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A (probably) simple RC circuit question

  1. Apr 29, 2007 #1
    Its been 30 years since my last Physics course so while I have the basic concepts I'm uncertain how to solve my current problem: I need to operate a pair of 12V solenoids (.5 seated amps, 4 watts each) with a periodic on/off cycle that gently ramps to full voltage (over about .5 to 1 sec) and then shuts off, repeating every 5-10 seconds (I'd love to be able to adjust this once my device is built and tested). It sounds like an RC circuit to me in theory but I'm lost as to how to proceed in reality at these voltage/amperage levels. I've looked into programmable controllers but this seems like over-kill for what strikes me as an essentially simple operation.

    I'd love any help or thoughts on where to go to educte myself on this topic.

  2. jcsd
  3. Apr 29, 2007 #2
    one 555 and a two bjts will do what you want, use one after the 555 (5-10 second adjustable pulse) with a drain resistor and cap that determines the rc time ramp of the base of the second bjt.
    Last edited: Apr 29, 2007
  4. Apr 29, 2007 #3
    what is ".5 seated amps"
  5. Apr 29, 2007 #4
    light_bulb: Thanks so much for the prompt response--it sounds like its very do-able but I'm strictly an amateur as far as real-world applications--would you mind taking a minute and breaking it down for me (or point me at a resource for understanding your response). don't know 555 or bjts and how do I figure out where to start to estimate values for resistor and capcitor. (sorry). thanks again.

    and to moe_3_moe: seated amps refers to the steady state draw when the solenoid is fully activated and in the held state--
  6. Apr 29, 2007 #5
    i made the circuit, should be adjustable from 3-15hz, and the ramp time also. didn't test it but i figure it's good for illustration. it is an astable circuit off the data sheet with a series resistor-cap off the discharge leg that is adjustable to turn the square wave into a slight saw. once again didn't test.

    Attached Files:

  7. Apr 30, 2007 #6
    wow. thanks very much-that's phenomenally helpful. I'll dig in and see if I can put this together. I really appreciate your time.
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