Re: very simplistic potentiometer question
- From: byron@xxxxxxxxxxxxx (Byron A Jeff)
- Date: 1 Jul 2005 12:44:20 -0400
In article <hqqua1ppunk1cvuno2sl67q0d292q025rn@xxxxxxx>,
Barbara <idontwant@xxxxxxxxx> wrote:
>I am just beginning to tinker with electronics.
Congratulations!
> I've decided to start >very simply and work my way up to the fun stuff.
Define fun stuff?
> I like the thought of playing with LEDs for some reason so I'm starting
> there.
Not a bad start.
> I've figured out already that without a resistor that LEDs last about .2
>seconds before they fry.
But have you figured out why? I'll take a minute to explain. LEDs required
a minimum voltage to light. Also there is a maximum amount of current that
can flow through is before it burns up. Without the current limiting resistor
the LED sees too high a voltage and allows too much current through and it
blows like a fuse. POOF!
A good discussion of this is here.
http://www.palosverdes.com/pvarc/2002QRO/Sept2002QRO.htm
> 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.
Correct.
> 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.)
You can do that, but you're not going to get the expected result.
> I
>bought an assortment pack of pots and was astonished to find they have
>3 leads.
They all do. The two outer ends are like a regular fixed resistor. The
middle lead is known as the wiper and it's resistance varies vs. the other
two leads. So if you use one outer lead and the wiper lead, you'll get
the variable resistance you're looking for.
> 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.
See above. That should be good enough to get started.
> I need some
>basic info on how to wire a circuit using one led, a resistor, a pot
>and a battery.
Well that's a different kettle. LEDs are highly non-linear with respect
to current and brightness. Also as you've already learned they burn up
if too much current goes through them. And they go out if there is too
little voltage.
What you can do is hook up a low resistance pot (50-100 ohms) as a voltage
divider. You do this by tying the outside two leads to the two leads of
your 9V source and pull the variable voltage from the wiper lead. Be aware
that if you do this with a 9V batter that it'll go out pretty quickly because
the pot will be burning 10-20ma of current the entire time you're
experimenting.
BTW dimming LEDs usually isn't done this way. It's usually done by turning
them fully on/full off very fast. It's a technique known as Pulse Width
Modulation (PWM).
Hope this helps,
BAJ
.
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