Richard Prosser wrote: >One other possibility - Is the feed to the relay decoupled? If there >is significant inductance in this line & it's not decoupled properly >then there will be an additional transient from this inductance. While >I can't quite see how it would damage a diode in parallel with the >relay, it could damage the switching device. > >Also - is there anything special about the relay. Relay coil >inductance does change as it operates & there could be "something" to >do with the relay that produces a waveform different from what we're >talking about. For example a polarised relay will produce different >waveform from a non-polarised one. > > > I know. No, it was a standard 12V DC Modem relay, like you see all over the place. 12V is applied while it is closed. Good idea, though. I didn't design it, I was just hired to find out what was going wrong. The failure <1% of the installed base, and didn't fail immediately after insertion into service. But for these guys, that is a VERY high number. You have been to a K-Mart, right? These relays actuate every time the call center is dialled up, about 500 +x per day. What worried us the most was NOT that the diode failed & took out the 12V supply, it was that the diode might OPEN and the RF might cause problems with the uP,thus flawing the transaction document being transferred. . The TVS solved both problems. --Bob >And it is DC energised isn't it? > >RP > > >On 08/02/06, Herbert Graf wrote: > > >>On Mon, 2006-02-06 at 23:04 +0100, Wouter van Ooijen wrote: >> >> >>>>I don't think that the problem is speed or current - the >>>>problem is the voltage. A 1N4148 is rated with 100 Volts and >>>>this could be less if you switch off a relay. >>>> >>>> >>>How can you even approach 100V when switching a low-voltage relais? When >>>the relais is energised there is just the power supply voltage (5, 12, >>>24V), when the transistor switches of the diode will conduct so the >>>voltage is just 0.6V. >>> >>> >>I'm no expert, but remember we are dealing with non ideal components >>here. >> >>The diode isn't an ideal device. When reversed biased the charge >>separation is basically a capacitor (very useful in FM transmitters). >>When the relay is turned off it'll take a while for this "capacitor" to >>discharge and the diode to become forward biased and start conducting. >> >>The relay, being an inductor, will "want" to keep the same amount of >>current flowing and will do anything it can to make that happen. It'll >>ramp it's voltage up so that the current continues to flow. It's during >>this time of going from reverse biased to forward biased that the >>voltage "spikes"; and you can EASILY see voltages WAY above the power >>supply voltage. >> >>Considering how widespread and successful the practise of putting a >>reverse diode across a relay coil is, calling it "bad practise" is a >>false generalization. I will agree that IN SOME CASES it is prudent to >>go for a "better" device, but if the problem were that bad we would >>notice. >> >>TTYL >> >>----------------------------- >>Herbert's PIC Stuff: >>http://repatch.dyndns.org:8383/pic_stuff/ >> >>-- >>http://www.piclist.com PIC/SX FAQ & list archive >>View/change your membership options at >>http://mailman.mit.edu/mailman/listinfo/piclist >> >> >> > > > -- 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