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I am looking into a solid state relay to solve my problem here. I think I have found something that might work, but I am a little concerned as to weather it will work bidirectionally (I need DC current to flow in either direction).

The relay I am looking at is here. It has a maximum current of 9A (I need 6.5), and it also has a reasonable price tag.

Can someone please tell me if it could possibly be the solution that I am searching for, I have scoured the datasheet, but I cant find any mention of polarity on the switched side of things. If it isn't the right relay, can someone please point me in the right direction.

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  • \$\begingroup\$ I can't answer the second part, but the pin configuration shows a single MOSFET so it doesn't look bidirectional if that's complete and not just indicative. Also I'd image they would mention AC operation if it was capable of bidirectional operation. \$\endgroup\$
    – PeterJ
    Commented Apr 5, 2013 at 8:04
  • \$\begingroup\$ Could I use an AC relay? Those are available here:digikey.com/product-detail/en/S108T01F/425-2395-5-ND/720455 and here:digikey.com/product-detail/en/S108T02F/425-2396-5-ND/720456, but I just thought you wouldnt be able to use them for DC. \$\endgroup\$
    – Reid
    Commented Apr 5, 2013 at 8:10
  • \$\begingroup\$ The TRIAC based ones like that need to pass through zero volts to turn back off, so probably not. That's the most common for AC but I think what you're after does exist and uses a pair of MOSFETS but I'm just not aware of any off-hand. I'm sure someone will have some good advice on a suitable device or way to go about it, I was just commenting that particular one probably won't be suitable. \$\endgroup\$
    – PeterJ
    Commented Apr 5, 2013 at 8:14
  • \$\begingroup\$ I think the first link that I posted might work, as it says specifically in the datasheet that it is not zero cross, but it says "lower voltage DC", and I don't know how low that would be. (argh, going to keep looking) \$\endgroup\$
    – Reid
    Commented Apr 5, 2013 at 8:55

2 Answers 2

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The Solid State Relay linked in the question will not work for loads which could operate in either direction. For such purposes, search for SSRs specified for bidirectional DC or AC/DC loads (as distinct from just DC, AC or AC/Zero-crossing types).

While narrowing down to a specific such part will involve price, availability, voltage, package, size and perhaps other considerations, here are a few examples to illustrate that such relays do exist. Note how they are specified, to help extend the search for the ideal part:

  • Tyco MS14 SSR Solid State Relay with Bi-Directional Output
    • Notice that the Vload pin is indicated with a +/- in the diagrams: SSR configurations
  • From the same family as the CPC1709 SSR in the question, IXYS CPC1909 would work, with the constraint that its maximum rated current is 6.5 Amperes, just at the limit for the application in question. Hence, it is inadvisable to use this part.
    • Notice how the relay's switching portion is indicated in the diagram, with two FETs as opposed to the single FET in the CPC1709: CPC1909

Unfortunately Digikey does not seem to carry any bidirectional load SSRs specified for greater than 6.5 Amperes, so the search needs to continue elsewhere.

Alternatively, consider limiting the current across each SSR to below the maximum load current rating of the part.

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This may be a stupid idea but... the cells are grouped in series to produce the optimum output voltage, then there are parallel groups to give more current capability right? What if all the cells were paralleled to produce a very high current 3V6 battery? Then use a switching regulator to produce an output voltage that is appropriate for your system?

Because all the cells have a common 0V connection each can be isolated (via that 0V connection) with a single N channel FET.

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