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### Topic: How do I know what resistors to use? (Read 2308 times)previous topic - next topic

#### WazzupDrewfus

##### Jan 18, 2014, 07:42 am
I understand Ohm's law and how to calculate the resistance, but I don't know where to get the information for the equation. To be specific, I don't know how to figure out how many Amp's my output is drawing. (Example: Red LED) How would I find that. I've read in other forums that it's 20mA, but I need to know HOW they know that, how they found that information.

-Drew

#### Grumpy_Mike

#1
##### Jan 18, 2014, 08:26 am
Quote
but I need to know HOW they know that

Every component has a data sheet associated with it. That figure is to be found in the data sheet, normally under maximum forward current. The figure of 20mA is typical of thousands of LEDs. You can also run an LED at lower currents than this but it will not be as bright. However running it at say 10mA will not be half as bright it will be brighter than that say three quarters as bright because perceived light brightness is not proportional to current. A good data sheet will also have curves that show light output against forward current.

#### scottyjr

#2
##### Jan 18, 2014, 12:51 pm
Quote
However running it at say 10mA will not be half as bright it will be brighter than that say three quarters as bright because perceived light brightness is not proportional to current.

Interesting! Thanks.

#### MarkT

#3
##### Jan 18, 2014, 01:41 pm
For modern high-brightness LEDs 20mA is very intense, and you'll probably
find 1mA works.  Early LEDs were so poor that 20mA wasn't bright enough to
see in daylight!  The efficiency has increased by about 1000 times!

The eye's reponse to light is approximately logarithmic, 10 times more current
will look about 2 or 3 times brighter only.  A point source of light is really hard to
judge anyway.

For calculating the series resistor for an LED you need to decide what current you want,
and also need to know the forward voltage of the LED for that current.

voltage across the resistor = supply voltage - forward voltage.

You calculate the resistance using this voltage and the current.

LED forward voltage depends on colour,  IR are about 1.1V, red is about 1.7V, blue
3.2V.  White LEDs are simply blue/violet LEDs with a fluorescent lens and are 3.2 to 4V
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### batteryman

#4
##### Jan 18, 2014, 03:48 pm
Maybe not relevant to your application, but I'll mention it for completeness...
There is a type of LED which is used as a source of light, rather than just indication. These are CREE type LED's, and can have a forward current of several amps. They usually require dedicated driver circuits, rather than just current limiting resistors.

#### WazzupDrewfus

#5
##### Jan 18, 2014, 04:39 pm

Every component has a data sheet associated with it. That figure is to be found in the data sheet, normally under maximum forward current. The figure of 20mA is typical of thousands of LEDs. You can also run an LED at lower currents than this but it will not be as bright. However running it at say 10mA will not be half as bright it will be brighter than that say three quarters as bright because perceived light brightness is not proportional to current. A good data sheet will also have curves that show light output against forward current.

Ok that ^ brings a new light onto the subject for me. That would mean that I could use a 330 Ohm resistor if needed, because a 200 Ohm resistor wouldn't give enough resistance. (I=5/200 would mean that it would be getting a 25mA draw, right? Which is too much, isn't it?) Whereas a 330 Ohm resistor would have a 15mA draw. Do they even make 250 Ohm resistors? Which is the minimum resistance given in the equation with a 5v voltage and a 20mA draw. At this point I don't exactly know what I'm asking, or what I'm confused about. I'm new to this so I guess I'll learn more on my own over time. Experience is the best teacher.

#### Grumpy_Mike

#6
##### Jan 18, 2014, 07:59 pm
Quote
(I=5/200 would mean that it would be getting a 25mA draw, right? Which is too much, isn't it?)

It would be if that were the formula but you need to subtract the LED's forward voltage from the supply to get the voltage that will be dropped across the resistor. This forward voltage drop is again in the data sheet and mainly depends on the colour of the LED. Typical is 3V for a green one so a value of 200R would give a current of:-
(5-3)/ 200 = 10mA.

#### JimboZA

#7
##### Jan 18, 2014, 08:04 pm
Quote
brings a new light onto the subject for me

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