Inductive resistance management??

Discussion in 'The Projects Forum' started by flinthill, Sep 22, 2014.

  1. flinthill

    Thread Starter New Member

    Sep 22, 2014
    6
    0
    I think I just fried a Denkovi relay board due to my lack of knowledge about inductive resistance. I have been looking for reasons why the board went south and the best I can come up with is the contacts or the coils (or both) were exposed to voltage levels they were not designed for.

    I have done some research and have discovered that if I place a diode (IN400?) in the circuit that it will help absorb the inductive loop when the contact is opened. What I am hoping to accomplish with this post is to get some advice on the validity of my assessment and some suggestions on what steps I can take to avoid the problem happening again. Also, I used a 12VDC/500ma wall transformer to power the relay and am wondering if I should have used a more regulated power supply to prevent the initial spike when turned on. (the relay officially requires a 12vdc/300ma power supply)

    My apologies for not having specific knowledge. This relay is part of an effort to automate my observatory and telescope so it can operate unattended while I am inside enjoying the comforts of a warm home this winter. :>)

    Thank you for reading this post.

    Ron
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,153
    3,058
    Yes, a relay coil should have a reversed-bias diode such as 1N4007 across its poles to absorb the inductive voltage spike that occurs when the magnetic field in the coil suddenly collapses. The diode choice varies with the specs of the coil, but the one I named is more than enough and very inexpensive.

    What load are you switching with your relay? Obviously, the contacts must be rated for the load.
     
  3. flinthill

    Thread Starter New Member

    Sep 22, 2014
    6
    0

    Thanks for your reply.

    Several appliances: One .09a fan, two light box levels estimated at 1.2a and 1.5a, two external relays that separately control a 120vac pump and a 120vac TEC.

    Do you think I should have used a different power supply?

    Where do you think the best location will be for the diodes. I am thinking between the individual relay input pole (+) and the output to the appliance. Is that reasonable, or is there a better location. Also, (please forgive me here for my lack of terminology) should the diode direction arrow point to the input pole (before the relay contactor) or the output pole.

    Regards,

    Ron
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    the place for a diode is across the coil windings. if the points are burning, and those points are not connected to a relay coil, or are connected to ac, an RC snubber should be used, or the relay sized larger.
     
  5. flinthill

    Thread Starter New Member

    Sep 22, 2014
    6
    0

    The board is a Denkovi CB/Ro8/12v-USB. It has the heavier relays rated at 10a @ 12vdc. The relay contacts are activated slowly by command line batch files so I don't imagine there is a need for fast cycling. However, when the board initializes the FTDI driver cycles the 1/3/5 relays several times very quickly...almost so fast I can hardly see the status LEDs flash. The specs for the NI4007 diode says it will handle 250vdc kickback and I read the maximum voltage is 10x the input voltage. When you say the diode should be placed across the coil windings can you tell me what physical points that relates to on this solid state relay board?

    Thanks for your reply,

    Ron
     
  6. Brevor

    Active Member

    Apr 9, 2011
    279
    54
    Your board should already have diodes across the relays, It uses a ULN 2803 relay driver IC which has internal diodes. Most likely these are being used.
     
  7. flinthill

    Thread Starter New Member

    Sep 22, 2014
    6
    0
    You are correct, that is the number on the IC. I am wondering if it would be beneficial to install another diode across the leads to the individual loads to insure prevention of damage to the IC or relays.

    Thanks,

    Ron
     
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