On 23 April 2015 at 22:13, IVP wrote: > I've been asked to revise the design of an old product. One of > the queries is whether the relays can be replaced with solid state > > Basically, the PIC controls 3 relays. An SPST on/off, which > switches 12V power to two DPDT, with a reversible DC motor > as the final load. > ... > > The SPST relay is 5A, the two DPDT are 3A, the motor stall > current is ~2A. The DPDT are activated before the power so > that they don't actually switch any current, just pass it I'd be happy for people to point out if I'm wrong here - any good cost competitive "solid state" low cost relay replacements would be welcome. Every decade or so it seems time to again see if relays have finally had their day. I've been investigating relay versus SSR for DC and AC use at somewhat higher power levels than you mention. Relays are, alas, still very good solutions in many cases. At around a few amps such as you are using - it is easy to get low cost solutions with designable long life times. There are numerous good quality relays available at acceptable prices. Omron relays are available in NZ at OKish prices and are a good starting point. They are not the only suitable brand (of course). Electrical wholesalers charge an arm and two legs for them. Jaycar slightly less. Even Farnement14 and RS are possibly cheaper. Digikey & maybe Mouser cheaper still but minimum order prices apply to avoid crippling freight costs. DO NOT trust the ratings of the incredibly cheap opto isolated Chinese modules being sold for "maker community" use. Ask me how I know :-). (I used these for experimental purposes and they are superb value for money when used well clear of their rated limits). Also note the brain dead contact locations on all (that I have seen) Chinese relay modules where the centre changeover contact of mains switching relays is located on the PCB physically between the low voltage contacts so that what could have been 10-20mm of creepage distance is instead only a few mm. Very sad. And at higher power levels - say a few 10's of amps, the losses in SSRs/ FETS/TRIACS/ ... tend to be substantially higher than contact losses. It becomes harder to design relays well at a few 10's of amps using COTS parts as alternatives are fewer and $ higher. But still doable. For eg mains hot water heater switching at 3 kW / 15A a relay has far lower contact losses and much lower price than an AC mains rated solid state SPST switch. For SPST DC use you can get MOSFETS with Rdson of a few milliOhms and 100A, 30V rating for acceptable prices, but as soon as you want an H bridge worth plus control logic a relay solution again becomes competitive. 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 .