PWM Motor Controller

Discussion in 'The Projects Forum' started by samh93, Oct 12, 2011.

  1. samh93

    Thread Starter Member

    Oct 4, 2011
    36
    0
    Hey everyone. After some really helpful advice from everyone here I've decided to build a PWM circuit to control the speed of my motor.
    I have a circuit diagram I found on the internet that looks pretty simple to make. I'd just like some help to configure it with my motor settings which are:
    The motor runs on 8.4volts and 1.5amps and is a universal AC and DC motor.

    What do I need to change? What resistors, capacitors, diodes and transistor do I need to change to get it to work on my motor.

    Thanks guys

    Sam
     
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  2. #12

    Expert

    Nov 30, 2010
    16,348
    6,840
    Suggestion 1) Change the power supply to 9 volts.
     
  3. samh93

    Thread Starter Member

    Oct 4, 2011
    36
    0
    Thanks for the suggestion but changing the supply isn't an option unfortunatly. It'd make things so much easier wouldn't it. I just need to change that 12 volts in the diagram to 8.4. I'm not too familiar with physics and calculations so that's were I'm stuck :/

    Thanks

    Sam
     
  4. #12

    Expert

    Nov 30, 2010
    16,348
    6,840
    You only need to kill the extra voltage at the motor end. 3.6 volts is not the right answer because the driver transistor will eat up some voltage. The way to do this is E = IR
    Voltage = current times resistance. 4 instance, to kill 3 volts at 1.5 amps will need 2 ohms.

    For smoke resistance, P = IE
    Power = current times voltage
    1.5 amps x 3 volts across the resistor = 4.5 watts
    Us 'lectroheads always double the wattage rating to keep the resistors cooler, so buy a 10 watt resistor in the 2 ohm to 2.2 ohm range.
    Maybe Radio Shack has a 2 pack of 1 ohm, 10 watt resistors. Overkill, but cheaper than shipping costs.

    ps, install the resistance in series with the motor at the + terminal of the motor. Leave the diode connected from motor minus to the +12V line.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    If you don't want to waste power in a resistor, just limit the PWM duty-cycle to a 8.4/12 = 70% maximum. That will limit the average voltage at the motor to 8.4V.
     
    #12 likes this.
  6. samh93

    Thread Starter Member

    Oct 4, 2011
    36
    0
    Okay I've built the circuit and used a 10 watt resistor but It's not working. I've done everything correctly but the motor doesn't even spin. The 8 volt battery is fine and the motors fine because the motor spins when the PWM isn't in the circuit. I'm so confused, are there any diagrams that could help me here?

    Thanks

    Sam
     
  7. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    for one, if you followed the directions and added the resistors, it will not work, considering the circuit shows 12 volts as the power source, so these nice guys helped you to figure out how to get the power down to 8.4 volts, but now you say you are only powering it from an 8 volt battery!!! so after the circuit alterations, and the type of 555 you are using, you are not supplying enough power to the circuit for it to do what it needs to do to work.... if you are not using a low power 555, you need the 12 volts to run it.

    FYI : there are different manufacturers and variations of the 555 timer, so you may want to read the specs for the one you are using... http://en.wikipedia.org/wiki/555_timer_IC#Specifications
     
  8. samh93

    Thread Starter Member

    Oct 4, 2011
    36
    0
    Sorry guys I'm new to this. I'm not very good at explaining what I mean. When I said change the 12 volts in the diagram I should have said I was replacing that 12 volts with 8.4 volts.
    I'm gonna replace the battery with a 12 volt one I think, should make it easier.
    Thanks for the help everyone and for being patient with me :)

    Sam
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    You shouldn't have to change the circuit. Just apply 8.4V instead of 12V to it.
     
  10. samh93

    Thread Starter Member

    Oct 4, 2011
    36
    0
    thanks alot for the help guys, I appreciate it. I haven't got it working yet but i picked up the wrong capacitors so when I change them it should work.
    Thanks everyone

    Sam
     
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