HELP!! Resistor value for LED needed

Richard Sinkler · From: aka: Rusty Strings -- Missoula, Montana

I am building an A/B box that will have LEDs to tell whether A or B is active. I have tried the small LEDS with the 2 legs, but am having too hard of a time trying to solder a resistor onto one of the legs. So, I have decided to go with prewired LEDs, but they are 12 volt (see link below). They do not come with resistors. I have a 12v wall wart I can use, and it is rated at .15 amps, 1.8 watts, which will probably cook the LEDs. What value of resistor would I need?

I know I could just buy an A/B box, but I like doing this kind of stuff and want to build one. Decades ago, I built one to plug my steel and a synth (what I plan to do with this one) into and output to my volume pedal. It didn't have LEDs though, and would like this one to have them.

Thanks!

https://www.amazon.com/gp/product/B07WRQRXZD/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&th=1
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Carter D10 8p/8k, Dekley S10 3p/4k C6 setup,Regal RD40 Dobro, Recording King Professional Dobro, NV400, NV112,Ibanez Gio guitar, Epiphone SG Special (open D slide guitar) . Playing for 54 years and still counting.

Bill A. Moore · From: Silver City, New Mexico, USA

Richard, they are intended for automotive applications, so they will withstand more than 14V. I don't think the current matters, as a battery supplies a few hundred amps for cranking.
I have used LEDS as indicators in cars for years, usually add a 1K ohm resistor. (9V battery would also work, just remember to turn it off). Some use a shorting switch in the input, (like Boss pedals), to disconnect the grounded side.

ajm · From: Los Angeles

Richard: I would try to stick with using 9 VDC as your supply as opposed to 12v.
It is the standard for effects pedals.
If you decide to use your new box on a pedal board it will make things easier.

Your new LED/lamp in the link: Do you actually have them yet?
If so, try one with a 9v battery and see if it works. If it does, problem solved.
I don't believe that you need a current limiting resistor with those, but I'm sure that someone here will tell me if I'm wrong.
FWIW, these look like they should be available at an auto parts store.

**** Edited, after researching the link you sent........
It appears that they draw about 30 ma at 12v. (That is in the ball park of a general 9v effects pedal.)
One guy tried it at 5v and it wasn't bright enough for him.
So........I'd try one at 9v. It just might work for you.

However, if you decide to use a good old standard LED, you will need a resistor for those.
If you decide to go back to a standard LED let me know.
Figuring out the resistor value is a piece of cake.

Richard Sinkler · From: aka: Rusty Strings -- Missoula, Montana

I don't have the 12 v LEDs yet (in transit). I have all the parts needed to make this, including a 9v wall wart. I just thought to use the 12 volt wall wart because I have one and don't have the item it was used to power. But I think I will just return the 12 volt LEDs and use what I have. I found that I can use butt connectors that I have to mount in the leg of the LED and connect the wire to the other side of it. I have had no luck soldering a wire to the LED leg, thus the reason for getting prewired LEDs. It calls for a 1/4 watt resistor, but Parts Express only has 1/2 watt and 1 watt resistors (what a disappointment to me - They are my "go to" electronics parts store). I got 1/2 watt resistors from them, but found 3k ohm 1/4 watt resistors on Amazon that I ordered.

Here is the schematic I want to use:

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Carter D10 8p/8k, Dekley S10 3p/4k C6 setup,Regal RD40 Dobro, Recording King Professional Dobro, NV400, NV112,Ibanez Gio guitar, Epiphone SG Special (open D slide guitar) . Playing for 54 years and still counting.

Bill A. Moore · From: Silver City, New Mexico, USA

Richard, you may want to use 1/4 watt resistors for their size, but if you have a 1/2, 1, 5, or even 10 watt, the circuit doesn't care.

George Biner · From: Los Angeles, CA

If a 1/4W resistor is called for, you can use any resistor at or above that rating, so you have flexibility there.

LEDs always need a resistor in series -- the current (and resulting brightness) is set by the resistor (and the supply voltage).

When operating, the LED will have a forward voltage, about 2V roughly (look in datasheet for actual number) -- you subtract that from the supply voltage to get the voltage across the resistor -- pick an LED current -- then use Ohm's law to calculate the resistor value.

Example: 12V supply, 20mA LED current, 2V forward voltage:
V across resistor = 12-2V = 10V; Ohm's law is R=V/I, so R = 10V/0.02A = 500 ohms. (Pick nearest standard resistance value). Power dissipation in resistor is I^2*R = 0.2W

The Amazon LEDs I think must already have resistors built in, or else you wouldn't be able to put 12V across them. A downside is you cannot set the current, you just take what they give you, which might not be bright enough. (There seems to be some confusion on Amazon whether these have built-in resistors or not. I think they do but I can't prove it from the site. Product descriptions on Amazon are a travesty -- "buyer beware" doesn't even begin to describe buying stuff from them.)

Keep in mind there exist hi-brightness LEDs -- which are much brighter than the regular types and don't take any more current. Great for outdoor use of foot pedals.
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Richard Sinkler · From: aka: Rusty Strings -- Missoula, Montana

Looks like the 12v LEDs from Amazon have resistors built into them, according to an answer to my question on Amazon. Of course, someone else answered there is no resistor. I'll hook one up to my 12v power supply when they get here.
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Carter D10 8p/8k, Dekley S10 3p/4k C6 setup,Regal RD40 Dobro, Recording King Professional Dobro, NV400, NV112,Ibanez Gio guitar, Epiphone SG Special (open D slide guitar) . Playing for 54 years and still counting.

Bill Sinclair · From: Waynesboro, PA, USA

Almost every 9V wall wart I've measured with a volt meter reads 12-14 VDC unloaded. I'll bet the 12 volt automotive leds would work just fine with a "9V" supply.

Richard Sinkler · From: aka: Rusty Strings -- Missoula, Montana

Richard Sinkler · From: aka: Rusty Strings -- Missoula, Montana

The 12v LEDs do work with both 12v & 9v. Now to do some assembly.👏[/list]
_________________
Carter D10 8p/8k, Dekley S10 3p/4k C6 setup,Regal RD40 Dobro, Recording King Professional Dobro, NV400, NV112,Ibanez Gio guitar, Epiphone SG Special (open D slide guitar) . Playing for 54 years and still counting.

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