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Topic: How do I know what resistor to use?? (Read 15 times) previous topic - next topic

Westbam

Okay, NoObisch Q,

I wonder how I would know how many Ohm a resistor should be, when I am designing a circuit on my own.

Like the KnightRider example, they use 220 Ohm, why (and why use a resistor here anyway??)

I am also using Google offcours, but here I have a feeling I can ask stuff I don't get.
(I have a medical background, doing electronics as a hobby).

Thnx for the time and Greetz from the Netherlands!!

haesslich

Quote
Okay, NoObisch Q,

I wonder how I would know how many Ohm a resistor should be, when I am designing a circuit on my own.

Like the KnightRider example, they use 220 Ohm, why (and why use a resistor here anyway??)

I am also using Google offcours, but here I have a feeling I can ask stuff I don't get.
(I have a medical background, doing electronics as a hobby).

Thnx for the time and Greetz from the Netherlands!!


it's basically ohms law. all the time ;)
you need the 220 ohm in this case, because an LED equals a very low resistance as long as it is conductive. and the law of mister ohm says: amperage = voltage / resistance. the voltage on our digitaloutput is 5Volt, the resistance is very little, so we get a huge amperage. our microcontroller digital output is designed to offer ~20mA. it will may get destroyed when an amperage larger than that flows through it. so we use a 220ohm resistance to force an amperage below 20mA.

Westbam

:) thanks, so, because the LED has almost no resistance, and the voltage stays the same, the amperage gets very high, but our microcontroller can not provide more than ~20mA (without overheating it) so we us a resitor. I understand that, thank you!! :)

My calculations make it a 250 Ohm resistor, but a 220 Ohm should work as well??

(Now, we want to keep it below 20mA (=0.02A) at 5 volts, so we use Ohms law: R = V / I = 5 / 0.02 = 250)

daveD

#3
Sep 09, 2006, 03:58 am Last Edit: Sep 09, 2006, 03:59 am by daveD Reason: 1
You can use a standard value thats close to what you need. In this case use a 330 ohm and you'll be fine.

koji

hello: I bought today my first electronic components for my arduino. there were also resistors on my wishlist. I bought 1kOhm resistors. I had to choose between 1/8 and 1/4 watt resistors. I took the 1/8 ones. Is that correct and what is the different besides the componentsize?

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