Re: very simplistic potentiometer question

Barbara wrote:
> I am just beginning to tinker with electronics. I've decided to start
> very simply and work my way up to the fun stuff. I like the thought
> of playing with LEDs for some reason so I'm starting there. I've
> figured out already that without a resistor that LEDs last about .2
> seconds before they fry. Now that I can keep one going for more than
> 5 seconds I'd like to add a pot to control the brightness. I know a
> pot is a variable resistor that basically adds resistance. I had this
> idea that I would simply insert it between my resistor and the
> positive lead on my power source (a 9 volt battery in this case.) I
> bought an assortment pack of pots and was astonished to find they have
> 3 leads. This doesn't fit into my notion of how they work so I am
> obviously missing the big picture here. I figure the answer is simple
> sow I've done a lot of searches on Google and yahoo and I can't find
> simple enough circuits to make sense of how they work. I need some
> basic info on how to wire a circuit using one led, a resistor, a pot
> and a battery.
>
> Barb

So you have already calculated the resistor: If it is a white LED, it might
have a forward voltage of 3.3V at 20mA current. The 9V -3.3V= 5.7V must be
dissipated. R=Voltage/Current = 5.7V/20mA = 285 ohms. So 270 will also do.
Now you can take a pot of low value (1k) in put it in series with that
resistor and you can vary the current from 4.5mA to 21mA. But 0mA can not be
reached.
If we connect the pot in a different way, we can vary the LED-current from
18mA to 0. (3.3mA run through the pot)
The best is to use a 1W wire-wound pot for this purpose to not burn the
wiper or the carbon trace. Also a linear not log characteristic should be
chosen, they have the best power rating.
.---------.
| |
| .-.
--- | |
- | |270
--- '-'
- |
--- 9V |
- .-.
--- | |<------.
- | |1k |
--- '-' |
- | V white LED
--- | -
- | |
| | |
| | |
'---------o--------'
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
view\fixed font
--
ciao Ban
Bordighera, Italy


.

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