At 06:34 PM 12/12/2013, David VanHorn wrote: >We used to use the diode across coil method, but I've switched all=20 >our designs over to the zener across transistor method. Most of our=20 >products drive larger motors (100-300W) and they are heavily=20 >inductive, so relay arcing is significant if the relays don't release >quickly. Even for DC resistive loads, I don't see any reason to=20 >torture the contacts. :) I suspect that I would be doing the same as you if I were driving=20 those loads. In our case, the relay loads *are* inductive: they are=20 hot-water recirculating pumps that consume perhaps 60 - 70VA @ AC=20 mains voltage (120 or 230 Vac). The JS1 relays are rated at 10A and=20 277V and I have not seen any relay failures out of more than one=20 hundred thousand relays used over the years. > > FWIW - most of our stuff uses 5V Vdd with an unregulated supply > > driving the relays - the unregulated supply comes from either 12Vac > > or 24Vac (nominal +16 or +32 Vdc on the relay coils). > > > The excess coil voltage is dropped in the (series) indicator LED and > > either a series dropping resistor *or* by applying PWM to the=20 > relay coils to > > reduce the average voltage to the proper value. PWM is nominally=20 > 1ms OFF and 4.1ms ON (~80%) - works very well. > > > >So are you using relays rated for significantly lower voltage than your >relay supply voltage? I use relays with 12Vdc and 24Vdc coils and the incoming low-voltage=20 AC that powers the card is 12Vac or 24Vac. I usually don't have a=20 choice in what the incoming AC supply voltage is: our cards are part=20 of larger systems that either supply or require one of those voltages. The relay data sheets say that the relays will tolerate the rectified=20 DC voltage that results from those input AC voltages but they run=20 much hotter than I like. So: I do whatever is necessary to drop the=20 voltage on the relay coils down to the rated value. Older boards / products that use JS1 relays have a 10-pin bussed SIP=20 resistor array (100R) for each group of 4 relays. The unregulated=20 supply feeds the common pin on the resistor array; one resistor feeds=20 the anode of the indicator LED (cathode to relay coil +) and another=20 resistor from the array also feeds that same relay coil +=20 terminal. The net result is a 100R resistor in parallel with the=20 series combination of a 100R resistor and LED - this gives a nominal=20 12Vdc across the relay coil when the circuit is powered with a 12Vac=20 transformer. The relay drivers are almost always 2003 drivers with=20 the catch diodes tied to the relay supply rail. Modern boards use the PWM method I spoke of earlier. The relay=20 drivers on those boards is either a 2003 / 2803 or a TPIC6595 shift=20 register with integrated MOSFET drivers. The relays that use the ULN=20 parts use the built-in catch diodes with the common line tied to the=20 relay supply voltage; the relays driven by the TPIC use the built-in=20 avalanche clamps (no catch diodes). The TPIC parts are specifically=20 rated for avalanche transient suppression and it works exceedingly well. >I have seen some pretty interesting failures. One was a mosfet that=20 >was failing with only 60V (peak) applied to a 70V part. It turned=20 >out that the reason that I only saw 60 was that the part was driven=20 >into avalanche so fast that a 300MHz scope might have been able to=20 >see it. Testing with a 1000V part in that circuit showed the real=20 >flyback energy that was killing the parts. > >Current probes are wonderful tools, in that you can see what is=20 >really going on, as opposed to what ought to be going on. I have a very old Tek DC current probe that has bandwidth up to=20 50MHz. It has a range selector switch that gives full-scale input=20 from 1mA through 1A (1-2-5 sequence). It is *wonderful* for seeing=20 exactly the kinds of problems that mention. That and my DSO make a=20 formidable troubleshooting team for tracking down those kinds of issues. Luckily, I haven't had do deal with the issues that you brought=20 up. But I will keep your comments in mind for the future. I consider myself to be extremely lucky to have become part of the=20 PIClist community. I've been doing electronics since before I was a=20 teenager and I've become fairly good at what I do because I've had=20 the extreme good fortune to be able to learn from people who have=20 more experience than I. Dave - you are one of those treasured=20 resources. The experiences that you have shared, especially in=20 regards to RF stuff and emissions compliance, has taught me much. I=20 *really* appreciate it. Many thanks! dwayne --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .