What value resistor for LED?

[rutledj]

Can someone tell me what the correct values of series resistors to use with the super bright LED's.

Thanks,
Rut

[80k]

i recently used 15K for a superbright White LED as well as for a blue LED.  For the blues, i have gone as low as 3.3K, but usually i choose 10K or so.

A superbright violet LED, i used 4.7K.

try different ones.  and not all are the same.

[bassmeister]

If you really want your LED to shine, you could go way lower. Look in the datasheet for the LED for value "If max" (that should be f as an index, can it be written in some way, anyone?). Make your resistor a bit larger than Vsupply/If (in the case of a stomp box, normally 9/If). I use a clear blue LED in one pedal, having an "If max" of 20 mA. Using a 470 ohm resistor for this one gives about 19 mA of forward current. However, it's not an option if you don't run your pedal on an adapter. This high a current will make your pedal use up more battery than necessary. The life length of the LED will also decrease if you decrease the resistor value. For some people, the light might also become too intense.

[rutledj]

Thanks. Battery life would be a concern so I think I'll experiment and go try higher r values.

Rut

[spudulike]

The correct formula for series resistance is

Rs = (Vs - Vf)/If

where :

Rs = Series resistance in ohms
Vs = Supply voltage (usually 9V)
Vf = Forward voltage of the LED from the data sheet
If = Forward current from the data sheet

The series resistance restricts the current flowing through the LED and this should never exceed the data sheet max If. Be careful, because (for example) a blue LED can have a Vf of 5V whereas a red LED is typically 2V.

[niftydog]

heaps of these types of applets around that might help.

[bassmeister]

The correct formula for series resistance is

Rs = (Vs - Vf)/If

Of course it is :oops:. Double checking my values revealed that I really had used 220 ohms, having a Vf of 5.1 V. Perhaps I could blame this one on what time it was when I wrote that, taking a faulty example from my head...

[jabez]

Go to this link. http://www.stewmac.com/cgi-bin/hazel.cgi/hzpi/u/HzSt01140m151716140m160U0n0n0n0m15110n17170m/hazel.cgi?action=SERVE&sku=I-1561&item=freeinfo/print.html  Go to bottom of page.

I do it this way & it works very well. Lights shine bright & last long.

[Mark Hammer]

Over and above something that prevents the LED from being burnt up immediately, the "correct" value will be one that provides adequate illumination given the brightness of the LED, its desired visibility, its context, and the type of power supply.

For instance, if you play in dark clubs without a whole lot of spotlights, and the LED is a superbright situated in a vast expanse of 1590BB surface area, against a black background, with a 9v battery for power, then there are several reasons for using a resistor in the range of 15k-20k with a "superbright".

On the other hand, if you have a powered pedalboard that you use in a well-illuminated room (e.g., studio), the LED is crammed in between controls, and you foolishly used a "red" LED  in a pedal you painted red or orange, then you'll need it to be a little more visible, so think more in terms of 6.8k to 10k for the same LED.

I know that those chrome finish LED mounting things are very popular these days, and an indicator of "boutiqueness", but consider that the more traditional black plastic LED mounting brackets provide higher contrast by being a darker surround.  Now, I'm not saying you should always use black or indigo as your chassis colour.  Just note that your goal is not to make the LED work electronically, but to make it work visually, and that involves consideration of what would help and hinder its visibility - i.e., don't expect that an identical value limiting resistor can be stuck in the board each and every time, regardless of other factors.  Contrasting colours play a role there.

At the same time, another goal is to have the remainder of the circuit, which depends on the very same battery for its life-support, not have to fight for current, so one aims for optimal visibility conditions so as to be able to offer the most battery current to the rest of the circuit for the longest period of time.  Poor choices of factors affecting visibility will necessitate lower resistor values and higher LED current consumption, and good choices can keep battery drain to a minimum via higher resistor values.

Finally, keep an eye out for mcd (millicandle) values when you buy LEDs, higher values permit use of higher-value resistors for equivalent brightness.  Plenty of devices with at least 2000mcd of potential illumination are available at reasonable cost (traditional "garden variety" red LEDs are in the 300mcd and under range).

[scratch]

Mark, you sure have a way with words too!!! That's what I call a big picture, all around answer.