[RE-wrenches] Solid state relay (SSR) questions

Wed Apr 29 11:55:27 PDT 2009

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.

cfreitas at outbackpower.com <mailto:cfreitas at 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-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Mick
Abraham
Sent: Wednesday, April 29, 2009 10:21 AM
To: re-wrenches at 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

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