is there a way to increase the light of an LED with a dial?

"Arfa Daily" wrote in news:3q3mj.5020$ snipped-for-privacy@newsfe1-win.ntli.net:

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And the pots don't work so well, once you let out some of the magic smoke, as I have found out. :)

I once put a reostat on a 6v lantern. It did save battery life. I did find out that the dimmer lamp worked better at shinning through the water at night, and full brightness just produced a glare. Yellow does not refract light like whiter light. Guess thats why fog lamps are yellow.

greg

Ok, I did some tests, borrowing a variable resistor and it works great

- now here is the concern : I am not sure what the maximum output of the SINGLE LED is - so I am trying to match up the MAX of the variable resistor with the MAX of the LED, so it doesn't blow. The power sources is TWO button batteries : CR2032 (3V) ontop of each other to the LED. It works great. Now, when I add the resistor, how can I tell which VARIABLE RESISTOR to buy because I want one that is going to give me the maximum output (without it burning out) at the top of the resister cycle of turns (I guess the best example is 1-10 on a volume of a stereo because I am a non-techincal person when it comes to this stuff). So I want the resister to top out at '10' (brightest) but also be able to dim down (but it doesn't necessary have to go to 0).

Thanks, I h> "Arfa Daily" wrote

innews:3q3mj.5020$ snipped-for-privacy@newsfe1-win.ntli.net:

If the batteries drive the LEDs directly, then there's no way that you can burn them out by turning the pot all the way up -- in either case, the net resistance in series with the LEDs is zero.

If there's a dropping resistor between the battery and the LEDs, the same is true, if you leave the resistor in place.

Yes a pot is a variable resistor. Don't they have something around 5K? That's a very common value. You can salvage this sort of thing out of all sorts of old junk. Any junky old boom box or other audio equipment will have a number of pots in it. Some are board mounted trimmers, some are panel mounted, they all do pretty much the same thing.

If you get one with too low a resistance (25 ohms is too low) you won't be able to dim very far at all. If you go too high, only the upper portion of the rotation will be useful and below that the LED will be off. Anything from 1K-10K should be usable. Rat Shack used to sell an asortment of mixed trimpots for a few dollars, not sure if they still do.

You might want to read up on this stuff so you understand the theory behind it rather than just asking for a specific part to get. Ohms law is really simple stuff if you understand basic algebra.

Probably not the actual volume control, as this will have a log taper, which will bunch the control operation up one end, in this application. Tone controls should be fine.

Arfa

Ok, I tried something with a 10K variable resistor today at a school today (they were kind to indulge me). When I hooked up the 2 3V batteries in series to the resistor, it dimmed immediately when I turned it only slightly. The falloff of power to the LED went to the dimmest setting right away. On a whim, we hooked it up to 12V and the dimmer worked all the way through the entire turn. It was great. I am a little confused why this happened? The LED comes with 2 3V batteries in series from the $ store and is quite bright. When hooked up to the 10K resistor, it isn't as bright. So, the next basic question is - what happened? Should i get a resistor that is less than 10K?

Thanks

LEDs are current driven. Which in practice means a different value resistor according to the supply voltage as you need to keep the current through the LED constant for the same light output.

You can work out this series resistor value easily. The formula is:-

Vs - Vf R = ------------ I

Where Vs is the supply voltage, Vf the forward voltage drop of the LED and I the current in amps. You'd need to know the spec of the actual LED to be accurate but making Vs 3V and I 0.020A will do for most white LEDs.

Which gives 150 ohms for 6 volts and 450 ohms for 12

The pot needs to go in series with a fixed resistor of the above value to set the maximum current - otherwise you could burn out the LED. The pot should be about 4 x the fixed resistor value to get a reasonably smooth dimming range.

I forgot to add - with the 12v, the LED went all the way to it brightest and dimmest while turning the resistor. It was perfect, but with the 5V, it peaked near the top of the resistors turn, and then started to burn out. Not sure why with 5V it would burn out, but with

Well that formula explains it all. To put it in really simple but not completely accurate terms, the resistor drops the voltage actually going to the LED. The bigger the resistor the lower the voltage that reaches the LED. So if you put a larger resistor in series before the LED you need higher input voltage to achieve the same brightness because your output voltage to the LED would be lower with a larger resistor. If you check this whith a voltmeter while you are doing this it will be apparent what is happening. Higher voltage= brigher light. Higer resistance = lower output voltage and dimmer light. Does it make sense to you now?

Mike

Mike

Try this site:

If you don't know the diode(LED) forward voltage and current go to RadioShack and get a new one. The info is on the LED packaging and the website under Tech specs. for the Power supple voltage, if in series add the volts if in parallel its the mean(NOT RECOMMENDED to use more than one cell(battery), unless they are all the same) i.e. voltage, size and amount charged. or that more powerful battery will try to charge the others.

In order to change or lower the brightness simply, take the output of the calculator measured in Ohms, and get a pot or (variable resistor) with the same value. that way you will get max power and brightness with your chosen LED.

Hope this Helps

Mark

And exactly what led you (no pun intended !) to this odd conclusion? I can't think of any practical or theoretical reason why this should have been the case. 12v and full range control, yes, no problem. 5v, and range of control bunched up one end. Yes, no problem. Then it was "burning out" ? What was, the pot or the LED ? If the pot, then I don't know why it should have been giving trouble at the lower voltage rather than the higher, but it does rather neatly underline the problems that can occur when you use a simple pot to control DC, rather than adding a 'helper' transistor as I suggested

Arfa

''

I'd probably have written something like that, even just a few years ago, but my feelings have changed. It's too easy to forget how all of us were ignorant at one time. If you feel a poster doesn't deserve help, then don't respond.

I'd like to offer the OP some advice, though... learn how to express yourself clearly. Your original post was atrocious, verging on the incoherent.

I'd also suggest the OP buy one of those 100-in-1 electronic-experiment kits to play with. They're fun, and you'll learn something.

Victory wrote:

Possibly the 12 volt output was current limited and the 5 v not so

Ron(UK)

Hmmm. It's a thought, Ron, but we're only talking a pretty small current draw here, and I'd be surprised if any current limiting on a schools type power supply would have the 'delicacy of touch' required to be able to limit the current to something tha would not burn out a white LED i.e. less than

Arfa