> So where are we up to with this ? > > Options so far - > > TVS across transistor > RC snubber across relay coil > RC on transistor base to slow switch-off > Diode (of some sort) across relay coil > > Is there any consensus as to the preferred order ? > > eg if cost is not an issue / if cost is an issue > eg if components are not to be grossly over-rated Probably not a consensus but my summary in terms of price and performance would be... 1. Diode across relay coil (cheapest option) - Time honoured method. Must be suitably current rated ( = coil current), voltage spikes don't exist here, diode turn off time not an issue either, diode turn on time has been an interesting discussion but I am yet to be convinced that a lazy-as 1N4004 etc wouldn't be perfectly acceptable here. 2. TVS across transistor (pricier) - great for lowest EMI option, better than above, but needs a little more design attention to select best TVS voltage for lowest EMI. Potentially problematic if PSU voltage is unregulated or spikey (will conduct and pass current through relay coil when voltage rises above threshold). 3. RC on transistor base - Even just increacing base resistor value can slow down turn off time appreciably for no extra component cost, slower EMF spike => lower EMI. Works with BJT and MOSFET. Add C where necessary. There will be dynamic power dissipation in the transistor as it turns off, need to check SOAR carefully. 4. RC snubber across relay coil. Adds the finishing touch to reducing EMI. Low cost anyway. Add this in addition to any of the above. I rank this last because it's two extra components which eat PCB real estate and needs to be used in addition to some kind of diode. -- 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