PWM with 555 problem

Discussion in 'The Projects Forum' started by oopsSmoke, Jun 16, 2009.

  1. oopsSmoke

    Thread Starter Member

    Jun 16, 2009
    23
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    Hi all,

    I am working on a PWM circuit using a 555 timer to drive a 3.6V DC motor from a Black and Decker Versapak screwdriver. My power source is a 9.0V, 100mA wall wart. The circuit itself is breadboarded and outputs about 8.5V and .15mA with the potentiometer turned up. The problem I seem to be having, though, is that I can't even get it to run a cheap hobby DC motor.

    This is what my circuit is based on: http://www.dprg.org/tutorials/2005-11a/index.html. I've substituted the 100K potentiometer with a 35K pot and the IRFZ46N with an IRF640. I really don't know what all of the components do, I'm just putting them together to get a project to work. My best guess at the moment is to replace C1 with a larger one like a 470uF capacitor.

    If anyone can offer any assistance, insight, education, etc. I would greatly appreciate it.

    Kevin
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You would be better off with the original values - especially for the pot. That has a major effect on pulse width and rate. Adding a resistor in series with the FET gate migh help, too.

    I find it very odd that the FET drive does not come from pin 3.

    Here is a link to a tutorial on 555's - http://www.uoguelph.ca/~antoon/gadgets/555/555.html
     
    Last edited: Jun 16, 2009
  3. franzschluter

    Active Member

    Jun 1, 2009
    95
    0
    You need to know how much voltage you have in between Drain and Source of Q1 and you need to know your supply voltage for the 555. Upon this data you can compute for C1. This is must not be greater than 0.1uF.

    In any case try measuring the gate voltage to ground.. How much do you get?
    If you give up...

    Try this circuit...
    http://forum.allaboutcircuits.com/showthread.php?t=5524
    It is very similar.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    Part of the problem is you are trying to use a MOSFET on a 9V circuit. The 555 also doesn't go to rail, with 9V in it can only put out 7.8V, even worse. MOSFETs, unless they are design for it, need 10V gate voltage. There are MOSFET families that will do it, which is what they showed.

    You could replace the MOSFET with a BJT NPN power transistor, something like this...

    [​IMG]

    I just noticed their design left out the swamping diode across the motor, a major oversite.
     
    Last edited: Jun 16, 2009
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    I think your little power supply provides not nearly enough current to run a motor. Its voltage drops too low when the motor is powered. Measure it to see.
     
  6. kan_sai

    New Member

    Jun 16, 2009
    1
    0
    THANK 4 ALL
     
  7. oopsSmoke

    Thread Starter Member

    Jun 16, 2009
    23
    0
    Thanks for all the replies! I didn't get a chance to check this thread yesterday evening, but I will try some of the suggestions after work today.

    Bill_Marsden, I have a couple of TIP31B's. Will they work in the diagram you provided?

    Kevin
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    It should be fine, keep an eye on the case temperature though. It might need heatsinking (or not).
     
  9. ale2121

    Active Member

    Mar 20, 2009
    71
    0
    Hi! I did this exact same circuit. I consulted this forum for help, even. First off, there is some problem with the pins 7 and 3. There's and error in the schematic and they are switched. Also, I think you really have to use the 100k pot. i used a different diode as well. all the info is here, i got it working. http://forum.allaboutcircuits.com/showthread.php?t=22026
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    Actually their schematic will work, as will mine. Pin 3 is the output pin for a 555, pin 7 is the capacitor discharge pin for monostable timing, which goes to ground exactly in sync as pin 3. Their schematic works by grounding the gate of the FET, which will turn it off. They are using a logic FET, which takes much lower gate voltage to turn on. I prefer my technique, it uses less power overall.
     
    Last edited: Jun 17, 2009
  11. ale2121

    Active Member

    Mar 20, 2009
    71
    0
    i was referring to the pins in the original schematic, not the one you posted. someone explains why in the link i posted, and this is how i got the original schematic to work.
     
  12. Wendy

    Moderator

    Mar 24, 2008
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    I understand, but the original schematic will work. Pin 7, while not an output, does have ½ of driver in the form of a switch going to ground. They use a pull up resistor on the gate of the MOSFET. The same scheme would work on a BJT. Not the way I would do it, but it would work.

    The only thing I see wrong with their design is the lack of a back EMF diode across the motor, a major problem. The diode will keep the motor from blowing the transistor, which can happen to either MOSFETs or BJTs.
     
  13. oopsSmoke

    Thread Starter Member

    Jun 16, 2009
    23
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    I did some fiddling with it last night, but ended up throwing my hands up in frustration again. I swapped out the pot with a 100K, replaced the IRF640 with a TIP31B, and checked the votage between drain and source on the MOSFET and then again on the transistor between the collector and emitter (not sure if I should have done that, but it didn't seem to hurt anything). I don't think I checked the current, though.

    In the end, I still ended up getting 8.5ish volts on the leads to the motor with the pot turned up. Still no motor spin. I don't understand how I can hook a battery pack up to the motor and that works just fine, but the same motor hooked to this circuit does nothing.

    Oh yeah, when I was checking the voltage on Q1 my multimeter started giving me flakey readings after a while. Could be time for new batteries. Just one more thing to do...
     
    Last edited: Jun 18, 2009
  14. Wendy

    Moderator

    Mar 24, 2008
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    Are you using a protoboard? If so, have you seen this?

    555 PWM Oscillator

    It is a work in progress.

    The LEDs shown are a quick and dirty diagnostic.
     
  15. oopsSmoke

    Thread Starter Member

    Jun 16, 2009
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  16. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    It is called dirty circuit because they use the discharge transistor to drive the motor.
    I have not found the maximum allowed current in the datasheets.
    If the current gets to high the chip will go hot.

    Greetings,
    Bertus
     
  17. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    ST Microelectronics' datasheet for the NE555 gives a current limit of 15mA for the discharge pin 7. Attempting to sink current from a motor using pin 7 would not be a good idea.
     
  18. Wendy

    Moderator

    Mar 24, 2008
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    Use the transistor. The 555 will thank you, and if you oops it the transistor is cheaper (though not by much).
     
  19. Wendy

    Moderator

    Mar 24, 2008
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    I had never noticed that spec. Seems a capacitor would dump a lot more than that on the discharge.
     
  20. oopsSmoke

    Thread Starter Member

    Jun 16, 2009
    23
    0
    I swapped pins 3 and 7 so that now 7 goes to ground and 3 goes to the motor, per Bill's diagram. I assumed that 5 and 7 went to ground since they didn't seem to go anywhere. This changed did give me some positive results. I was able to get a small hobby motor to run, but the 3.6V motor that I am shooting for still isn't doing anything. At this point, I'm really tempted to pull it all apart and start over. Good thing the weekend is right around the corner!
     
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