Solid State Latching Relays

Discussion in 'General Electronics Chat' started by washer, Apr 23, 2011.

  1. washer

    washer Thread Starter New Member

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    Hey all,

    It has been a while since I've posted here but I haven't seen this in the forums yet.

    I'm working on my senior design project and one part of our system involves individually controlling each plug in a standard power outlet (120VAC 20A). We were initially looking at a SPST electromechanical or solid state relay, leaning towards solid state if we could find one rated at the right amperage for the right price. After a few hours on Digikey, I'm guessing the industry standard is 16A and we should just say our project is rated at 15A. Is that what most products are rated at? I haven't been able to see anything mentioned on things like surge protectors.

    Since the goal of our main project is to encourage energy efficiency, we are trying to be as low-power as possible. We think a latching relay would be the best way to go so you aren't burning power in either state. Problem is, I haven't been able to find any off the shelf latching relays rated for 120VAC and at least 16A.

    I've been looking around at some circuits and thinking we may want to make our own discrete latching solid state relay. I think I have a good grasp on how a non-latching ssr circuit works, but I'm still a little confused about how we would implement a latching one. We hope to switch 120VAC at 20V with as little power possible from the controller. We have a few optocouplers on hand, and are thinking about picking up some of these SCRs and will be using an 8051 as our controller.

    Do you think a latching solid state relay is the right solution? And could you help explain how to implement the latching circuit or point me to a good reference? Also, I may just not be talking nice to Digikey today and there really is an off the shelf solution that fits our bill perfectly.

    We would greatly appreciate any help! We are all a tad bit embarrassed at how rusty we have gotten since our intro circuits classes...

    Thanks a ton,
    washer
  2. Adjuster

    Adjuster Well-Known Member

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    Personally I would not recommend a latching relay in such an application, even if you can find one. A normally-open type may be considered more fail-safe, unless you are considering appliances like freezers where it is important to avoid loss of supply (such things might anyway be better not connected to your system).

    Perhaps more importantly, non-latching also reduces the possibility of the relay state getting out of step with the controller requirement, for example due to a glitch caused by a momentary power outage.

    The power required to hold in a solid-state relay is likely to be of the order of 100mW, which may be pretty insignificant in comparison with the power consumed by the mains devices being controlled. Note that at full 20A load the SSR switch will likely dissipate about 30Watts, which is a bit more serious.

    There seem to be plenty of parts out there rated at 25A or more, but maybe your budget will not allow you to buy them. Here is an example:
    http://www.crydom.com/en/Products/Catalog/s_1_120.pdf
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=CC1002-ND
  3. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Most SSR (Solid State Relays) are simple. There is a LED circuit that can take a wide range of input voltages on the input, and a optically triggered TRIAC on the other side. I would not mess with this simplicity, instead using external circuitry to create effect you need. I am a fan of SSRs, their simplicity makes them relatively fool proof.
  4. ifixit

    ifixit Well-Known Member

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    Hi washer,

    Since you want to consume no power in either the on or off state then a solid-state solution is not possible. Refer to Adjuster's post.

    Google: latching relay mechanical

    Potter & brumfield have some relays that might work for you. There are two basic types, one coil that you pulse for a brief period, and one that has two coils (one for each state).

    Regards,
    Ifixit
  5. tracecom

    tracecom Well-Known Member

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    Any relay can be made to latch through its own contacts, however it does draw power when it is latched. Google latching relay schematic and look at the images.
  6. washer

    washer Thread Starter New Member

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    Thanks for the feedback guys. I guess I didn't really think through how a solid state relay could be bistable without using any power if the LED needs to be on in the closed state.

    I saw some of those Crydom relays on Digikey but we would really like to be able to build each outlet for under $50, which would be difficult if each one included 2 $30 relays.

    Our other constraint is size. We may not get past the breadboard for senior design, but the idea is to eventually get this small enough to embed inside an outlet box. With that restriction, I'm leaning towards the discrete parts or a solid state this one from Sharp.

    Are latching mechanical relays also always pulling power in one state? So the only difference in having it latching/bistable is that your control only needs to worry about it on the changing edges, right?
  7. beenthere

    beenthere AAC Fanatic!

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    Last edited: Apr 23, 2011
  8. Adjuster

    Adjuster Well-Known Member

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    Latching relays really do physically latch, in some cases due to the action of an internal permanent magnet. The drive requirement is for a pulse lasting a certain minimum time to effect a change of state. No power is required to maintain either state once it has been set up.

    Typically there are two coils: one to latch the relay, and the other to reset it. Some rarer designs may use a single coil requiring to be energised with current in one direction to latch, and in the opposite direction to reset. You would need to arrange your controller to generate the required drive pulses, and with the usual two coil arrangement this would include barring driving the two coils simultaneously.

    Frankly I would not recommend making your own mains switch circuit from scratch. This is risky territory, and the safety policy on this forum means that you are unlikely to be given any advice about that. Furthermore, the design of a good safe switch is not a trivial task. If you are going to build this in any practical form, use a commercial part for the switch, and accept that you may not be able to build a full-scale 20A design if you cannot afford more highly rated devices.
  9. tracecom

    tracecom Well-Known Member

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    I was describing a relay that is latched through its own contacts. In such an arrangement, the only way to release the relay is to interrupt the power to the coil.
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