need solid state relay

Discussion in 'The Projects Forum' started by RainbowVideo, Sep 21, 2012.

  1. RainbowVideo

    Thread Starter New Member

    Apr 10, 2012
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    I have a digital device that responds to programmed triggers and and outputs +3.3vdc 500ma upon the trigger command.

    I need to control a 12vdc .08ma LED.

    Attempted this using an SSR 5vdc 1A NO (plus 12v supply) to switch the LED (DigiKey Z2521-ND)

    Upon the trigger, the relay operates and the LED illuminates, but when the trigger opens, the LED remains on.

    The bottom load voltage on this particular SSR is 70v. It that the reason it does not open when the trigger is removed?

    Can anyone suggest an SSR that will work properly for this application and is reasonably priced (I need 56 of them)?
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Whoa, there are easier and cheaper ways to switch an 8mA load than an SSR. For instance, if you could live with a bit less current - say 5mA - you could use a quad comparator to control 4 LEDs. Or a hex schmitt trigger to control 6.

    My point is, there are a lot of easy options for switching LEDs. Are you certain you need a SSR?
     
  3. crutschow

    Expert

    Mar 14, 2008
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    What is the "bottom load voltage"? :confused:

    Need a circuit diagram of what you are doing.
     
  4. RainbowVideo

    Thread Starter New Member

    Apr 10, 2012
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    WayneH I just went with an ssr because it was the only single component solution my limited knowledge and understanding could think of. I'm very open to any solution that I can solve for a couple of dollars per node. I can live with a lot less current. The LEDs are 12v .08 per circuit. I may need to add up to 3 per control circuit but still within the 5ma range.

    Cruschow That particular component gave a 'load range' in the specs. 70v was the bottom of the range (well over the 12v I'm using)
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Do you have any more details on the LEDs you want to switch? Your first post said 0.08mA, which is impossibly small. 0.08A is 80mA, which is possible but large for a standard LED and large enough to eliminate some options (such as the comparator). Making the right choice requires knowing the details.
     
  6. RainbowVideo

    Thread Starter New Member

    Apr 10, 2012
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    sorry about that. the salesman said .08ma, I didn't even question it. Wasn't thinking in those terms. Went back to the specs, they are .125A and they work with either polarity. Beyond that, I don't know much about them. These are high output accent lights that will be used as part of a museum display.
     
  7. crutschow

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    Mar 14, 2008
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    If you look at the Z2521-ND SSR specs you will see that the output is AC (which means it likely has a TRIAC to control the current). That is why is won't turn off with a DC load. A Triac needs to have the current go through zero, as with an AC signal, to turn off. It won't turn off with a DC load. You need a DC out type SSR.
     
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  8. RainbowVideo

    Thread Starter New Member

    Apr 10, 2012
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    Thanks. To be honest, I was looking at input voltage and price as I swam through the sea of options. Should have been more careful. Unfortunately, I can't find one that is DC that meets the specs and is in the $2 to $3 range. Thus I am looking for other options.
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Either polarity? That's odd. Are they meant for 12v AC? A normal DC LED definitely needs a proper orientation.

    Anyway, now we're talking about switching a 12V, 0.125A load (your LEDs), right?

    For a single LED, or a few, just about any NPN transistor would work for you as long as it's rated for a continuous current of 0.2A or so. Eg. 2N5550.

    You could choose to use a logic-level MOSFET, a special transistor often used for this kind of switching. Eg. 2N7000

    Since you have so many lights you want to switch, you should be looking for multiple transistors on a single chip. This will save room, money and assembly effort.

    Hopefully someone here has experience with one of these and can make a recommendation.
     
  10. RainbowVideo

    Thread Starter New Member

    Apr 10, 2012
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    Thanks for the input. yes, it is definitely a DC device. I'll wait and see if anyone else has input.
     
  11. wayneh

    Expert

    Sep 9, 2010
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    Something like the UNL2803A is interesting. 8 transistors in one package.

    "Transistor array" is the term for what you're looking for.
     
  12. RainbowVideo

    Thread Starter New Member

    Apr 10, 2012
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    The uln2803A looks like the perfect solution, particularly since I'll be controlling groups of 8 lights on each of 7 installations. The only question left is on input voltage. The specs say 5v, the control device outputs 3.3v. I'm not sufficient knowledgeable of such things to know if that will work. The good news is, they're .35 each, so I can afford to play around a bit if needed. Thanks again for the input and direction.
     
  13. wayneh

    Expert

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    I'm not sure which spec you read, but the TI datasheet makes it fairly clear that the transistors will be fully "on" at 3v, so you should be good to go.

    These things are amazingly cheap, aren't they. I guess it shouldn't be surprising, considering they put billions of transistors on computer chips these days.
     
  14. RainbowVideo

    Thread Starter New Member

    Apr 10, 2012
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    It could be that the catalog spec may be conservative. For the price, I can hardly afford to not give this a try. This will be a far less expensive solution for no more labor. I'm looking forward to putting one together. Many many thanks for your expertise in guiding me to a solution.
     
  15. RainbowVideo

    Thread Starter New Member

    Apr 10, 2012
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    Bought some ULN2803A and it sort of works but not completely. On the input side, when +3.3vdc is applied, the output changes but not is the manner I expected. I have +12 from the power supply on pin 10. Pin 16 (11-16 all exhibit the same behavior) is connected to one side of the LED fixture and the other side of the fixture goes to common on the power supply.

    When the +3.3vdc is applied to pin 1 and pin 9 to ground, the fixture barely illuminates. I attempted to measure voltage on pin 16 when it was operating and it bounced around between .005 and .5.

    Remember that this LED fixture is 12vdc but operates with either polarity.

    I'm stumped. Any other thoughts or solutions?

    I'm probably back to an SSR if I can find one with a DC output.
     
  16. wayneh

    Expert

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    I assume you mean pin 18 (the one output that corresponds to pin 1 input)?

    If the voltage is close to zero as you observed, and LED is connected to V+, the LED should light.
     
  17. RainbowVideo

    Thread Starter New Member

    Apr 10, 2012
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  18. wayneh

    Expert

    Sep 9, 2010
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    Can we assume it's working?
     
  19. RainbowVideo

    Thread Starter New Member

    Apr 10, 2012
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    Works like a champ! I really appreciate the effort, information and expertise that have been shared to help me with this project.
     
  20. wayneh

    Expert

    Sep 9, 2010
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    Great! Thanks for coming back with feedback on the end result.
     
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