<html><head><style type="text/css"><!-- DIV {margin:0px;} --></style></head><body><div style="font-family:tahoma,new york,times,serif;font-size:10pt"><div>We've used Optoblocks (SSR's) for years in traffic signal work, very reliable.....<br><br>Max Balchowsky<br>SEE systems<br></div><div style="font-family: tahoma,new york,times,serif; font-size: 10pt;"><br><div style="font-family: times new roman,new york,times,serif; font-size: 12pt;"><font face="Tahoma" size="2"><hr size="1"><b><span style="font-weight: bold;">From:</span></b> Marv Dargatz <mdargatz@enphaseenergy.com><br><b><span style="font-weight: bold;">To:</span></b> RE-wrenches <re-wrenches@lists.re-wrenches.org><br><b><span style="font-weight: bold;">Sent:</span></b> Wednesday, April 29, 2009 10:33:51 AM<br><b><span style="font-weight: bold;">Subject:</span></b> Re: [RE-wrenches] Solid state relay (SSR) questions<br></font><br>
<title></title>
<font face="Verdana">Hey Mick,<br>
<br>
We used a lot of SSRs in wind turbine controls. Found them to be
extremely reliable. Overrating the device is a very good idea. Also
be careful of very low temperatures, as the Current Transfer Ratio
(CTR) of the optical isolation can degrade over time and eventually
cause failure to operate at very low temperatures. When used within
their ratings, they are extremely robust and reliable with a long
lifetime.<br>
<br>
<a target="_blank" href="http://Opto22.com">Opto22.com</a> is a very good resource.<br>
<br>
<br>
</font>
<pre class="moz-signature">See Ya!<br><br>Marv<br>Enphase Energy<br>707 763-4784 x7016</pre>
<br>
<br>
Mick Abraham wrote:
<blockquote type="cite">Hello, Wrenchers and special greetings to the silicon
gurus to whom I address these questions about solid state relays.<br>
<br>
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.<br>
<br>
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.<br>
<br>
Question 1: Are SSR's suitable for the variety of loads (including
motors) that this circuit might carry?<br>
<br>
Question 2: Should the SSR be massively over-rated relative to the
expected load (as with typical electronic design practice)? <br>
<br>
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.)<br>
<br>
Question 3: Should I instead bump up to model 7456K44 with a 50 amp
rating for this 30 amp pass through situation?<br>
<br>
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... <br>
<br>
Question 4: ...is it bad strategy to even consider using an SSR for
this application?<br>
<br>
Thanks in advance,<br>
<br clear="all">
Mick Abraham, Proprietor<br>
<a rel="nofollow" target="_blank" href="http://www.abrahamsolar.com">www.abrahamsolar.com</a><br>
<br>
Voice: 970-731-4675<br>
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