### Author Topic: Simple resistor question  (Read 1009 times)

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#### Melcin Powell

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##### Simple resistor question
« on: August 11, 2010, 04:15:20 PM »
Hello once more i have another simple question pertaining to resisters and ohms law. I created an extremely simple circuit i used a 9v battery and a 340 ohm resistor to power a vibration motor from an xbox 360 remote i just wanted to see how fast it spun without the resistor, and then see how much it slowed down with it. The difference was drastic, but now i wanted to see how many volts were hitting the motor with the resistor attached (Correct me if im wrong i thought resistors also affected how many volts the motor got.) I used ohms law to discover there is 265 mA going through my circuit when i tried to figure out how much the resistor dropped the voltage i got 9v V = I X R. So to sum it up do resistors affect how much voltage goes through the circuit, and if so what did i do wrong?

Sorry for such a long question

Thanks in advance i appreciate it ~ Melvin Powell

#### Webbot

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##### Re: Simple resistor question
« Reply #1 on: August 11, 2010, 05:05:40 PM »
You have a 340 ohm resistor in series with a motor that has an unspecified resistance - lets call it Rm
V = I x  R

9 = I x (340 +Rm)

I = 9 / (340 + Rm)

But since you dont say what the resistance of the motor is then I cant see how you've estimated that I = 265mA

This is where a meter helps. Use it to measure the actual current 'I' and then you can calculate 'Rm'.

Rm = (9 / I) - 340

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#### Melcin Powell

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##### Re: Simple resistor question
« Reply #2 on: August 11, 2010, 07:30:46 PM »
Well i was just wondering how much does the 340 ohm resistor decrease it? Not including the motor can i do that?

#### Webbot

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##### Re: Simple resistor question
« Reply #3 on: August 11, 2010, 08:00:38 PM »
Well i was just wondering how much does the 340 ohm resistor decrease it?
What is "it" ? Voltage, current etc?

You wont know the answer to either unless you know the resistance or current draw of the motor on its own.
eg if you just connect the motor to the supply, with no other resistors, then how much current does it draw - you'll need a meter.
In practice its more complex...If you apply a load to the motor shaft (try grabbing it in your hand) then you'll see it needs more current. Since V is fixed at 9V then if the current goes up then ohms law indicates that the resistance of the motor is not constant.

You are probably just running the motor with no 'stress' and you will no doubt find that the resistance of the motor is quite small. Hence adding an equivalently 'huge' resistor means that the voltage drop across the resistor is quite big - hence starving the motor of voltage so it runs slowly.

Before others answer then "Yes - this is a very basic answer" to a basic question.
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#### waltr

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##### Re: Simple resistor question
« Reply #4 on: August 11, 2010, 08:30:22 PM »
Measure to voltage drop across the resistor. This with Ohm's law give the current flowing in the current. Since current is the same every where in a series circuit then that is also the current thought the motor.
One could also measure the voltage drop across the motor, or subtract the voltage drop across the resistor from the source voltage, and the current found earlier then use Ohm's law to calculator the equivalent motor resistance (motors are not purely resistive).

The resistor by itself does not effect the voltage, only the current. Only when a resistor is in series with another resistive element does the voltage across one resistor change. This is the common resistor divider.

#### Melcin Powell

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##### Re: Simple resistor question
« Reply #5 on: August 11, 2010, 09:51:50 PM »
Oh ok you guys are awesome. I appreciate all the help. I will get a multimeter and see how much resistance the motor gives. Its also good to know that voltage doesn't drop unless it is in series with another resistor.

Sorry if I am slow to pick up on things but i appreciate all the advice.