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 > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
