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

[Michael Kennedy]

Hi Mike.
Well, if you reckon on a maximum current of say 20mA per LED, then for 3,
that's 60mA. Depending on what rail voltage he's figuring on running it
from, there's a good chance that if he was just using a pot on its own -
even allowing for still using any original limiting resistor - that when
he got towards the low end of his pot, he might be getting towards the
dissipation limits of the track and the wiper structure. Most carbon track
pots that you are likely to pick up at a 'general' electronics store, are
only rated to a few mW, and are not designed to carry current. There are
exceptions such as wirewounds, and some plastic tracked types. It just
seemed to me that the addition of a 20c transistor guaranteed that the pot
would not be damaged the first time it was turned up.

Someone mentioned pulse drive, which is the better way to do it, both from
linearity of control and LED life angles, but you are then into either a
specialist driver IC, or something like a 555 timer IC and a couple of
extra Rs and Cs and *still* the pot as well.

Arfa

I wasn't saying that your idea was bad, but just thought it might be a
little over complicated considering the OP's apparent electronics knowledge.
I agree that using a transistor would be best for reliability. The only
reason I questioned you is that in the past I have used pots for exactly
this, but then again that was before all of these high power blue / white
LEDs exsisted too.

 

[Arfa Daily]

I wasn't saying that your idea was bad, but just thought it might be a
little over complicated considering the OP's apparent electronics
knowledge. I agree that using a transistor would be best for reliability.
The only reason I questioned you is that in the past I have used pots for
exactly this, but then again that was before all of these high power blue
/ white LEDs exsisted too.

No probs. It's just that I have used pots before in temporary 'power' lashup
applications, and have found todays minuscule tracks to really not be up to
the job. If the OP is just using say 6v worth of batteries - and he's gonna
need at least 4 volts worth to run a white LED - then it would probably be
OK, but the wipers on these modern pots are pretty delicate, and the contact
area on the track is very small, so I just figured that even given his
apparent limited electronic knowledge, a 'helper' transistor, which really
is very simple to connect into the circuit, would be a good idea, if the
scenario was that this thing was going to get turned up and down regularly.

Arfa

 

[bz]

.....

No probs. It's just that I have used pots before in temporary 'power'
lashup applications, and have found todays minuscule tracks to really
not be up to the job. If the OP is just using say 6v worth of batteries
- and he's gonna need at least 4 volts worth to run a white LED - then
it would probably be OK, but the wipers on these modern pots are pretty
delicate, and the contact area on the track is very small, so I just
figured that even given his apparent limited electronic knowledge, a
'helper' transistor, which really is very simple to connect into the
circuit, would be a good idea, if the scenario was that this thing was
going to get turned up and down regularly.

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


--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

[email protected] remove ch100-5 to avoid spam trap

 

[GregS]

No probs. It's just that I have used pots before in temporary 'power' lashup
applications, and have found todays minuscule tracks to really not be up to
the job. If the OP is just using say 6v worth of batteries - and he's gonna
need at least 4 volts worth to run a white LED - then it would probably be
OK, but the wipers on these modern pots are pretty delicate, and the contact
area on the track is very small, so I just figured that even given his
apparent limited electronic knowledge, a 'helper' transistor, which really
is very simple to connect into the circuit, would be a good idea, if the
scenario was that this thing was going to get turned up and down regularly.

Arfa

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

 

[Victory]

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 hope this is clear.

 

[William Sommerwerck]

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.

 

[Victory]

Ok, thanks for that. I just want to make sure I buy the variable
resistor that will work through the range the first time. I do not
like taking things back if I don't have to...
Does anyone know a part number for Radio Shack that will work with
what I want?

 

[Victory]

Ok, I called Radio Shack and they have 'potentiometers', which I hope
is the same thing. I just don't know which to pick because the one
they told me about is rated for 25 ohms...

 

[James Sweet]

Ok, I called Radio Shack and they have 'potentiometers', which I hope
is the same thing. I just don't know which to pick because the one
they told me about is rated for 25 ohms...

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.

 

[Victory]

Darn, I just threw away a stereo a month or so ago. I will have to go
to Value Village or goodwill and see what they have. Will any stereo
volume tuner do for a variable resistor?

 

[Arfa Daily]

Darn, I just threw away a stereo a month or so ago. I will have to go
to Value Village or goodwill and see what they have. Will any stereo
volume tuner do for a variable resistor?

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

 

[Victory]

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

 

[Dave Plowman (News)]

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?

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.

 

[Victory]

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
12V it did not.

 

[Michael Kennedy]

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
12V it did not.

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

 

[Mark Bovinich]

Try this site:
http://led.linear1.org/1led.wiz

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.

http://www.radioshack.com/search/index.jsp?kwCatId=&kw=led&origkw=led&sr=1

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

 

[Arfa Daily]

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
12V it did not.

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
*might* be a good idea.

Arfa

 

[Victory]

At the highschool it was on one of those 'tester' boards with
different voltages and such.

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
*might* be a good idea.

Arfa

''

 

[Smitty Two]

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
*might* be a good idea.

Arfa

The reason I suggested the OP secure a primer on electronics is that,
from his original post, and several subsequent ones, it's clear that he
doesn't even know that ohms law exists. Look at the topic -- he wants to
*increase* the brightness by adding resistance.

When he says "burn out," he means that the LED is no longer
illuminating. I wouldn't waste time with this guy, not because he's
ignorant, but because he's clearly choosing to remain ignorant through
laziness.

 

[William Sommerwerck]

When he says "burn out," he means that the LED is no longer
illuminating. I wouldn't waste time with this guy, not because
he's ignorant, but because he's clearly choosing to remain ignorant
through laziness.

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.