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how do you figure out what resistor to use for a 500k pot and a white 2.1v / 30mA LED with a 9V power supply? Answered

Topic pretty much describes it.


The thing that makes this tricky, is figuring out what to do with the 500K pot.  It's resistance is not really well matched to the range of resistance you'd want in series with your LED for the purposes of dimming it.  I mean there'd be just this tiny little range of angle, circa 1 degree (or 2*pi/360 radians) or so,  where you'd be able to do any adjusting. 

I think the thing to do would be to use the pot as a voltage divider, and then use the old NPN-transistor-as-voltage-controlled-current-source trick.  Also use two of them in a Darlington pair, because they're small... I mean because you need an input impedance that's bigger than the output impedance of the 500K pot as a voltage divider (which would be 250 K at best).

I actually built this thing, and it seems to work. 

Links to the full sized pictures:


oh, wow man. Thank you so much for taking the time to help out a new hobbyist like this. You have definitely gone above and beyond.

I think the important thing is that I had fun doing it.

thanks for the help, but I already know how to do the calculations for single or multiple LEDs, my problem is with potentiometers. I salvaged one from an old guitar and I want to use it on an LED, but i think the rating is too high and if i even slightly turn the dial it goes from green LED to red LED, and I was trying to see if I can add a/some resistors so i can get an led from off and slowly turn the dial to bright green.

Mm your question refers to a white LED - is there more to this story than your telling us?

haha yes I lied, it was really a green LED. but all in all, it's pretty irrelevant.
Steve had a great answer, but unfortunately I'm the type of person that needs to see things in action before I can understand it.

The LED is a current drive device. That is it operates because of the number of electrons flowing through it. More electrons (higher current ) you get a brighter light. Less current you get a dimmer light.

Your set up with the Pot is varying the voltage. As the LED has a very low resistance small changes to the voltage make HUGE difference to the current through the LED (Ohms law) so this is a poor way to dim an LED.

However both Steve and Jack have give forms of constant (but variable) current circuits that control how many electrons flow through the LED. With a constant current the voltage source doesn't matter and the LED will respond in a fairly linear manner to increases and reductions in the current supplied.

You are of course limited by the relevant characteristics of the LED so need to look at the data sheet. At some point the current gets to low to light it and at the other end so high it burns out.

Forget the pot.

A fixed resistor is (Vsupply-Vled)/ Iled.

If you want to make the LED adjustable, then a circuit like this works really well.

Copy of maths.jpgtrimmable current source.JPG

ok, how do I figure out what pot to go with an LED ( or anything for that matter )
There has to be some kind of logic, and I can't find it anywhere.
or is there a way to make the pot that I have to let's say a 250k or even a 10k pot (maybe with resistors?)
(I'm assuming that when a pot says "500K" it's talking about maximum Ohms.)

Topic still un-intelligible. What's the 500K pot go to do with it ? You can't control the brightness of the LED with a 500K pot, because a.) It might burn out b.) Your whole control range will be about 1/5000 of the range of the pot.