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### Topic: How do I know what resistor to use?? (Read 30621 times)previous topic - next topic

#### Westbam

##### Sep 06, 2006, 02:14 pm
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

#1
##### Sep 06, 2006, 03:11 pm
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

#2
##### Sep 07, 2006, 01:22 pm
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 amLast 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

#4
##### Sep 20, 2006, 04:11 pm
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?

#### haesslich

#5
##### Sep 21, 2006, 09:11 am
the difference between the 1/4W and 1/8W type is the maximum power which the resistor can stand. the electrical power is amperage * voltage. so in the case of a 1k resistor connected between an arduino output and ground:
5V is the output voltage of arduino, so the amperage is 5V/1kohm = 1mA. so the power is 5V*1mA = 5mW (V*I = Power -> V*V/R = Power). the 1/8W would be strong enough to stand this.

#### koji

#6
##### Sep 21, 2006, 11:37 amLast Edit: Sep 21, 2006, 11:42 am by koji Reason: 1
sorry:

I = V/R = 5V/1kOhm = 5mA      // how do you get 1mA?

Power = I * V = 5mA * 5V = 25mW     //  would still be enough.

?

best regards, koji.

#### haesslich

#7
##### Sep 22, 2006, 08:52 am
Quote
sorry:

I = V/R = 5V/1kOhm = 5mA      // how do you get 1mA?

you are right. my fault. sorry

#### koji

#8
##### Sep 22, 2006, 10:22 amLast Edit: Sep 22, 2006, 10:22 am by koji Reason: 1
cool  8-), thanks.

#### Nescafe

#9
##### Sep 26, 2006, 08:47 am
Hi,

I have read through and I am still having problem understanding which resistor to use. Currently, I have created a simple circuit with a 12V battery producing 23A (stated on the battery) connecting to a 150ohm resistor and then to a LED. This works fine...

Now, my question is, if I change the battery to a 12V 60A (car battery), can I still use the 150ohm resistor or will the resistor overheat...? Do you take the Amp produce by the baterries in to the calculation...? How do you apply the fomular to this...? Please advise, thanks...

Regards,
Sim

#10
##### Sep 26, 2006, 09:12 am
the Amp rating on the battery is misleading... that value is the amount of amps that you can potentially get from the battery but the amount of current that flows in a circuit depends strictly from the load (I'm sure it says A/h no just A which i something slightly different)

in this case the load is the LED which normally operates around 20mA so 0,02A @1,7V (red led)

150 Ohm is too low even for a 5v power supply

so what we need to "get rid of" is the difference from 12v and 1.7v Vr = 12  - 1,7 = 10.3
the resistor that we need will have 10.3V across it when the current is about 20mA

R = Vr / I = 10.3 / 0,02 = 515 ohm -> closest value = 560 ohm

there you go

massimo

#### Nescafe

#11
##### Sep 26, 2006, 09:47 am
Thank you. Your explanation is very clear. So, can I say that, if I use your calculation on my circuit, regardless which 12V batteries (with different potential Amp amount), it doesn't really matter, right...?

Actually, I am a telecommunication person and doing electronic stuff as a hobby too like the first person who posted this thread. My intention is change the light bulbs in my car to pure white LEDs. Similar to the website I have attached. Is there any advise that you can provide if I want to do this?

http://s40concepts.net/forum/index.php?t=msg&th=6736&start=0&rid=&S=8886a75ac3024342c68db556196a3cc4

I have one more question. When you say 150ohm is to low, does it mean that it will spoil the resistor or the LED? When will the resistor heats up..? Thank you again...

Regards,
Sim

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