At 08:19 AM 11/9/2006, Larry Taft wrote: >The darlington diode doesn't protect against a high plus voltage at the >collector. The collapsing magnetic field in the K17 relay coil at >turnoff produces a reverse polarity pulse across the coil which turns >out to be a big positive voltage at the collector. ( I think) > >Larry K2LT
I would suggest that before embarking on such a design that you read what the relay manufacturers have to say about transient suppression (hint, a diode across the coil isn't the recommendation). http://relays.tycoelectronics.com/appnotes/ has a series of appnotes: The first one (application of coil suppression with DC relays) is a good one, as is the "Coil suppression can reduce relay life" They recommend either a suitable zener across the transistor (so that the voltage doesn't exceed the Vceo breakdown) or a resistor. Typically a 20-30 V zener is appropriate. Some commercially available driver devices include the zener, or have the output transistor designed to tolerate the breakdown (the latter is particularly common in electronic ignitions, where you have a 400 V transient to deal with). Jim, W6RMK _______________________________________________ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://www.mail-archive.com/flexradio%40flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
