[Amps] Solenoid polarity on DC reed relays

[Paul Christensen]

I use a Viz triple-output bench supply for this kind of testing and I'm always crossing leads by mistake when bread-boarding something.  The supply has an adjustable, electronically-switched current trip point so no matter how abusive I am, the supply silently trips.  I just press the reset button until the next fault occurs.

Paul, W9AC

  ----- Original Message ----- 
  From: Jeff Carter 
  To: Paul Christensen 
  Sent: Wednesday, March 10, 2010 2:27 PM
  Subject: Re: [Amps] Solenoid polarity on DC reed relays


  Makes sense.  I especially like the bench supply solution!

  Thanks for the quick reply!

  Jeff/KD4RBG


  On Wed, Mar 10, 2010 at 2:23 PM, Paul Christensen <w9ac at arrl.net> wrote:

    Jeff:

    That depends on the coil resistance.  The resistance check may not work on relays with low coil resistance.

    Another check is to apply DC coil voltage with a current-limiting bench supply.  Reverse the coil leads and if the diode is present, the supply will fault.

    Paul, W9AC


    ----- Original Message ----- From: "Jeff Carter" <amps at hidden-valley.com>
    To: <amps at contesting.com>
    Sent: Wednesday, March 10, 2010 2:16 PM
    Subject: Re: [Amps] Solenoid polarity on DC reed relays



      I don't have one here to easily check, but it seems to me that a resistance
      reading would just read the coil in either direction, since that's the path
      of least resistance.

      Does this actually work?  If so, why?

      Jeff/KD4RBG

      On Wed, Mar 10, 2010 at 2:13 PM, Paul Christensen <w9ac at arrl.net> wrote:


        Bill,

        DC coil polarity is an issue because polarized relays generally contain a
        reverse-biased snubbing diode across the relay coil to: (1) protect a
        solid-state driving circuit from damage resulting from the rapid coil flux
        collapse; and (2) to decrease the relay .  Measure the coil resistance with
        a DVM or VOM, then reverse the test leads and measure again.  This may give
        you an indication if the snubbing diode is already part of the relay.
         Equal
        readings generally signifies a non-polarized relay coil.


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