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

"Victory" wrote in message
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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.