# How to calculate resistor values

• beefertoo
In summary, to control the speed of a 12V motor that draws 10 amps, using a series or parallel resistor will not be effective without using a large, impractical resistor that consumes a lot of power. The most common way to control the speed of a DC motor is through pulse width modulation (PWM). To do this, you will need an integrated PWM controller, which can be obtained at a low cost. This controller will output an on-off signal for a heavy duty transistor to supply the motor. With this method, you can effectively control the speed of your motor without wasting excess power.

#### beefertoo

if my 12v motor draws 10amp when running am i right to think if i put 6v on the same motor it will run at half speed? how would i work out the resistor needed to do it?

i think i need to be using ohm's law somehow but can't remember just how,its been a while since I've needed to do something similar...

thanks craig

p.s its for my car with only one speed at the moment

I'm afraid its not as simple as you may be imagining, a resistor in series or parallel with the motor won't have the effect you desire without the resistor being completely impractical and consuming just as much power as the motor. The most common way to control the speed of a DC motor is through pulse width modulation.

If you don't mind 'wasting' a lot of power - for instance, if you are only running slow for a short while - then a series resistor would be OK. Old Electric railway trains used resistors to eliminate over current on startup, for instance.
The other point is that you would need something more than 6V for the motor to run at half speed. 6V would represent only 1/4 Power into a resistive load.
If you want to go for a 'brute force and ignorance' approach (not a bad idea to start with) you could obtain a selection of series high power resistors (tens of Watts rating) of 0.1, 0.3 and 0.5 ohm sort of values. Don't touch them 'cos they'll get hot! Not very pretty but it could do what you want. You could even use a long length of ordinary electrical flex (no coiled up - it may melt the insulation if it is) Use both blue and brown conductors in series and measure the resistance with a multimeter. It's not difficult to get 0.5 ohms that way, using thinnish wire. Cheap cheap, too.

beefertoo said:
if my 12v motor draws 10amp when running am i right to think if i put 6v on the same motor it will run at half speed? how would i work out the resistor needed to do it?

i think i need to be using ohm's law somehow but can't remember just how,its been a while since I've needed to do something similar...

thanks craig

p.s its for my car with only one speed at the moment

What do you mean that this motor is for your car? What does it run?

To control the speed of the motor, you should be using pulse-width modulation (PWM) of the 12V drive signal.

thanks for the replies, looks like then its more complicated than i thought... it is for the car,the heater motor. i'll hold my hand up and say I'm not that up on electronics so making a 'pwm' is probably beyond me.

cheers craig

It's really not too bad. You can get an integrated PWM controller. All you have to do is give it a supply, attach some resistors and capacitors according to the data sheet, then it will output an on-off signal for a heavy duty transistor that supplies your motor. Turning a supply on and off can be just like splitting the voltage.

Look at the http://focus.ti.com/lit/ds/symlink/tl5001a.pdf" [Broken]. The data sheet shows a typical configuration that should work for you. Just be sure to replace the transistor with something that can handle 12 amps. Also, they're 1.65USD at Digi-key. It comes in a DIP package (Dual-inline-package) which means it can be mounted on a simple breadboard.

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## 1. How do I calculate the resistance value of a resistor?

To calculate the resistance value of a resistor, you need to know the color code or the resistance value in ohms. If you have the color code, you can use a color code calculator or a color code chart to determine the resistance value. If you have the resistance value in ohms, you can use the formula R = V/I, where R is the resistance value, V is the voltage, and I is the current.

## 2. What is the purpose of calculating resistor values?

The purpose of calculating resistor values is to determine the resistance needed to achieve a desired voltage or current in an electrical circuit. This allows for the proper selection and placement of resistors to ensure the circuit functions correctly.

## 3. How do I determine the tolerance of a resistor?

The tolerance of a resistor can be determined by reading the color bands on the resistor. The fourth color band represents the tolerance, and each color has a specific tolerance value. Alternatively, you can refer to the datasheet or manufacturer's specifications for the tolerance of a specific resistor.

## 4. What is the difference between series and parallel resistors?

In series resistors, the resistors are connected one after the other, creating a single path for current to flow through. The total resistance in a series circuit is the sum of each individual resistance. In parallel resistors, the resistors are connected side by side, creating multiple paths for current to flow through. The total resistance in a parallel circuit is less than the smallest individual resistance.

## 5. Can I use resistors in AC circuits?

Yes, resistors can be used in both AC and DC circuits. However, it is important to consider the frequency of the AC circuit when selecting resistors. High frequency AC circuits may require special types of resistors that can handle the rapid changes in current and voltage, such as surface mount resistors.