DPRG mailing list archives: [DPRG] A Simple PWM Circuit Based on the 555 Timer

Mon Aug 18 15:09:00 CDT 2008

I applied a 4-AA battery pack which ended up being a little over 5 volts.
Anyways, I am getting erratic results from the motor. In particular, when I
turn the pot past a certain point, the motor makes a loud buzzing sound and
it seems the motor vibrates less. Any ideas as to what is going on with the
motor?

Also, does the MOSFET magnify the voltage or what is it doing? When I
connect the motor to pin 7 of the 555 timer, I get finer control of the
motor, meaning I can run it at slower speeds. With the motor's ground
connected to the MOSFET, the starting speed is very high. Also, it seems the
MOSFET inverts the pin 7 signal, which makes sense due to Ed's explanation
that the higher the voltage, the lesser the resistance between drain and
source (which is connected to ground).

Thanks again for helping me to understand this circuit more!
Peter

On Mon, Aug 18, 2008 at 12:28 PM, Ed Paradis <legomaniac at gmail.com> wrote:

> Hi Peter!
>
> On Mon, Aug 18, 2008 at 11:50 AM, Peter Chung <peterchung1031 at gmail.com>
> wrote:
> > I'm not sure if this is the correct spec I should be looking at but it
> says
> > the minimum Gate Threshold (VGS) voltage is 2V (max is 4V)... so the
> voltage
> > supplied shouldn't affect the MOSFET since this circuit requires a
> minimum
> > voltage of 3V... is this assumption correct?
>
> I've realized since my original reply that my description of MOSFET
> behavior is somewhat inaccurate.  What I described is fairly practical
> information, but don't use it to answer any questions on your
> Electronics 101 exam.
>
> You are right in that the MOSFET datasheet says 2v to 4v for the Vgs.
> The threshold voltage refers to the point at which conduction starts.
> You'll notice that the sheet says ID (the current through the Drain)
> is only 250 microamps.
>
> Googling around, I saw similar pager motors to yours spec'd at 30
> milliamps.  You'll want the Drain Current to be at least this much, so
> that the MOSFET isn't limiting the motor.
>
> Check out Figure 3 on that datasheet.  You'll see that the lowest Vgs
> they show on the graph is 4 volts. The slope of those curves is the
> resistance of the MOSFET.  Notice how steep the curves get when they
> get close to 4 volts.  This means the lower the voltage, the more the
> resistance.  Notice also that at 6 volts, the MOSFET would allow 60
> amps to flow through it!  That's a huge amount of power, which means
> the resistance must be very low.
>
> In person, I could draw you some graphs and show you how to compute
> exactly what voltage you'll need.  I'd look for a good MOSFET tutorial
> if you're interested in designing circuits like this in the future.
>
> But back to the questions.
>
> > So are you saying the higher voltage I provide to this circuit, the
> better?
>
> For this specific problem (turning on the MOSFET), yes.  At some
> point, you provide too much voltage and everything catches on fire.
> Don't go above 15 volts; the 555 can't handle any above that.
>
> I'd say 6 volts will make that MOSFET have very little resistance when
> turned on.
>
> > The motor I am using is a 3VDC Micro Vibration Motor from Radio Shack.
> Would
> > the higher voltage damage the motor?
>
> It might.  If a higher voltage doesn't out right cook the motor, you
> might be able to get away with a higher voltage by using the motor
> less.  You can only PWM up to a duty cycle where the motor is seeing
> an "average" of 3v.
>
> For example, if you PWM at 25%, the motor is only seeing an "average"
> of 25% of the supply voltage.  This is really for large inductive
> loads, which your tiny motor is not.  You might be able to get away
> with it.  Experiment!  Remember to write all the settings down so when
> you blow up your motor, you can remember the setting right be fore it
> blew up.  ;)
>
> Perhaps someone with more experience with these little motors can speak up?
>
> > Thank you again so very much for helping me out!
>
> You are very welcome!
>
> This is all rule-of-thumb sort of "engineering".  Some of the other
> people on this list that do this sort of thing for a living could give
> a more precise answer.  That being said, I think 6 volts will work if
> you don't crank up the duty cycle too high.
>
> Ed
>
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