Regulating output voltage

Discussion in 'The Projects Forum' started by jreeves231, Sep 28, 2010.

  1. jreeves231

    Thread Starter New Member

    Sep 28, 2010
    Good afternoon guys,

    I was hoping someone might has some insight for me. I have a cordless stick vacuum and i am trying to lower the RPM of the brush roll when it is on the lo setting. The battery for the vac is a 12v, 9 cell pack. There are 2 settings, a high and a low. I have already done some analysis and i just need to regulate the voltage going to the brush bar motor to 5 volts to get the required rpm. I need to know what i would use or add to the circuit to accomplish this? I was thinking a Zener Diode, but i'm not sure.

    If you can help it would be much appreciated.

    Thanks in advance,

  2. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    As you lower the voltage you will also lower the torque. I would suggest you do a little research on PWM motor control. This way you will not have the same loss of torque.
  3. jreeves231

    Thread Starter New Member

    Sep 28, 2010
    That thought had crossed my mind but since there is a no loss of suction motor also on the vac then i'm not to worried in the loss of torque from the brush bar motor. I really just need to know what type of component to add to the power nozzle assembly to limit the input voltage to the brush bar motor to just 5 volts. What about a variable resistor?

    Thanks in advance for all your thoughts and help,

  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
    Varying the supply voltage is not how DC motors are controlled, it's more complicated. If you reduce supply voltage to drop the RPM you are basically stalling the motor.
    Last edited: Sep 29, 2010
  5. wayneh


    Sep 9, 2010
    Look up the datasheet for a 7805 regulator. A big concern dropping voltage this way, though, is heat. Whatever power the motor is taking will also be burnt off as heat by any resistive (linear) device like the 7805, and actually a bit more. 7/12 versus 5/12ths. IF you're motor draws less than an amp at 5v, AND it actually works at 5v, you might have a chance. You know, you might have more luck dropping to 9v instead.
  6. DonQ

    Active Member

    May 6, 2009
    Your motor current is probably fairly high, so the biggest part of the answer to your question is power.

    Just putting a zener in series is not generally a good solution. For example, if your motor will be drawing 5 amps (not an unreasonable number, maybe even somewhat low), you will also be dropping 12-5=7 volts across the zener. This is 7 Amps * 5 volts=35 watts, all of it as heat. You would have to dump 35 Watts of heat out of the system somehow. And besides, Radio-Shack doesn't even carry 35W zeners :D.

    This is the same for a proper resistor. 35Watts. It may be somewhat easier to find a 35 watt resistor, but it probably won't have a knob on it for adjusting so you'll probably have to do some measuring and calculating to determine what you need. In the end, this works even less well than the zener because as the load goes up, the current goes up, the voltage drop across the resistor goes up, and the voltage across the motor goes down. Exactly what you don't want as a response to a larger load.

    PWM is a slightly more complicated method of reducing the drive to a motor without dissipating hardly any heat. I'll leave that explanation to already available resources, but suffice it to say that it is probably more effort than you want to expend on slowing down your vacuum cleaner. However, as a learning project it would be a good thing, just quite a bit more effort than a Power Resistor or Zener.
  7. Pich

    Active Member

    Mar 11, 2008
    If you have an old variable speed cordless drill around that is of no use you can use the trigger to do the job.