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@hidden-valley.com>
To: <amps@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@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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