Spehro Pefhany wrote: >A diode across the coil has been suggested, and is the simplest way of >handling the coil inductance. A 1N4148 is fine for any relay you'd be >likely to use. > > Alas, sorry, it is NOT fine and is no longer considered good engineering practice. I see this idea all over the place, and this is bad information. Even relay manufacturers include a diode, but it is not good practice. The 1N4148 is a "faster" general purpose silicon diode, used at one time because nothing any better was available. The 1N4148 cannot switch fast enough to suppress the edges of an inductive spike, so was most successful when a cap in series with a resistor is in parallel with the coil as well as the 1N4148. Moreover, it cannot handle the high current pulse of the inductive collapse. I've seen wholesale device failures with this setup, even with tiny 5V relays. Use a Transorber, Varistor, or TVS device, with a voltage rating just above the voltage being applied to the relay or motor. If need be, just trust me on this one, its the truth. Go to the technical documents page of any transient voltage suppressor maker and you will understand what people are doing. These were originally invented by GE. Slow oscilloscopes won't pickup the unsupressed spike, so this can't be easily proven, but if you own a small portable radio, you can pick up the unsuppressed spike on the AM spectrum. With a 1N4148 it will still be present, with little improvement. If you then use a transorber or TVS placed AT THE RELAY COIL, the spike will be so suppressed that you can't pick it up (but if the suppressor is not placed as close to the relay as possible, the RF will still be generated by radiating through the conductors going from the relay coil to the suppressor). Try it. --Bob >Best regards, > >Spehro Pefhany --"it's the network..." "The Journey is the reward" >speff@interlog.com Info for manufacturers: http://www.trexon.com >Embedded software/hardware/analog Info for designers: http://www.speff.com >->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff > > > > -- Note: To protect our network, attachments must be sent to attach@engineer.cotse.net . 1-520-850-1673 USA/Canada http://beam.to/azengineer -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist