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

[Jack Smith]

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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