Hi Russell, I have designed a number of low voltage disconnect units for telecom power supplies. All "48V" DC (but bidirectional as they connect to batteries) with disconnect ratings up to 1000A or so. The larger units just used suitably rated (expensive) contactors but a couple of smaller ones used automotive type relays with a parallel diode bridge + mosfet. Disconnect ratings for these was around the 50-100A area. For basic operation, the idea works. Problems occur under fault conditions however - especially as under a fault condition the supply voltage will fail and the relay will drop out anyway. So you have to provide hold-up capacitors for several 100mS as well as a way to turn things off in a controlled manner. Major faults are less of an issue as the protection devices (fuses, circuit breakers etc.) will switch quickly but moderate faults (5 to 10 times normal) are a real pain. The thermal cbs & fuses will eventually blow but only after a while and magnetic cbs may not act at all. You also generally have to arrange the relay coil drive circuit so that the coil will drop out as quickly and consistently as possible. Placing a diode across the coil terminals stops the voltage spike, but the circulating current slows the drop out. Zener + diode or switched resistor or MOV works better as long as you can handle the voltage spike. And you have to make sure the swiching control circuit is stable and not susceptable to switching noise with a variety of load and battery types. (don't ask how I know about this one). RP On 3 January 2015 at 06:59, Dwayne Reid wrote: > This isn't the same thing, but many years ago I did a H-bridge at > 28Vdc , 15A using 2- T90 style relays for the positive DC rail and 2- > N-channel current-sense MOSFETs for the negative DC rail. The > application was a large DC linear actuator acting as a servo that > needed to operate anywhere from several times per minute to a few > times per hour. > > Timing was such that the relay closed first, then PWM the MOSFET to > full-on. Turn off was similar: turn the MOSFET off, wait, turn the > relay OFF, wait, turn both MOSFETs on for braking. We built many > hundreds of these units and all worked reliably. The major failure > point was the feedback position sensor, which we had no control over. > > dwayne > > > At 05:35 AM 1/2/2015, RussellMc wrote: > >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 pow= er > >dissipation than a a semiconductor switch alone will allow for a given a= ll > >up cost? DC operation is of primary interest, but AC gains should also b= e > >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 ne= ed > >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 perhap= s > 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 no= t > 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 maj= or > >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 t= he > >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 th= e > >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, th= e > >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 contac= t > > > > 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 > >-- > >http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > >View/change your membership options at > >http://mailman.mit.edu/mailman/listinfo/piclist > > > -- > 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 > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=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 .