555 PWM Fan controller start up

Discussion in 'The Projects Forum' started by Sam__, Dec 22, 2012.

  1. Sam__

    Thread Starter New Member

    Dec 21, 2012
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    Hey,

    I'm new to this forum so please go easy on me. A little about me: I'm a second year EE student, but oddly not spent much time actually making circuits, I just seem to be doing maths all the time. Anyway I like playing with electronic things and figuring stuff out.

    I'm currently reacquainting myself with my breadboard and all these free TI chips I have by making a nice simple 555 PWM fan controller.

    I have designed and tested a nice controller that allows for 11-96% duty cycle via a VR. All well and good. But I was thinking what about the fan start up. If the duty cycle is set low then the fan may not spin up and this is not what I want.

    I've been trying to think how I could create a pulse to the fan for the 1-2s it needs to get moving but I'm stuck. I thought about having a monostable 555 that gives a pulse on power on but can't figure out how to get it to only give the pulse when the system is powered up. Is there a way with a monostable 555 or is there another way I'm completely missing that would be easier? I just need a pulse for 1-2s to help the fan spin up at the start.

    I was wondering if anyone could help me? Thanks in advance.
     
    Last edited: Dec 22, 2012
  2. crutschow

    Expert

    Mar 14, 2008
    12,974
    3,220
    You could use a MOSFET, a resistor and a capacitor. Does the PMW switch the high-side or low-side of the fan motor power?
     
  3. Sam__

    Thread Starter New Member

    Dec 21, 2012
    10
    1
    That sounds like an idea, what would the circuit for that look like? I've not got any experience using MOSFETs.

    The current circuit is a 2n2222 switching the low side but the plan is to get a slightly higher rated transistor in there to power fans up to 10-15W.
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,965
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    Is the fan in the Drain side or the Source side, and is it an Nfet or Pfet,

    if its an Nfet then use the Drain to drive the fan to the positive supply, also you could use another 555 as a one shot monostable that puts a positive pulse to the fet's gate via a diode to give the start up pulse.
     
  5. Sam__

    Thread Starter New Member

    Dec 21, 2012
    10
    1
    The fan is on the emitter side of the 2n2222 npn at the moment.

    I'm not currently using a FET and don't have any to hand so I'd prefer a solution that doesn't use one. Of course if that's the best/only option then I'll try and source some.
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Well put the fan in the collector side, then it will run better, you can still use the one shot to start it up if you need it with the diode + 1K resistor to the base .

    Can you post a diagram?

    PS Mosfets are easier to use than bipolar as they fully turn on and dont need a drive resistor,
     
    Last edited: Dec 23, 2012
  7. Sam__

    Thread Starter New Member

    Dec 21, 2012
    10
    1
    Might be a silly question but why will it work better on the collector side?

    I will get a diagram up, also good to know about the mosfets. Are there any models in particular I should look at? I'm going to run the fans at 12V and the max amperage will be 2A.
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    If you use the emitter it will give out a lower voltage, as for Fet numbers here are a few i use
    IRF830, IRF630, IRF520, IRF5210 ,IRF9620,
     
  9. Sam__

    Thread Starter New Member

    Dec 21, 2012
    10
    1
    Here's the diagram.

    [​IMG]

    Thanks for the codes. I'll look into them. Is there any way I could use a transistor to do the one shot start the in the meantime?
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    You need to put the load in the collector side, and use a 1K resistor as the transistor base feed,
    As for the 1 second pulse use the circuit here to add on to your 555, increase the 100k for longer pulse,
     
    Last edited: Dec 24, 2012
  11. Sam__

    Thread Starter New Member

    Dec 21, 2012
    10
    1
    Sorry I forgot to edit my diagram after I changed it on my breadboard. I did as you said and moved the load, also I forgot to add the 470 resistor I had as the transistor base feed. Will it make any difference if I change that to 1k?

    thanks for the diagram, I'll give that a go later.
     
  12. Sam__

    Thread Starter New Member

    Dec 21, 2012
    10
    1
    Just ran a simulation with your diagram and a 100k and 100uF and I get just over 1s pulse. Thanks a lot!
     
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