[Amps] Solenoid polarity on DC reed relays

[Paul Christensen]

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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