555 one shot HELP

Discussion in 'General Electronics Chat' started by thaas386, Sep 26, 2006.

  1. thaas386

    Thread Starter New Member

    Sep 26, 2006
    Hi, I am building a 555 circuit to trigger a relay to source a 30 amp 120v load. I am wondering how to use a transistor on the output(pin3) to power the relay if neccesary. I also need the duty cycle of the relay to be variable from about 500 mSec to 2 Sec, can a relay handle that? Any thoughts would be helpful. The circuit is to control a small homemade spot welder.
  2. Søren

    Senior Member

    Sep 2, 2006

    For such an application, I'd certainly not want a set of relay contact points to weld together (a likely scenario, if not when new, then later). A semiconductor solution is IMO better and will be more precise in the timing, since there's no physical mass to move against springs that *will* age over time.
  3. hewad


    Sep 26, 2006
    I take it you are controlling a 30 amp 120 volt AC circuit and not rectified AC. I haven't looked at the control voltage requirements for a solid state relay but they usually require a small DC control voltage of 3-32 volts at pretty low current. The timer chip may run this control directly without any additional interface. Solid state relays are real cheap on Ebay. The duty cycle won't be exact since you don't really know when the AC waveform will shut off the relay after the control voltage has been removed. It should be more than precise enough for a spot welder though.
  4. mrmeval

    AAC Fanatic!

    Jun 30, 2006
    Ebay relay

    It says 240 but they work from 48 up to 240. If you're driving a large load you will want a heat sink.
    This one has one but is more money.

    Very simple to drive it this circuit is an example.
  5. thaas386

    Thread Starter New Member

    Sep 26, 2006
    Thanks to everyone, you solved all my issues with this project. I bought a 40Amp solid-state relay and the output driver circuit is EXACTLY what I was looking for, thanks again for your help, This is the best forum ever!