No. You don't want a zener diode; just an odinary silicon diode rated to carry about twice the motor's maximum (stalled) current.

...how would an ordinary silicon diode regulate the voltage to 3-3.3V? Isn't that what sets Zeners apart from regular diodes? #confused...

 

Okay, did a bunch of reading. I see why a regular diode would work since a silicon diode drops 0.7V - didn't know that. However, I would prefer to have a fairly stable voltage to power the motors so their intensity doesn't weaken as the battery drains. Right now, I'm trying to figure out what has a smaller dropout - a LDO linear regulator or a Zener diode. I'm trying to determine exactly how much higher the input voltage of a Zener regulator has to be to the output voltage, but I'm not finding a consistent answer. Is this a value listed in datasheets somewhere that I just haven't yet understood?

 

See here: http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00003758.pdf

VI control is one way of doing speeds. There used to be chips that did it. It does not require a sensor.
the idea is Vm=Vw-Im*Rw; m=motor and w=winding; Vw you can't measure directly, but Vm is proportional to RPM.

For an unloaded brushed motor, the DC voltage generated when the shaft turns is proportional to RPM when you draw virtually no current.

 

Here are some ideas for this task - a kind of Switch Mode Power Supply.

If there is access to the peripheral modules, you can use a PWM output (Pulse Width Modulator) to control a transistor. PWM preset is calculated by an ADC (Analog to Digital Converter) measurement and a little math.

I would say that it is enough to make this calculation just before starting the motor.

The components for this solution will be:
Medium power MOS transistor
Medium current Diode (flyback diode across the motor)
Two Resistors (voltage divider from the battery input)

 

Okay, did a bunch of reading.

If you take your PWM circuit and put a zener from the large cap to ground, the circuit will charge to the point where the zener starts to conduct and stay at that level. It's not variable, so you'll have to choose your zener wisely. Choose one to give you and average of 3.2 to 3.6V (you can set the average by sound). It should be fairly constant as the battery discharges. Here's what the output looks like during the turn-on period - the slow ramp-up, then steady state:

 

Try E-Bay.
http://www.ebay.com/itm/Auto-DC-DC-...174186?hash=item33a34eac2a:g:s1sAAMXQLs5Ru~~9

 

If you take your PWM circuit and put a zener from the large cap to ground, the circuit will charge to the point where the zener starts to conduct and stay at that level. It's not variable, so you'll have to choose your zener wisely. Choose one to give you and average of 3.2 to 3.6V (you can set the average by sound). It should be fairly constant as the battery discharges. Here's what the output looks like during the turn-on period - the slow ramp-up, then steady state:

Your scope shows 12V. I know that's what you originally designed the PWM for, but will this work with a lithium battery (4.2-3.2V range)? Besides guess and check, how do I "wisely" pick a zener?

 

Hook everything up to your battery (or how you plan on powering the final design). Use a pot with one end on VCC, the other to GND and the wiper on the input to the opamp with the big cap (remove the cap from the input) and vary it until you get the speed you want. Measure that value and select a zener as close to that voltage as you can.

Remove the pot and reconnect the cap and the timing resistor and put the zener across the capacitor. The zener will prevent the capacitor from ever reaching full charge (and thus full speed). The peak voltage will still be VCC, but the average voltage will be closer to 3.2-3.6V. AND, your motor will still soft-start.

 

You're awesome, @SLK001 !!!

 

You're awesome, @SLK001 !!!

I know!

 

. . . the humblest man who ever lived.

 

. . . the humblest man who ever lived.

Ah, shucks... that's me!!