On Mon, 31 Aug 2009 02:47:34 GMT, (Doug Miller)
wrote:

In article m, David Nebenzahl wrote:
On 8/30/2009 1:58 PM Doug Miller spake thus:

In article
,
JIMMIE wrote:

It does not affect the current that the wire COULD carry or the
AMPacity. A long cable, compared to a short cable, will the current
in the total circuit because it adds resistance to the total
circuit.

Adding resistance to a circuit changes voltage, not current.

Changes current, too. E.g., a current-limiting resistor in series with a
LED, sized so that the LED won't draw excess current and burn itself out.

Wrong.

If you want to limit the current passing through an LED, you put a resistor in
*parallel* with it, not series.


I guess you didn't try that.

The resistor in parallel wastes power, and will have no effect on
current through the LED (other than if it creates excessive voltage
drop, a really inefficient way to dim the LED).

With no resistor in series, current (from a constant voltage source
greater than about 2V) will approach infinity, until the LED is
destroyed. I've seen that happen.

Total current in the circuit remains the same.

Not when you add series resistance.
LEDs regulate the voltage across them, unless the current is
excessive. Since that voltage is around 2V, a 120V source would
require a really big series resistor. To avoid that, 120V would
usually be applied to a series string of multiple LEDs (in series with
one smaller resistor).

A string of 50 LEDs (common in holiday lights) using 20mA from a 120V
source would require a resistor of 1000 ohms (that's assuming 2V
voltage drop on a LED). There would be 2V across each LED and 20V
across the resistor. Use multiple strings in parallel for more light.
Current is 20mA and power dissipated by the resistor is .4W.

Changing that resistor to 2000 ohms will change the current to 10mA.

Changing it to 500 ohms will change the current to 40mA. The resistor
will need to handle .8W. You may need a bigger resistor.

--
Mark Lloyd
http://notstupid.us

"How could you ask me to believe in God when there's
absolutely no evidence that I can see?" -- Jodie Foster