Potentiometer

[cholin]

This is a quick question answer situation.

[ChadTower]

Any pot that can handle 5v or greater will work.  If you go a ton higher than that, you'll simply see the max brightness be farther and farther from the end of the range.

[BobA]

The proper way to configure your LED for dimming is to put the proper resistor in series with the LED and POT.  That way the minimum resistance when your POT is at 0 Ohms is the resistor.  The value of the pot should be about double the value of the resistor to give you a range of dimming.   If your 5V led is made to work with 5V without a series resistor then you might have to experiment with a 100 or 200 Ohm pot.  Remember if you need a limiting resistor and you use only a pot you can fry the LED.  At minimum resistance the pot will allow more than max current to flow and you LED will pop.

BobA

[cholin]

I will have a resistor, heres a picture of what I want, this way I just want to know which potentiometer can output 5v MAX, 0v MIN when 5v is supplied.

[BobA]

Any pot can do that because it goes from 0 ohms to max res of the pot.  If you size it at 50 ohms then the max resistance of you circuit will be 100 ohms and the min resistance will be 50 ohms.   When the pot is at 0 ohms it has a OV drop and thus your circuit gets the full 5 V.   

If 50 ohms resistor will allow your LED to have max current then a 50 ohm pot will cut that in half.  A 100 ohm pot will give you about a third.  However since light output is not proportional to current you will have to try one to see if it give you what you want.  A 1000 ohm pot will for sure make sure your LED does not have any light output for about 3/4 of its range and thus the pot will probably only work in about 1/4 of its rotation. From No LED to Full Brightness in less then 1/4 of a turn.

BobA

[cholin]

Okay thanks alot, now here comes the problem, its not displayed in Ohms.  It says like 1k pot, 2k pot, etc.  Look at this page as an example:

http://futurlec.com/PotRot.shtml

Which one of THOSE should I buy then?  They are in 1/2 watts, although I know nothin about pots so I cannot say if thats sufficient.

[tetsujin]

I will have a resistor, heres a picture of what I want, this way I just want to know which potentiometer can output 5v MAX, 0v MIN when 5v is supplied.

Well, the potentiometer doesn't really output voltage at all..  it's a variable resistor.  The LED turns off when there's no longer enough current flowing through it to produce an appreciable amount of light.

Most likely your series resistor will be 1/4 watt, so 1/2 watt rating on the pot should be fine.

I'm not really sure how much resistance you need to put in-line before your LED will be unable to operate.  That's the main thing you want to match to the value of the potentiometer.  You might be able to get the minimum operating current for the LED from its datasheet, if you have that.  Otherwise, you might just need to experiment.

I'd start with a pot with maybe 4 times the resistance of your series resistor.  That'd probably cut it down low enough.  A higher rating on the pot will make the LED able to go dimmer, but it'll also mean more of the turning space will be in the "dim" range, so you won't have as much fine control over the brightness.  (In other words, all your "on" range will be too close together.)

[cholin]

Come to think of it, the LED doesnt have to go off at all, it just needs to have a fine brightness control.  It does not need to go super dim, but maybe half of what it could be, so if it operates at 4 volts now with a 50 ohm resistor, I should but maybe a 100ohm pot so that it just fine tunes.  I still need to know though, because it doesnt say "100 OHM POTENTIOMETER", it says "1k POTENTIOMETER" so I don't know the conversion.

[tetsujin]

Come to think of it, the LED doesnt have to go off at all, it just needs to have a fine brightness control.  It does not need to go super dim, but maybe half of what it could be, so if it operates at 4 volts now with a 50 ohm resistor, I should but maybe a 100ohm pot so that it just fine tunes.  I still need to know though, because it doesnt say "100 OHM POTENTIOMETER", it says "1k POTENTIOMETER" so I don't know the conversion.

1K = 1000 ohms.

[cholin]

So thats WAY to much then, whoa!  Are you sure about that though, because anything lower than 1k is measured in R, such as 10r, 600r, 999r, then it goes 1k, 100k, etc.

[tetsujin]

So thats WAY to much then, whoa!  Are you sure about that though, because anything lower than 1k is measured in R, such as 10r, 600r, 999r, then it goes 1k, 100k, etc.

I guess "R" must be for "Ohms of resistance" or something, then...

[ShinAce]

R is the equivalent of a decimal.

R10 is 0.1 ohms

500R is 500 ohm

as mentionned, k = 1000
m = 1 000 000

I'd say anything between 100 - 1k will be fine. Because if you use a 50 ohm pot, you will not be able to dim it by half. You'l dim it by about 10%. Vision is logarithmic. Better yet, get yourself a 1k audio(log) taper pot.

[cholin]

Are you sure?  Well, I know you're sure because Ive seen your ideas in my other topics.  Would this do though?  How much would it dim?

http://futurlec.com/Potentiometers/POT1KA.shtml

[ShinAce]

That's the one. It'll dim them down to about 30%. If you want even lower, go with a 5k. Hell yeah.

Are you sure about the 50 ohm resistor? Sounds like you're going for about 40+ mA. I used a 100 ohm, white led(measured 19 mA max) and a 2.5 k pot just now. Running off 5.2 V.

[cholin]

Im running off 5 volts and using Japala's LED calculator to figure it out.  I put 5 volts as the supply, 4 volts as the drop and 20 mA for current which came out to 50 ohms.  This way, the MAX that can possible go to the LED is 4 volts @ 20 mA which is the highest rated for the LEDs I was looking at, although this may change because Im buying different LEDs from a different place, so I bought some 68 ohm ones just to be safe.  I obviously dont understand pots because I thought that the highest it would turn is the highest voltage output and the lowest it would turn would be 0 volts, I had no idea there were so many types!  I don't see how its relevant to get a 5000 ohm resistor basically   I thought that 1k would be good enough so that after like 1/4 of a turn it would already run out of current because there would be 250ohms resistance.  The two things I can't seem to understand:  Relays and Potentiometers.

[ShinAce]

Are they blue?

[cholin]

White, very bright.  Look at the brightest ones from www.lsdiodes.com, it should be like those.  It said maximum voltage was 4 so thats where I got the figures from.

[Trimoor]

Cholin, cholin, cholin.  Yet another electronics post?

If you want to be certain the LEDs have the dynamic range of fully on to fully off, use the largest value potentiometer you can find.  2K will probably work fine.

A 1Mohm potentiometer will definitely be able to achieve zero brightness, but as a consequence, you will have very coarse tuning.

If you are obsessed with the brightness and want to fine tune it to the point of a 1 photon difference, use a 10-turn 5K potentiometer.

I'd recommend the POT2KA.  If you are unsatisfied, you can always change it later.

Remember how I explained wattages before?  If this potentiometer is to be used for a certain circuit mentioned in a previous thread, the wattage will be a product of the combined amperages of all the LEDs and the voltage.
Watts = .02A * X * 5 = .1X
Where X = number of LEDs

Relays are simple.  It's merely multiple switches controlled by a single electromagnet.  Wait till you get to transistors.  Those things will drive you nuts.
How relays work.

[cholin]

Trimoor, I love you   Simple as that, thanks.