Changing LED - best to keep current Resistor or change?

[StuKeeler]

Hi everyone,

Sorry if this is a 'noob' question but I'm looking for a simple answer/general advice regarding stock or new resistor for new LED....

The pedal in question is my Ibanez LF7 (actually will be all the '7' series pedals I own - same principle), and the schematic can be viewed here

I have some new 3mm Green LEDs I want to put in, which have forward voltage of 3.4V and come with 330 ohm resistors for the 9V circuit (presuming it is all 9V right?).
But the stock LED (D9) has a 10K ohm resistor (R62) in place, which is much higher than 330 ohms!

So my simple question is - would it be advisable/any problems in swapping in the new resistor, or would it be better to leave the 10K one there? i.e. does that resistor affect anything else in the circuit/pedal and dropping it would cause problems?

Thanks in advance!

Stu

[mth5044]

Changing the 10k resistor won't do anything to the circuit, it goes from VCC to the resistor to a transistor set up as a switch to ground. VCC is shown as 9V (or whatever you put in the power jack) on that schematic. Using the 10k in the circuit won't hurt your new LED. I'd try it with the 10k and see if it's bright enough. Is the resistor SMD? If so, you're going to have trouble 1) getting it off and 2) putting a new resistor in it's place - that's if you can even identify what resistor it is.

There are a bunch of LED calculators online, do a google search for one. Enter the forward voltage, ma and supply voltage and it will let you know the resistor to use. I usually use something around 4k7, so 10k isn't too much higher.

[GGBB]

^ +1

I think the 10K might be a little too big - it'll give you about half a milliamp to drive the LED - but try it first.  I find that with the superbright LEDs I've been using, running them at about 1mA is just right for me - not too bright - which usually ends up being around the 6.8K mark.  On the other hand, 330 ohm will give you about 17mA which should drive the LED close to as bright as it can go, which if its a super/extra/ultra bright type, will be blinding.  Plus that's a lot of current which will drain a 9V battery significantly faster than 1mA.  But LEDs vary a lot and we don't know exactly what you've got - you'll need to test - breadboard the LED and try different values of resistors to see what you like best.

[StuKeeler]

Many thanks for the quick responses - that's clarified the few things I was wondering.

The new LED is pretty much a perfect match in size for the stock one and the stock resistor is a standard 4 band soldered to the board nearby, so replacement of both will be easy.

The forward current is stated as 20ma for the new LED, so calculators give the result of 330 ohms, but that does give a stated brightness of 14000 MCD so maybe it will be crazily bright (although they are 'wide' angle LEDs).

I'll be using the pedal(s) via power supplies on a pedalboard so batteries aren't really an issue, but I do appreciate the extra unnecessary drain factor.

[GGBB]

14000 MCD @ 20mA is crazy bright!  I think the 10K is probably going to be fine.