Solid State Switching Circuit

Discussion in 'The Projects Forum' started by LEDtech, Jul 20, 2009.

  1. LEDtech

    Thread Starter New Member

    Jul 20, 2009
    5
    0
    Hi,
    I'm trying to piece together a circuit which when triggered by 3Vdc will drive 3 purchased Solid State Relays, with at least 5 seconds delay between each one. This is because the equipment will draw 240Vac 20~30Amps from each, so I dont want all the equipment starting at once. The SSR trigger is 4~32Vdc.

    I though of some sort of a LED sequencer circuit with a DC output of about 4V ??

    I need the SSR to stay HIGH until the trigger voltage is dropped off. This could be up to 14 hours per day.

    Would really love to hear your ideas and help.
    thanks
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    A simple set reset flip flop, reset by your trigger voltage going low, and set by a simple timer (a 555 IC comes to mind).
     
  3. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    If you can measure the turn-on voltage of the solid state relay and it stays consistent, another way would be to charge a capacitor through a resistor. The capacitor's voltage is connected to the control lug on the SSR. When the capacitor's voltage is high enough to turn the SSR on, it does so. Use one resistor and capacitor per SSR. This should probably be powered through another switch like a relay so that when the power is shut off, the control lugs of the SSRs automatically all get grounded. The resistors can be made pots so you can adjust the startup delay.
     
  4. LEDtech

    Thread Starter New Member

    Jul 20, 2009
    5
    0
    Well yes this sounds ok ...
    Just to fill you in ... the trigger voltage will becoming from an LED driver board which is used in a big message display. The software has provisions to turn porwer on/off at a predefined time through a Solid State relay..

    So the DC output from this board is 3V 200mA and stays high as long as the "ON" state exisits ... then obviously drops to 0V when OFF ...

    The input voltage of the SSR ranges from 4~32VDC
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    SSRs are gravy, they are among the simpliest components to use I've ever met and used, quite like a true relay. The isolation between the high voltage triacs and DC input is excellent (optical), so even a lightning strike probably won't make it through.

    I love em.

    So I'm going to ask some basic questions, designed to gauge how much help you need. You are messing with house current, but I've met a lot of competent electricians that weren't into electronics.

    How would you rate yourself as an electrician?

    How would you rate yourself as an electronic technician?

    Are you up to designing this yourself, or are you needing help in this area.

    If you are able to build something from a minimum of information, how complete does the information have to be? For example, you recognize the ground symbol and understand it as it applies to an electronic circuit.

    We like to help, but sometimes it is hard gauging how much help is really needed.

    Another approach I was thinking of, if the display has a true digital output (of 3VDC), you could put simple comparitors with different RC constants to do the same thing. Since there are 4 comparitors to a package on a LM339 this might do the trick nicely, with minimum circuitry. It would also depend on the off state for the display lasting a while. Do you need something drawn up?
     
  6. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    That's the specification (probably printed on the SSR). If you plan to use the RC method I suggested, you'll want to measure the actual turn-on voltage of each relay. I had a box of them given to me (with the same turn-on spec) and, if I recall correctly, they all turned on between 2.5 and 3 volts. Of course, this was years ago and, in my chronologically-gifted state, I could be remembering the weight of my belly button lint in grams...
     
  7. LEDtech

    Thread Starter New Member

    Jul 20, 2009
    5
    0
    Thanks Someonesdad,
    Measurments were taken yesterday, and turned on @3.75V as I've got the ones with an LED mounted on them. From a bit of research, the ones without the LED triger at 3V

    The output of the control card of the Display is 3v 200mA
     
  8. LEDtech

    Thread Starter New Member

    Jul 20, 2009
    5
    0
    Hi Bill,
    My expertise in Electronics is;
    • Made PCBs from scratch, not all worked for the past 30 yrs and worked with countless kits...
    • Basic electronic knowledge (self taught), it would perhaps take me a couple of weeks to design this with the help of building block gathered from the internet and a lot of trial and error.
    • If I'm presented with a schematic I am able to then produce that circuit
    • and definately know how 240V bites !!!!
    Having said that, would really appreciate if help could be forwarded, as this circuit will be used in a customer's display.
     
  9. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    OK, here is a concept circuit of what I was describing earlier. I'm figuring on a LM339, which is open collector. Without the pullup it won't register an output.

    [​IMG]

    I tried for a straight RC for seconds, figure R1=47KΩ and C1=100µF is 4.7 seconds, etc. Vcc is 3VDC. I am also assuming a hard drive for the input, you may need to add a signal conditioner.

    I did this in a rush, so double check the polarity of the comparator inputs.
     
  10. LEDtech

    Thread Starter New Member

    Jul 20, 2009
    5
    0
    Bill,
    Much appreciated, I'll play arounf with this and see how it goes
     
Loading...