In article , "Arfa Daily" wrote:

"Michael Kennedy" wrote in message
...

"Arfa Daily" wrote in message
...

"Michael Kennedy" wrote in message
. ..

"Arfa Daily" wrote in message
...

"Michael Kennedy" wrote in message
. ..

"James Sweet" wrote in message
news:Y7Alj.8931$4b6.560@trndny08...

"Victory" wrote in message

...
I guess I am looking for a dimmer or something for it - just a dial
that can turn the light up to full or down a little depending on
what
I want. But I want it to be controllable with a dial or something.
I
hope that is clearer.
Thanks for the quick response and looking forward to the next one!



You want a variable resistor. The value depends on the particular LED
but you might try 2.5K or 5K.


Yes, James ie right. You don't want a varible resistor like I said.
You want a variable resistor. haha. I'm terrible at typing.

Mike


It may not work with a simple variable resistor because at higher
settings, the current will increase enough to probably just burn out
the small piece of the pot track that's still in circuit. A fixed
resistor will also still be needed to limit the full brightness
current. In order to get a decent control range, without risk of
over-running the pot's track and wiper, I would suggest that a small
transistor needs throwing into the mix to control the LED current,
whilst the pot is left the low-current job of controlling the
transistor's base. If I get time today, I'll give it a try.

Arfa


Arfa, How much current are we talking about here? If this is what I'm
immagining in my head with 2 or 3 led's it shouldn't draw much power. I
guess he could have one of those 5w led flashlights, but that was not
what I had in mind when he said dollar store.

Mike

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.


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