# How do I choose the right resistor?

#### Sulvek

Joined Aug 27, 2022
20
Hello all! My question is when I look at my circuit and all the components how do I select the proper resistor? What is the formula? I assume I would need to know the max wattage of each component and the current supplied but not sure what the easiest formula would be. Also for example if I have a 10k ohm resistor at 1/2w, 5% will all 10k resistors be 1/2w, 5%? Thanks for the help!

#### WBahn

Joined Mar 31, 2012
27,903
Hello all! My question is when I look at my circuit and all the components how do I select the proper resistor? What is the formula? I assume I would need to know the max wattage of each component and the current supplied but not sure what the easiest formula would be. Also for example if I have a 10k ohm resistor at 1/2w, 5% will all 10k resistors be 1/2w, 5%? Thanks for the help!
You size a resistor based on what you are trying to achieve with it. Since you refer to "my circuit", the place to start is to ask yourself why you put a resistor there in the first place. Was it to limit the current? Produce a voltage? Set a gain? Establish a cutoff frequency? What?

As for your second question, of course not all 10 kΩ resistors will be whatever power and tolerance as whatever one you happen to have.

#### Marley

Joined Apr 4, 2016
489
So to specify a resistor:
• The resistance (in this case 10k = 10,000ohms)
• The tolerance (the accuracy of the resistance value). The importance of this depends on its function in the circuit. Closer tolerance (smaller %) costs more so is important for a volume manufacturer.
• Power dissipation (W). Depending on the voltage across the resistor and the current it is carrying, power will be dissipated and its temperature will rise. It is important that the resistor can cope with the power dissipation without getting too hot. High temperature will damage the resistor and other items near it. Also shorten its life.
• There are other characteristics that are important in some situations.
But of course all this is secondary. Why do you need a resistor in the first place? Usually to control the amount of current flowing in a section of the circuit. Sometimes (often in pairs) to create a certain value of voltage (from a higher voltage). Sometimes to attenuate a signal. Sometimes simply to create some heat!
Once you have decided you need one, than calculate the resistance required. Then knowing the operating voltage and current you can specify a power dissipation rating.

#### Jerry-Hat-Trick

Joined Aug 31, 2022
208
What is the formula?
Mot of the time, just Ohms Law! - V = IR. Power = VI = I^2 x R or V^2 /R
If capacitors are close by, it's maybe a filter, slightly more complex.
Many times, if two resistors are in series with the centre point going somewhere else, it's a potentail divider

#### dl324

Joined Mar 30, 2015
15,462
My question is when I look at my circuit and all the components how do I select the proper resistor?
It would be helpful if you posted a schematic of your circuit so we can see how they're being used.

If it's a circuit that you designed, you should be able to answer any questions regarding its usage.

#### Audioguru again

Joined Oct 21, 2019
5,420
The circuit he designed is completely wrong and is shown and is discussed in his other thread:

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#### Sulvek

Joined Aug 27, 2022
20
So to specify a resistor:
• The resistance (in this case 10k = 10,000ohms)
• The tolerance (the accuracy of the resistance value). The importance of this depends on its function in the circuit. Closer tolerance (smaller %) costs more so is important for a volume manufacturer.
• Power dissipation (W). Depending on the voltage across the resistor and the current it is carrying, power will be dissipated and its temperature will rise. It is important that the resistor can cope with the power dissipation without getting too hot. High temperature will damage the resistor and other items near it. Also shorten its life.
• There are other characteristics that are important in some situations.
But of course all this is secondary. Why do you need a resistor in the first place? Usually to control the amount of current flowing in a section of the circuit. Sometimes (often in pairs) to create a certain value of voltage (from a higher voltage). Sometimes to attenuate a signal. Sometimes simply to create some heat!
Once you have decided you need one, than calculate the resistance required. Then knowing the operating voltage and current you can specify a power dissipation rating.
Thanks! I understand now that resistors do a lot of things. At the moment my main focus is use resistors to protect components from my self. I really want to start playing with arduinos and shields. I can do some programming but hardware is my downfall. I want to make circuits and not destroy anything. I know its just a temporary fix untill I gain more knowledge but I am hoping resistors will be my helping hand to protect everything. I don't know any formulas though. For example if I have a simple led. How would I know what resistor to use? If I know the ratings of a component how do I use a formula to determine the proper resistor?

#### dl324

Joined Mar 30, 2015
15,462
For example if I have a simple led. How would I know what resistor to use? If I know the ratings of a component how do I use a formula to determine the proper resistor?
Assuming a simple circuit like this:

Assuming an LED forward voltage of 2V and a current of 10mA, the MOSFET is on and it's drain voltage is 0V:
$$\large R = \frac{V}{I} = \frac{V_{SUP}-V_{LED}}{I_{LED}}=\frac{5V-2V}{10mA}=300\Omega$$
$$\large P = I^2R = 10mA^2*300\Omega=30mW$$
300 ohms is a standard value and you could use a 1/8W (or larger) resistor.

#### WBahn

Joined Mar 31, 2012
27,903
Please don't respond to multiple posts with the exact same response. Either pick one post to quote, or quote all three, or don't quote any. Your comment is completely fine within the context of the thread discussion.

#### Audioguru again

Joined Oct 21, 2019
5,420
The audio power amplifier circuit you previously posted is completely wrong since the transistor and speaker will smoke and be destroyed.
Now you are talking about a completely different circuit to switch an LED on and off.

You must learn about electronics and learn about electronic parts to design those circuits.