Mick -
I would advise using a mechanical relay - probably a MDR (Mercury Displacement Relay) as they are very quiet, no arcing and very reliable. There still will be some noise made when the inverter's internal relay switches over - so not sure if using a SSR or MDR will really make that much difference. SSRs can be hard to troubleshoot when they do have problems... and they are expensive. One extremely good relay I have had very good luck with over the years is the old Potter & Brumfield PBR type of relays - big open frame type clunkers. Grainger stocks them and also offers a nice NEMA 1 "dust cover" metal enclosure to house them which has nice knockouts but is very compact. Only downside is a fairly good clunk when it switches. This is a DPDT type 30 amp relay - and would be the cheapest reliable option. Here is a link: http://www.grainger.com/Grainger/items/5X848 <http://www.grainger.com/Grainger/items/5X848> available also with 120VAC or 12 & 24 VDC coils. Relay: $ 34.50 Dust Cover: $ 16.00 Christopher Christopher Freitas Director of Research and Development OutBack Power Systems, Inc. cfrei...@outbackpower.com <mailto:cfrei...@outbackpower.com> Tel 360 435 6030 Cell 360 202 4239 19009 62nd Ave NE Arlington WA 98223 USA www.outbackpower.com <http://www.outbackpower.com/> ________________________________ From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Mick Abraham Sent: Wednesday, April 29, 2009 10:21 AM To: re-wrenches@lists.re-wrenches.org Subject: [RE-wrenches] Solid state relay (SSR) questions Hello, Wrenchers and special greetings to the silicon gurus to whom I address these questions about solid state relays. A client wants a gizmo to connect grid "AC In" to their inverter/charger when the battery voltage is low...resembling the LBX function on the good ol' Trace SW inverter. The grid power would feed into a 120/240 Magnum inverter/charger which is rated for 30 amps pass through on each hot leg. I may use a solid state relay for the heavy lifting: double-pole, single-throw, normally open...with DC for the control voltage. Using a voltage controlled switch, I'll send battery DC to the SSR when the battery voltage goes too low for too long; then I'll remove that DC trigger when the battery voltage recovers. Question 1: Are SSR's suitable for the variety of loads (including motors) that this circuit might carry? Question 2: Should the SSR be massively over-rated relative to the expected load (as with typical electronic design practice)? McMaster-Carr stocks solid state relays with built in heat sinks; model 7456K43 is published for 30 amps @ 230 volts AC. If protected with a 30 amp double-pole breaker, would this relay suffice long term or would it instead produce expensive smoke? (Let's assume free air conditions around the heat sink and a 90 degree F ambient temperature.) Question 3: Should I instead bump up to model 7456K44 with a 50 amp rating for this 30 amp pass through situation? I like the idea of no moving parts/no arcs/no clunks compared to a mechanical power relay, but I also want no customer callbacks. High speed switching (pulse width modulation) is not one of our requirements, so... Question 4: ...is it bad strategy to even consider using an SSR for this application? Thanks in advance, Mick Abraham, Proprietor www.abrahamsolar.com <http://www.abrahamsolar.com> Voice: 970-731-4675 MESSAGE CONFIDENTIALITY NOTICE: This message (including attachments) is covered by the Electronic Communication Privacy Act, 18 U.S.C. sections 2510-2521, is confidential, and may also be protected by attorney-client or other privilege. If you believe that it has been sent to you in error, please do not read it. If you are not the intended recipient,you are hereby notified that any retention, dissemination, distribution, or copying of this communication is strictly prohibited. Please reply to the sender that you have received the message in error, then delete it. Thank you.
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