Electrical schoolwork...

Ti Kan ti at amb.org
Tue Feb 10 00:39:25 EST 2004

Louis-Alain RICHARD writes:
> If I were to install some red 3V LEDs to backlight the numerous switches
> around the instrument panel of my 1983 urQuattro, what is the resistance
> value I must introduce to this circuit to lower the 12V to something =
> more
> acceptable?
> Resistance value of the LEDs: 150 000 ohms=20
> Ideal voltage at the LEDs: between 2.5V and 3V.
> Electrical voltage at the switches: 13.6V, permanent, switched by the
> ignition, not the IP rheostat.
> My electrical knowledge is so far away (20+ years), I don't remember =
> anymore
> how to compute series resistance in a simple circuit like this!


The purpose of the series resistor is not to reduce voltage, but to
limit current.  An LED that's rated for 3V when forward biased is
going to maintain that 3 volts across it regardless of what value resistor
you use (within a certain operating range).  Beyond that range, if the
resistance is too high, you will lose voltage but then the LED will
become very dim.  If the resistance is too low, the LED will just burn
up.  Within that range, you should select the resistor so that the
*current* flowing through is within the LED's safe operating rating.

For example, if the LED is rated to typically operate to at 20mA
(or 0.02A) at 3V, and you know your car's battery voltage is going
to be around 13V, then the resistor should be:

	R = V / I		(simple Ohm's law)

where V is the 13V - 3V = 10V, and I = 0.02A, thus your resistor should
be 500 ohms.  Now, you can use a resistor somewhat higher in resistance
and get a little dimmer output, or a little lower resistance for a little
more brightness, but be sure to stay within the LED's rated operating
safe current range.

When selecting the resistor, be sure that its rated power is high enough
given the voltage and current.  The formula is:

	P = V * I

Or 10V * 0.02A = 0.2W.  You should use a resistor at least double (if
not higher) that wattage to ensure that it won't burn up.  In this case,
a 0.5 resistor is marginally ok, but I would feel better with a 1W

2003 A4 1.8T multitronic
2001 S4 biturbo 6-sp
1984 5000S turbo
1980 4000 2.0 5-sp
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