Resistor Value

L

lstrike

New Member
I am working on a project for school and would like to figure out the value I'd need to drop 4.5 volts down to 3 volts using only a resistor. According to Ohm's Law Voltage/Current = Resistance, I am using a 3 x AA battery pack as the main voltage supply, which is capable of outputting approximately 4.5 volts, but I want to drop that to 3 volts to safely operate 2 onboard motors. I know my voltage is 4.5 volts and I'm not sure about the current of AA batteries but when I checked 2 AA batteries I had using a multimeter, they had outputted about 159 mA.

I divided 4.5/159 and got about .0283 ohms.

I could use some help as to whether I used a good value for the current of the AA Batteries so it'll safely drop the voltage to 3V.
 
B

BeanieBots

Moderator
Unfortunately, it is nothing like as simple as that.
You can use a potential divider to reduce a voltage but only as long as that divider is not loaded by anything else.
When you put motors on the divider, the motor changes the lower part of the divider.
What's worse, is that the effective resistance of the motor changes with the mechanical load.
Realistically, it is not possible to use a potential divider to drop a voltage for use with motors.
You will need to use an active regulator. What type will depend on the current the motor draws.
We can get onto that later but let's get Ohm's Law right first.

You say you checked the current of the batteries to be 159mA.
How did you do that.
Batteries do not put out a current. They supply a voltage.
The LOAD draws a current which relates to the battery voltage and the resistance of the load.
 
erco

erco

Senior Member
Resistors steal torque at low rpms for DC motors. Instead, use two silicon diodes in series to drop 4.5v down to 3.1v. You can use more or fewer diodes for different voltages. Each diode drops about 0.7 volt. For finer control, Schottky diodes drop about 0.3 volt.
 
W

westaust55

Moderator
Further to the above, AA batteries are not intended for high current draw though 159 mA is not too bad.
The internal resistance of the batteries will result in some voltage drop at the battery terminals when the motors are operating.

Is the 159mA when one or both motors are operating?

Is the same batteries powering the PICAXE and the motors? If so then at least some isolation with a diode and electro cap will be required for the logic side to avoid resets when the motors start.
 
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