Mike Keitz wrote: > > On Sat, 1 May 1999 12:37:03 +0200 Tjaart van der Walt > writes: > > >Remember the usually-forgotten diode between the negative side of > >the=20 > >relay coil (on the collector) and GND. This way you guarantee that > >no=20 > >excessive - voltages get on the GND line. > > How will installing a diode that way keep negative voltages from the > ground line? It is connected to the ground line in such a way to route > any negative voltage right to it. Such voltages won't occur in normal > operation anyway. ...and the rest of the electronics won't fail in normal operations either. The high Q of a coil guarantees very, very nasty voltages. We've tested a range of diodes, and only a few could conduct quickly enough to short these spikes. The neg side of the relay can go quite negative. > If they did, the transistor would start to conduct in > "inverted transistor" mode and shunt the voltage to the ground line (and > to the drive resistor, which may be more of a problem). The Vec rating of a transistor is usually much lower than the Vce rating. This one diode may (or may not) prevent comebacks. > really necessary, use aluminum ones instead. They are much more > resistant to damage from overvoltage. But it's not only a problem of > capacitors failing, as spikes of at least 20-30 V on the 12V supply are > bound to mess up something else. ...and they usually find the one components you don't want to fail :( -- Friendly Regards /"\ \ / Tjaart van der Walt X ASCII RIBBON CAMPAIGN mailto:tjaart@wasp.co.za / \ AGAINST HTML MAIL |--------------------------------------------------| | WASP International | |R&D Engineer : GSM peripheral services development| |--------------------------------------------------| | Mobile : tjaart@sms.wasp.co.za (160 text chars) | | http://www.wasp.co.za/~tjaart/index.html | |Voice: +27-(0)11-622-8686 Fax: +27-(0)11-622-8973| | WGS-84 : 26¡10.52'S 28¡06.19'E | |--------------------------------------------------|