[Elecraft] Can a 1N4007 Be an Effective Relay Snubber?

[Mel Farrer]

Hi, I have used the diode snubber in all designs of relay control from small reed to large contactors.  I usually check the series resistance of the relay and the non solid state device activation and measure the voltage "kick".  I then use ohms law to determine the peak current that would occur against the relay resistance and size the diode accordingly.  Most of the time the 1N4000 series are well suited for small relay applications.  When in doubt, measure the relay parameters..

Mel, K6KBE

--- On Wed, 4/8/09, Jack Smith <jack.smith at cliftonlaboratories.com> wrote:

From: Jack Smith <jack.smith at cliftonlaboratories.com>
Subject: Re: [Elecraft] Can a 1N4007 Be an Effective Relay Snubber?
To: "Paul Christensen" <w9ac at arrl.net>
Cc: "'Elecraft'" <elecraft at mailman.qth.net>
Date: Wednesday, April 8, 2009, 2:43 PM

The quick test I did today used a common 2N7000 MOSFET to switch a 
physically small relay typical of those used in ham gear. I think this 
is a reasonably common arrangement and I've used it in the past myself.

The turn-off voltage exceeds the 2N7000's maximum permitted source-drain 
voltage of 60 volts. What happens is that the 2N7000 breaks down at 
70-75 volts and acts like a Zener diode.

I'll leave it to the designer to decide whether it's a good thing or a 
bad thing to have the 2N7000 break down with every diode switch cycle. I 
don't recall seeing a caution in the 2N7000 data sheet recommending that 
the device not be operated in a mode that causes it to break down but it 
seems to me that it isn't a good idea for a reliable design. The relay 
coil resistance will limit the peak current through the 2N7000 when in 
break down mode, but it still leaves me uncomfortable.



Jack


Paul Christensen wrote:
>>> To answer, I hope definitively, the question "is a 1N4007 power diode
>>> too slow to be used as a relay coil snubber,"
>>>       
>
> Just curious -- over the years, I've seen references to the main purpose of 
> the snubber diode being used; (1) to decrease the action time of the relay, 
> since the decaying flux of the relay coil tends to keep the contacts engaged 
> (NO), or disengaged (NC) longer than necessary after voltage is removed from 
> the coil; and (2) to primarily protect a solid-state switching device (e.g., 
> an open NPN collector) from damage, also being caused by the snap of the 
> decaying flux of the relay coil.  I realize both uses of the diode are 
> valid, but it seems that unless there's an extremely high flux developed by 
> coil, the primary purpose of the diode is for relay speed, rather than 
> switching device protection.
>
> Paul, W9AC
>
>
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