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Basic electronics question

Former Member

Hi guys, this is my first post and I'm sorry if it's super basic -

 

I have a wall mounted ac -> dc adapter with current rating of 1200mA set to 12V. I wanted to create a 4-6v voltage to run a small DC motor.

 

I created a voltage divider on my bread board with 2x 1M ohm resistors. This gave me a 6V reading, but probably a very small current.

 

I kept dropping the R1 and R2 values from 100k to 10k to 4.7k and finally found that the motor only moved at 2 100 Ohm resistors (both R1 and R2)

 

While this worked, the resistors got really really hot. what am I doing wrong?

 

thanks in advance!

  • Hi Keshav,

     

    You cannot use a voltage divider, it is not intended for that purpose, try with a 7805, it is a cheap way, but not much efficient, and depending on the current that your motor needs it also can get hot.

     

    Hope it helps,

    Miguel

  • try with 5 watts power rated 100 ohm resistor to avoid heating up the resistor.

  • It's important to understand what you are doing when you hook up the motor in this way. See the figure below as I discuss this. Initially you are making the voltage divider and when you place your voltmeter in parallel with the resistor you are placing a 10 MOhm resistor in parallel with the original resistor. See the left side of the figure. If Ry is small enough, then the resistance of the multimeter doesn't appreciably affect the parallel combination of Ry and 10 Meg. When you remove your meter and hook up the motor, you are effectively hooking up about 10 ohms in parallel with the resistor Ry. Why 10 ohms? It's the resistance of the windings in the motor. The "resistance" of the motor is going to change as you apply more load to the motor. This will only work if 10 ohms is significantly larger than Ry, and since 10 ohms is already rather small, the current flow will be large and you will need high wattage resistors.

     

    So, you have two options: You can either use lower value resistors with high wattage, which will likely waste a lot of power, or you can use a regulator to provide the required voltage. Both options are good designs if they meet the requirements.

     

    image

  • in reply to michaelwylie

    thank you for the explanation and patience! Not entirely sure about the voltmeter creating a voltage divider, but I understand that I need to read up on some basics. Any resources would be gladly welcomed image