Summary: What are the issues when using a semiconductor switch across an electromechanical contact to (seek to) obtain higher voltage operation than the mechanical contact would otherwise allow and substantially lower power dissipation than a a semiconductor switch alone will allow for a given all up cost? DC operation is of primary interest, but AC gains should also be possible. _____________________________ Electromechanical power relays are low cost for typical "domestic" AC switching situations, have "long enough" lifetimes when used within spec (100,000 + operations) and have an Rdson [tm] contact resistance) that most semiconductors struggle to meet - or do so only at very substantially greater cost. . At 230 VAC (or 110) switching a 15A load a 10 milliOhm "contact resistance" dissipates 2.25 Watts. And around 12 Watts at 50 milliOhm. Semiconductor device suitable for these VI combinations are liable to need substantial heatsinking and the all up cost starts to climb (device, heatsink, mounting, housing, ...), but an eg 15A 230 VAC relay from a reputable manufacturer is well under $10 in volume and needs no heatsinking or special feeding - they can be eg DIN rail mounted with push in connections or eg lug mounted (2 bolts) with eg push on tags - quick, easy, low cost - choose any 3. PCB mounting is also common but slightly less maintenance friendly if things do go wrong. The DC situation is not so good. A 230 VAC 30A rated relay may be specified at 20A up to 28 VDC and then derated very substantially as voltage increases - and is good for perhaps a few amps at 250 VDC or not rated at all for 250VDC operation. A 230 VDC 20A relay is both rare and expensive. Once operated and the contact solidly closed a relay good for 20A at 28 VDC should (it seems) be good for 20A at 250 VDC if insulation issues are not a factor. The problem is arc formation on break and, to a lesser but still significant extent on make. Even with pure resistive loads this is a major issue and any sort of inductive load component adds to the problem. [[I can "arc weld" with feed from a single 30Vmp 250 Wmp PV panel - only about 8A !!!. At eg 400 VDC (or higher) high Wattage DC is a somewhat fearsome beast]]. An "obvious" solution (and many decades old) is to place a semiconductor switch, say a MOSFET (or probably an IGBT at higher voltages), across the contact to handle make/break (with high Rdson compared to the relay contact resistance and short term large dissipation) with the mechanical contact operating just after the FET and the FET being deactivated just after the contact opens. The FET needs to be able to handle any inrush current, short term dissipation and and turn off transients. Even allowing for this, the device 'size', cost and heatsinking should be modest compared to using a semiconductor only solution. Relay voltage ratings should be much enhanced. Even if the FET was left activated throughout its dissipation would be minimal due to shunting by the relay contact - allowing an extremely simple switching sequence. A web search for examples using various terms turned up very little. Here is an example of AC switching with a TRIAC bypassing a relay contact http://www.echola.com/power/Hybrid-Relay.html Questions: How common is this? Any examples? What sort of performance gains are realised in practice for DC and AC switching? Why is it a bad idea compared to alternatives? / What have I missed? Russell --=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 .