Optoisolator for AC

Discussion in 'General Electronics Chat' started by cactusjack, Mar 10, 2004.

  1. cactusjack

    Thread Starter New Member

    Mar 10, 2004
    3
    0
    Hi,

    I am wondering if anyone knows of a optoisolator or something that I can hook the input side directly to 120V AC and get a sinking logic signal out the other side?

    I am having a tough time finding one of these on Digi-Key. I guess I don't quite know what I am supposed to look for.

    I have worked with optos plenty on low DC voltage applications; I just am not sure how to detect whether the AC power is on or not.

    Thanks
     
  2. Harlan

    Active Member

    Feb 26, 2004
    46
    0
    Are you trying to determine that there is power at the input? or maybe that the circuit is drawing current? there are a couple things that would work beginning with a simple small step down transformer and then rectify that to feed the opto input, another possibilty is a current transformer where the current flow in the primary of the mains transformer developes a small signal on the secondary there for the opto. I will draw up the current transformer later but first lets determine what you are to try to use this for in application.
    ;)
    Harlan
     
  3. cactusjack

    Thread Starter New Member

    Mar 10, 2004
    3
    0
    I am simply trying have a TTL microprocessor detect whether or not a 120VAC solenoid is being powered at any given moment. I have no other simple way to accomplish this besides the power leads to it. The microprocessor is not running the solenoid but it needs to react to it being turned on and off.

    Thanks
     
  4. Harlan

    Active Member

    Feb 26, 2004
    46
    0
    Ok yeah!
    Let me ask a few simple specs then on the solenoid. I got the implied 120 VAC ok, but how much current operating does it take, can you get that for me. There are a couple of ways actually to do this. My next question is do you want to have knowledge of Electrical action or more from the mechanical action of the soenoid? Depends on the circumstances where I had one thjat Underwriters required me to show actual mechanical movement from the solenoid, rather than detect the triggered current. But BOth ways are possible and I like the current sense method the best really. I will draw up a small daig here shortly and post that as well.
    Harlan B)
     
  5. cactusjack

    Thread Starter New Member

    Mar 10, 2004
    3
    0
    I am not detecting motion, the only thing that I have to tap into, is the power wires going into the solenoid. Just the hot and nuetral.

    I do not have the current draw rating for the solenoid. It just says 120VAC 60Hz on it. I don't need to measure if it is drawing current. I just need to know if there is the presence of power on the hot and nuetral wires going to it. The wires go from being 0V (open circuit) to 120VAC.
     
  6. Harlan

    Active Member

    Feb 26, 2004
    46
    0
    Hey CactusJack:
    Ok here are a couple of circuits that may be of help. One is using a transformer which offers total isolation from power to the signal circuit. I implemented a Triad FS20-200 on the project I had, but select the transformer for your specific needs. I place the Secondary in series with the AC Line to the solenoid, and as current passes through the coil, the same current produces a feild in the transformer, and make sure the secondary current spec of the transformer is ample for the solenoid draw. So if the solenoid requires say 200 mA make sure the tranny is more than 200 mA in the secondary. The primary is then fed to the bridge rectifier, and you can add some small capacitance to the output. The voltage of the output will be determined by the amount of current passing through the secondary and the solenoid. So when the solenoid is active there is current and thus a voltage at the bridge. When there is no current to the solenoid then there is no output voltage.
    The second is using the same principle but with a small resistance for a voltage drop that is fed through decoupling capacitors (say 0.47 uF) non electrolytic rated at 250 volts or greater. That is fed to the bridge rectifier and again a small capacitance may be added to that. If I chose a 0.5 ohm resistor for the series or tap resistance in the AC line and there was 0.5 amp draw, I would see 0.25 volts across the resistor. When no current flows to the solenoid then there would be no voltage drop. I would use an OP amp to manage the DC output of the bridge here to adjust for proper DC level to the uP circuit.
    I do not prefer the Resistance method as the isolation is wary at best and if there was an isolattion cap failure, there would be a mess in the whole circuit.(Destructive) The transformer method offers the greatest safety to the circuit and is a bit more flexible really. Just choose the right transformer is all. One last possibility here is to use the resistor in series and then place the transformer across the resistance where you are stepping up the voltage of the Resistance drop.
    I will check back in a few hours and of course later on too. Hope this is helpful.
    Harlan :)
    I can see the images are way to big for here so here is a link to the images ok

    http://harlan.s5.com/forumstuff.html
     
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