Determining how many ohms resistor is needed

Is there a general rule of thumb when it comes to determining how many ohms resistor is required for a particular purpose you're trying to achieve.

If I could get an example of this that would be even better. Thanks

Is there a general rule of thumb when it comes to determining how many ohms resistor is required for a particular purpose you’re trying to achieve.

Tell us what particular purpose you want to achieve…

Finding the proper ohms of a resistor is just applying ohms law formula when you know the voltage applied and what current the circuit requires. R= E/I

Lefty

after a few times of calculating them... you get to the stage where you jusy slap in a rough estimate...

resistance = volts / current..

mrxyz:
Is there a general rule of thumb when it comes to determining how many ohms resistor is required for a particular purpose you’re trying to achieve.

No, it’s always a proper calculation.

Every component has limits of current/voltage. The resistors are there to heep things within those limits.

cjdelphi: resistance = volts / current..

While that is true, don't forget to check the power the resistor has to dissipate. Resistors are specified with their value, but also with a maximum power rating. Most common (i guess) is 0.25 Watts. If you would like to use up all USB power of most pc ports by a resistor, you want to "burn" 0.5 A. That would bring you to U =5, I=0.5 so R =U/I or 5/0.5 = 10 Ohms. P=U*I; 5*0.5 = 2.5 Watts. So in this case, when using a 0.25 Watt resistor, the question is what will be blown first: the internal pc fuse for that USB port, or the resistor (with a nice optical effect too).

It won't harm to also do this small step if you're going calculate the resistance you need.

on top of that remember that some resistors can only take up to a certain voltage due to the methods of manufacture. mostly applies to SMd where i'm guessing too much voltage will just cause the voltage to arc from one terminal to the other due to the size.

there are a few "rule of thumb values" for example 1K generally allows a standard LEd to light on 5v although 470R might be better. If you need to put something in series with a very high impedence input just for safety sake make it 1-10K like between MCu and gate of a mosfet. A pullup resistor is generally 1-10K, same for a pull down. specific values may vary considerably depending on circumstance.

cjdelphi: after a few times of calculating them... you get to the stage where you jusy slap in a rough estimate...

resistance = volts / current..

This is the magic formula. I just wanted to add something that wasn't obvious to me at first. Suppose you're trying to find a resistor to go with an LED, and that LED has a forward voltage of 2.4V according to the datasheet. That means that the voltage is going to decrease by 2.4V as it passes through the LED. So to find the resistor value, you subtract the forward voltage of 2.4V from the power supply voltage (5V usually with the arduino) and you get 2.6V remaining. That's the number that you want to use for finding the resistor, not the supply voltage. If you wanted to supply the LED with 15ma (0.015 Amps) to keep it a little below it's max rating, that would be R = 2.6V / 0.015A. So R = 173. That's not a standard resistor value, so you'd use the next step up - 180 Ohms. If you did the same thing without taking the forward voltage into consideration you'd get a resistor value of 333 ohms, and would probably be pretty disappointed by how dim the LED was.

Correct, the days when LEd's were mostly run off 12V and were just puny 5-10mA 1K was standard