheaper than 600 dollars for four units.
    All ideas welcome.

    Regards,
    John Crighton
    Hornsby
     
  2. Phil Allison

    Phil Allison Guest


    ** What drivel - the Farnell catalogue is FULL of 24 volt, 30 amp plus
    SPCO relays that are compact and would do that job for as little as $8 each
    plus gst.


    Forget the other idea ( ridiculously complex) using mosfets.




    ............. Phil
     
  3. Phil,
    I can see that I didn't spell out my friends requirements very well.
    He wants the contacts to handle direct current of 60 amps DC
    The ratings of the original relay were 28V 60 amps.

    The motor, when in use in a fishing boat auto pilot, continually
    starts and stops and the contacts arc when the relay opens.
    The relays don't last long, hence the solid state switch idea.

    I registered at the Farnell website and now I can't get on it,
    please choose a country, please choose a country, round and round.
    Probably finger trouble with me.
    I would like to see those relays that can handle 30 amps plus at 24V,
    for $8. My friend may be able to parallel them. I think he prefers
    not to parallel them but if the price is right, maybe.

    Anyway, if you or anyone else knows of cheap relays for 24V
    60 amp use, please point them out.
    My old work mate would be delighted. A lot less trouble
    than making a solid state switch but that would be better
    in the long run.

    Regards,
    John Crighton
    Hornsby
     
  4. Phil Allison

    Phil Allison Guest


    ** A very different requirement .


    ** 28 volts DC at high current ( more than 10 amps ) poses a serious
    arcing problem.

    If you place a strong magnet near as possible to the contacts it helps
    considerably to break any arcs.



    ** There is one rated at 40 amps (resistive load) for $8.

    But your problem is arcing - so the contacts need to open wide and fast.

    Series operation of the contacts may be better than paralleling.





    ................ Phil
     
  5. Phil Allison

    Phil Allison Guest


    ** A very different requirement .


    ** 28 volts DC at high current ( more than 10 amps ) poses a serious
    arcing problem.

    If you place a strong magnet near as possible to the contacts it helps
    considerably to break any arcs.



    ** There is one rated at 40 amps (resistive load) for $8.

    But your problem is arcing - so the contacts need to open wide and fast.

    Series operation of the contacts may be better than paralleling.





    ................ Phil
     
  6. paul

    paul Guest

    Hello

    The mosfet switching is not as easy as it sounds.

    I built a mofet H-bridge rated at about 50A 24V some years ago. We also
    tried 30A automotive relays which were cheap and easy to get.

    The load was a 500 watt 24V DC motor. The run current was about 13 amps
    but the peak start current was over 100 amps. Power came from two 12V
    40Ah batteries in series and a charger.

    Switching about 2000 times a day the relays lasted about about 6 weeks;
    usualy failed in a meltdown.

    The H-bridge had four 50A mosfets and a current driver for each leg. A
    very low-R power resistance monitored the current and shut off the drive
    if it exceeded a set point on each drive pulse (PWM drive). A bit of
    logic ensured that both directions were not enabled at the same time. We
    blew up some mosfets before getting it working.

    Let me know if you want some more details.

    It sounds like your application should go the mosfet way but I would
    recommend an off-the-shelf H-bridge motor driver if you can find one.

    Regards
    Paul Bealing
     
  7. Hello Paul,
    Yes, I am very keen to learn more about your H bridge
    motor driver. If one is available locally to do the job
    my friend will definitely be interested in that also.
    Regards,
    John Crighton
    Hornsby
     
  8. paul

    paul Guest

    Hello

    I should be able to dig out the circuit diagram and I've got an old
    prototype unit here but I don't know if it works. It's missing a small
    DC-DC converter that generates the high side drive voltage.
    The files will be archived somewhere here. The prototype board has a
    date on it of 1993. I'll do a search.

    Regards
    Paul Bealing
     
  9. paul

    paul Guest

    Hello

    I've found the circuit diagram and turned it into a pdf.
    It's about 1.5 MB.
    You can download it from the bottom of this page:
    http://www.pmb.co.nz/pub/index.htm

    The layout and construction is quite important if you are driving with
    PWM. For slower on/off relay replacement it shouldn't be a problem.

    Regards
    Paul Bealing
     

  10. Hello Paul,
    thanks very much for your time and effort there. I have told
    my friend where to find your circuit. So he will be studying it.
    Just one quick question.

    Can you remember what you used, part number or
    current rating, for the 15V isolated power supply?

    Regards,
    John Crighton
    Hornsby
     
  11. John
    One of the little 1Watt DC-DC converters from Farnell (NME2415 - cat no
    253-285) should do the Trick they cost about $25 each but that may work out
    cheaper than making one.
    Actually I suspect that given the fact you are replacing relays and are
    operating at (relatively ) low speed the driver circuit for the H bridge
    could be simplified somewhat from pauls design.

    Regards
    Richard Freeman
     
  12. paul

    paul Guest

    Hello

    Used the 1Watt NME1212 from memory. I had a 7812 regulator off the 24V
    to supply the logic etc. It acted as a pre-reg for the NME1212. The 24V
    supply was very noisy.

    Agreed, it could be simplified for low frequency switching.
    I think I was running it at about 15 to 20 KHz.
    The drive transistors deliver current to ensure that the mosfets switch
    fast enough to reduce losses.

    I'd leave the power-on-reset and logic. When you power up both
    directions at the same time without the protection, mosfets explode
    throwing bits of pastic around the room.
    The fuse is ceramic HRC motor rated so it doesn't protect the circuitry.
    It blows after the mosfets have failed preventing further damage.

    The mosfets used are MTP50N06; 50A 60V.
    The drive circuit shown will drive three or four mosfets in parallel on
    each leg.

    Regards
    Paul Bealing
     
  13. Hello Richard,
    I will pass that part number information on to my old work mate
    Les, up in Newcastle. He might want to try just an isolated supply
    and an opto coupler just to make one stand alone relay unit.

    If his SSR survives high side switching of the motor on and off all
    day long, he will be pleased, then an H bridge will be made up.

    Paul's H bridge circuit can then be used as a guide. There are
    lots of good features there in Paul's unit, current limiting,
    forward/reverse interlocking. All great stuff. Les will sort out
    what features he wants. He will probably keep it as simple as
    possible like you suggested Richard.

    Regards,
    John Crighton
    Hornsby
     
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