Brent Brown wrote: >>>>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. > > Yes, TVS switching times are around 0.1 to 1 pS. The 1N4148 switching time is 4nS. I _THINK_ that is 4000 x faster. --Bob -- 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