# Resistor size vs. LED brightness?



## Deane Johnson (Sep 19, 2016)

Does increasing the resistor size reduce the brightness of an LED? My thinking is that it doesn't, but I'd like to hear from the resident experts.

I'm installing some pole lighting (street lights) and one of them is a different brand and a little brighter than the others. It's a dual lamp pole with a supplied resistor that seems to measure about 470 ohms. The LEDs are rated at 3v. and I'm feeding it with 12v.

I have a dimmer on the group of lights, but want to balance this one to the others. My last ditch effort will be to try a little light yellow paint on the LED.


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## wvgca (Jan 21, 2013)

Nope ...
increasing the physical size of the resistor does NOT reduce the brightness ..
increasing the VALUE does, though , from roughly 470 ohm to 530 perhaps ??
it a try a see in most cases, if you know the values, it's a bit easier to use a calculator ..


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## highvoltage (Apr 6, 2014)

Deane Johnson said:


> Does increasing the resistor size reduce the brightness of an LED?


Are you talking about ohms or watts dissipated?



Deane Johnson said:


> I'm installing some pole lighting (street lights) and one of them is a different brand and a little brighter than the others. It's a dual lamp pole with a supplied resistor that seems to measure about 470 ohms. The LEDs are rated at 3v. and I'm feeding it with 12v.


Here's a link to an LED Resistor calculator. 

http://ledcalc.com/

I plugged in 12v as the supply voltage, 3v as the drop across the resistor, and played around with the desired LED current until I came up with a 470 ohm resistor. That resistor will supply about 20 ma to the LED.

Try different LED currents, you'll see that as you decrease the LED current (i.e. the brightness decreases), the resistor value goes up.


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## Deane Johnson (Sep 19, 2016)

When I typed this, I meant to say "increase the value". In other words, if the resistor is indeed 470 and I raise it to 680, does the brightness decrease.

To further complicate the issue, there are two LEDs in parallel with one resistor (470).

Probably the easiest way to find out is to substitute a 1k resistor for the 470 and view the result.


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## wvgca (Jan 21, 2013)

yup..
it might be 530 ohm or up...
easiest way to physically try


note: a phyically small [ 1/8 w] resistor is good for 125 milliwatts, about six leds


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## Lemonhawk (Sep 24, 2013)

Yes increasing the resistor value does dim them some what. LED's are current devices so there are limits to dimming the LED's in this manner. You get better brightness control driving them with a Pulse Width modulated control. Perhaps something like this would work better
https://www.amazon.com/FAVOLCANO-Si...844&sr=8-7&keywords=LED+brightness+controller


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## gregc (Apr 25, 2015)

Deane Johnson said:


> Probably the easiest way to find out is to substitute a 1k resistor for the 470 and view the result.


Since LEDs are diode, the voltage across them if fairly constant. The current will vary with the resistance in series. If the supply is 12V and the LEDs are 3V, there's 9V across the resistor -- 9ma for a 1k resistor. Halving the resistance to 500 results in 18 ma, which should be brighter.



Deane Johnson said:


> To further complicate the issue, there are two LEDs in parallel with one resistor (470).


the current thru parallel LEDs may be unequal. LEDs can be put in series so that the same current flows from each.

a smaller resistor can be used. There is 6V across the resistor if 2 LEDs are in series (6V). 6ma with a 1k resistor. 12 ma with a 500 ohm resistor.

with a 12V supply, you can put 3 LEDs in series. But there's no way to be sure what the current is if you put 4 3V LEDs in series w/o a resistor.


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## Deane Johnson (Sep 19, 2016)

Thanks to everyone who has helped here. In addition to possibly getting this fixed, it's a good learning experience.

Lemanhawk: I have all of my LEDs on dimmers so I can balance them for the most realistic mood. I hate overly bright LED that glare, so I try to balance them so they fit the scene at the best level. In this case, I had to add one pole from a different source and it needs the additional brightness control. When I'm able to find more of the original, I'll probably replace it. The originals have built in resistors, they're from the Netherlands.

If I can't find any of the originals, I could also put it on it's own dimmer and make life easy. It's not like I change them much, it's mostly a set and forget situation.


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## gunrunnerjohn (Nov 10, 2010)

FWIW, there's no real limitation on how low the light value from the LED can go by reducing the current, I can bring them down to where you need to have them in a dark room to see any glow from the LED looking at it directly. The light output is directly proportional to the current flow of most common LED's. This is with simply reducing the current with a linear regulator, you can also use PWM to dim LED's.


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