> > > But the flyback voltage generated by the relay coil turns the diode > > > on. The voltage rating of the diode only needs to match the power > > > supply requirement (the voltage accros the relay when on) - plus the > > > usual margin. > > > > And why 1N4148s are dying, 1N4007 not? > > Not too many scenarios here... > > Firstly, check if the 1N4148 has a current rating equal or greater than the > normal relay coil current (as other's have correctly said, this is the current the > diode sees at the instant when the relay is turned off, and it decays to zero > from there, there are no peaks above this). > > Secondly, if the power supply is ever reversed this diode will be forward > biased and it or the switching transistor will die! Could be you have a fairly > rugged transistor and the 1N4148 is the weak link. > > My take on this subject... inverse parallel diode is generally a more than OK > way of subduing back EMF of a relay when switched by a transistor. Keeps > EMF down out 0.6V. > > TVS or similar device.... allows EMF to rise to a higher voltage (must be > designed for) in order to allow current to decay as fast as possible. > > Good question...how does this effect EMI? If for example a 12V TVS was > used with a 12V supply... At turn off the coil voltage voltage would go > relatively quickly from +12V to -12V, hold there for a short while, then decay > to 0. The fast edge (24V spike) is what generates the EMI. > > Compare this with a simple inverse parallel diode...At turn off the coil voltage > voltage would go relatively quickly from +12V to -0.6V, hold there for a little > longer than the above example, then decay to 0. Hmmm, a 12.6V spike > compared with a 24V spike. > > I suspect that "slow" diodes (eg 1N400x series) might even produce less > EMI.... the slowness being evident as an equivalent capacitance that > suppresses dv/dt. I also could be totally wrong here :-) An RC snubber would > be a better idea. > > Interesting subject...simple, but quite a few things to think about. > > > But the flyback voltage generated by the relay coil turns the diode > > > on. The voltage rating of the diode only needs to match the power > > > supply requirement (the voltage accros the relay when on) - plus the > > > usual margin. > > > > And why 1N4148s are dying, 1N4007 not? > > Not too many scenarios here... > > Firstly, check if the 1N4148 has a current rating equal or greater than > the normal relay coil current (as other's have correctly said, this is > the current the diode sees at the instant when the relay is turned off, > and it decays to zero from there, there are no peaks above this). > > Secondly, if the power supply is ever reversed this diode will be > forward biased and it or the switching transistor will die! Could be you > have a fairly rugged transistor and the 1N4148 is the weak link. > > My take on this subject... inverse parallel diode is generally a more > than OK way of subduing back EMF of a relay when switched by a > transistor. Keeps EMF down out 0.6V. > > TVS or similar device.... allows EMF to rise to a higher voltage (must > be designed for) in order to allow current to decay as fast as possible. > > Good question...how does this effect EMI? If for example a 12V TVS was > used with a 12V supply... At turn off the coil voltage voltage would go > relatively quickly from +12V to -12V, hold there for a short while, then > decay to 0. The fast edge (24V spike) is what generates the EMI. > > Compare this with a simple inverse parallel diode...At turn off the coil > voltage voltage would go relatively quickly from +12V to -0.6V, hold > there for a little longer than the above example, then decay to 0. Hmmm, > a 12.6V spike compared with a 24V spike. > > I suspect that "slow" diodes (eg 1N400x series) might even produce less > EMI.... the slowness being evident as an equivalent capacitance that > suppresses dv/dt. I also could be totally wrong here :-) An RC snubber > would be a better idea. > > Interesting subject...simple, but quite a few things to think about. > Doh! My bad. I just drew out the circuit and discovered my mistake. When a TVS or zener is used it goes ACROSS the transistor, NOT accross the relay coil. Now I see that it is superior a simple diode across the relay coil. When the transistor is turned off the TVS conducts to maintain current flow from the coil (important to note that this current path now includes the power supply, whereas with the simple diode across the coil circuit the current flowed in a loop). If the TVS clamping voltage is close to the power supply voltage the EMF across the coil will be minimized. This does rely on the TVS switching very quickly, but as someone else said this is something they are very good at. -- Brent Brown, Electronic Design Solutions 16 English Street, Hamilton 2001, New Zealand Ph: +64 7 849 0069 Fax: +64 7 849 0071 Cell/txt: 027 433 4069 eMail: brent.brown@clear.net.nz -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist