At 11:19 PM 2/5/2006 -0700, you wrote: >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. It's fine from the pov of protecting the transistor. Hundreds of millions are in daily use. You won't see datasheet performance on the relay, though. What authority suggests that it is "not good practice"? And why would it change? BTW, some of my designs use relays by the sh..er... pallet-load. >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. Distributed capacitance of the coil will limit dV/dt, although in more sophisticated designs it may be desirable to add some external capacitance- but not for protection reasons. You don't necessarily need a series resistor, current will be limited by beta. For protection, even a slow-as-molasses 1N400x diode is fine. There are no nanosecond edges here to be concerned with. > 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. "High current pulse"? Where does this idea come from anyway? I have heard this misconception before. The peak current through the diodes will never exceed the steady-state current through the relay coil. There is no magic here, the coil is generally just acting as an inductor (with some small variations due to the magnetic stuff moving around, which can be ignored for this purpose). The "strange" thing that can cause transistor failures is violating the SOA of the BJT during "off" switching because of the inductive load. Small transistors often don't have the safe operating area explicitly defined, so it's best to err on the side of using a hefty part, vis-a-vis the coil current, as I suggested. See, for example, Figure 3 on this TIP31 data sheet: http://pubpages.unh.edu/~aperkins/pdf/TIP-devices/TIP31.pdf >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. Expensive and totally unnecessary from the pov of the transistor. They are actually *harder* on the transistor because the voltage goes higher. >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. A TVS is useful where high currents are involved- for power supply protection from load dump on automotive systems, where lightning may be involved (that's where fast clamping is good). For relay coils, the best approach if you feel like using a TVS is to use a diode or regular zener and send the price difference directly to me. ;-) One exception- if the layout is poor or if a crummy relay is used, it's possible to have coil-contact flashover, especially from lightning. In that case, a TVS can protect the driver (eg. telecom situation). But a properly rated relay with solid-state (Transil, TVS, MOV) or spark arrestors at the line is going to be more reliable long-term. 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 -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist