Zener 5v to 3.3v regulator

376

04:48:34

Please help.   I want to run a radio control model servo from a computer usb port at 3.3v (so it runs slower)

Testing the current draw with a multimeter from 3.24volts, it showed 0.06A so using ohm's law the servo load resistance should be 54 ohms right?

3.24 ÷ 0.06 = 54

When I test the resistance of the servo (by measuring the resistance across the positive and negative terminals of the motor) with the multimeter set to resistance 200 the reading is 08.0 which is 8 ohms right? Not 54 ohms.

Please explain what I am doing wrong.

published 12 years ago

glow2001

12 years ago

resistance on a stationary motor should be lower than when the the motor is running, SO I would imaging the same would apply for a servo

pip

12 years ago

When power is first applied the initial current would be 3.2 ÷ 8 for a few milliseconds and once the servo has started it's resistance will become proportional to load applied to the servo. Ie start current will be higher than load current. Make sure you have a large capacitor 1000uf should do across your zener so the motor can draw start current from this. You will need more current than you have aloud make your 28 ohm resistor at most 22ohm

waynewec

12 years ago

Servo's operate using internal control hardware. You'll have to read the data sheet for your particular servo to see if it can even work right at 3.3. Other than that I wouldn't worry about building a resistive divider like that - they are inefficient power wise and prone to having the voltage "droop" when the load has a current surge ie. Servo start up or high applications of torque (moving stuff). You can make a variable voltage supply with the same components plus an op amp and a mosfet that will perform perfectly. Or you can just get a lm317 from radioshack, a settable voltage regulator, that will do exactly what you need by the sound of it. I'll post the op amp regulator just cuz though.

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