Creating a DC input signal from AC power line.

Discussion in 'The Projects Forum' started by JonHopper, Jul 12, 2010.

  1. JonHopper

    Thread Starter New Member

    Jun 28, 2010
    Hello Everyone this is my first post here.

    I have a simple question that needs a fairly simple answer.

    I am creating a control system; there is a grinding wheel powered by 120 vac (60hz) that must be ON in order for certain functions to work (through RLL programming on an automation direct PLC).

    The DC input module accepts 12 to 24v DC input signals. My question is how to properly convert the AC signal into DC.

    I was planning on using a half wave rectifier, RC filter and perhaps a zener diode to bring me into the range i want (between 12 and 24v). I'm not sure if this is the easiest / cheapest / best way to go.

    Has anyone does anything similar? If so , please advise.

    Thank you in advance for any suggestions!
  2. BMorse

    AAC Fanatic!

    Sep 26, 2009
    A step down transformer + a bridge rectifier should do the trick for you.... you can get step down transformers pretty cheap that are used for powering 24 AC devices.... the transformer will provide better isolation from the mains power...

    B. Morse
  3. russ_hensel

    Distinguished Member

    Jan 11, 2009
    A 12 volt wall wart may be all you need. It is pretty much what B Morse described.
  4. JonHopper

    Thread Starter New Member

    Jun 28, 2010
    The wall wart's not a bad idea, actually. The only issue with these two options is that I do not want to use any power from the motor circuit - I am needing a control signal only.

    I was thinking a little harder and realized the absolute CHEAPEST and EASIEST would probably be a simple control relay... 120vac control signal on, run 24v line through it for continuity when the 120vac is present. Of course this is a mechanical solution but electromech relays last fairly long.
    Last edited: Jul 12, 2010
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    and if your smart you use a plug in style relay so if it ever goes bad you simply pop it out and plug in a new spare. Done all the time for machine controls,etc...
  6. John P

    AAC Fanatic!

    Oct 14, 2008
    Or, you could use an optically isolated connection. Maybe a capacitor in series with the LED in an optoisolator, and use the bipolar kind that has back to back LEDs, so it would conduct both ways? The capacitor would need to be rated for 200V or so.