pwm 555, motor speed

Discussion in 'General Electronics Chat' started by ale2121, Apr 10, 2009.

  1. ale2121

    Thread Starter Active Member

    Mar 20, 2009
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    hi again.

    I am using this schematic:
    http://www.dprg.org/tutorials/2005-11a/index.html

    I have replaced Q1 with a tip122

    and both diodes have been replaced with 1n4001.

    i can hear a slight slow down of my motor, but it's not nearly what I want. Is this because of the pot value, R1? Will lowering the duty cycle help? I'm guessing a much larger pot will help me out?

    ps, don't you hate when you have piles of potentiometers and can't make out the values on any of them? arg.

    ok, and sorry one more edit...this is gonna sound REALLY dumb, but does a higher or lower frequency make a slower motor?
     
    Last edited: Apr 10, 2009
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    A couple of the pin-outs on the schematic are incorrectly shown in their connections.

    Pin 3 is the LM555 output pin - not pin 7. Pin 3 is the pin which should be driving the FET gate.

    Also pin 3 should not be looped back to the Pot wiper as shown. So pin 3 goes only to the gate!

    Pin 7 should be connected to the supply through the 10k as shown, AND it is also the pin that should loop back to the Pot wiper.
     
  3. ale2121

    Thread Starter Active Member

    Mar 20, 2009
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    ok. now my chip is overheating... thoughts?
     
  4. ale2121

    Thread Starter Active Member

    Mar 20, 2009
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    ok, i figured it out. i had my positive voltage running through the transistor, rather than ground. it appears the chi is no longer overheating and the motor runs at a much more controllable speed. Thanks!
     
  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    What you have explained in word sounds OK for the diode connections.

    If your pot is the round type with a spindle and three solder terminals then the wiper is most likely the center terminal.

    If you have a multimeter you could disconnect the motor and on DC range observe the output voltage on pin3. It should vary over a wide range from a fraction of the LM555 supply voltage to almost the full supply voltage. If your multimeter measures frequency all the better as you should be able to see the LM555's output frequency.

    I've also just noticed that in the circuit, the motor does not have a diode in parallel to avoid back EMF damage to the FET. Your FET may no longer be FET.
     
  6. ale2121

    Thread Starter Active Member

    Mar 20, 2009
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    no, i got it working, thank you. it was just that i stupidly had the positive voltage running through the transistor rather than ground. ok here are some unnecessary geek questions. could i have used say a six position switch, with different resistor values in line with each switch? it seems to jump occasionally if i let it run for a long time, it could be a bad connection somewhere, but could there be voltage spikes happening? and is there a way to compensate for that? a capacitor as a filter somewhere? or a different transistor? and this is really unrelated, but what happens when you connect two pots in series? i'm assuming the resistance would add across the two, but is there any typical application for something like that?
     
  7. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    I'd seriously consider adding the diode in parallel with the motor - a reasonable precaution. The diode cathode end would go the the motor positive supply point. A 1N4001 should be OK.

    Occasional speed jumping - the diode may help. Could be a commutator issue - is it a "cheap" motor? Or as you say could be an intermittent circuit connection.

    Yes you could have a multi-position switch with different resistors selected to give speed steps rather than the continuously variable case with the pot. The switch common contact would go to pin 7 and the various resistors would go from individual switch contacts on one side and join in common on the other at pin 2. Bit of design required to determine the required R values. Check the huge range of resources on the LM555 here or on the web.

    Two pots in series - not sure what you mean exactly - do you mean to avoid having to use the diodes in this particular circuit configuration?
     
  8. ale2121

    Thread Starter Active Member

    Mar 20, 2009
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    that's a no on the two pots in series. it's just when i was thinking of using a larger pot, i didn't have one and it occurred to me to use two. I didn't have to go that far, but I was wondering what it would accomplish. I also saw some dual pots at radio shack and was just wondering what they're about.

    and yeah, it's a really cheap motor.

    when you say diode in parallel with the motor, do you mean an extra one, or the two that are already in the circuit?
     
  9. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    The problem with using different R values with a switch will be the frequency will vary as well - you would actually try to adjust the duty cycle and keep the frequency relatively constant. That's the beauty of the use of the diodes to steer the timing capacitor charging and discharging paths.
     
  10. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Re the diode in parallel with the motor - yes you'll need a third diode.

    Re the pots - is this (see attachment) what you might have in mind for the ganged pots? - they should be linear rather than log type by the way.
     
  11. ale2121

    Thread Starter Active Member

    Mar 20, 2009
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    cool. thank you so much for your help t_n_k!
     
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