Re: led questions

redls1bird wrote:
hello all. im interested in using some leds for automotive lighting
and such. i have a BASIC understanding of electronics and
electricity. i am familiar with diodes, but not leds. The very
small information i have found for auto use talks about using
resistors in parallel, but noone really goes into why. i believe its
to drop the voltage? but i wouldnt bet my next paycheck on it. can
anyone give me some basics on building led circuits? especially how
to connect multipes and determine the proper resistors to use? Thanks for the help in advance.


One and only one rule*: design the circuit to limit the current
through the LED to some particular value. The simplest way is
with a 1/2 watt (or larger) series resistor:

+Vcc-----[R]---[LED]---Gnd

Use the formula Resistance = (Vsupply - Vled)/Current
If LEDS are used in series: Vled = Vled(1) + Vled(2) + ... Vled(n)

* = until and unless you have a specific reason or reasons not to.

If you don't have specs on your LEDs, use these numbers for Vled:
Red LED ~1.8 volts; white (or blue, blueish white) LED ~ 3.4 volts.
and limit the current to about 20 mA  (or less). (Other color
LEDS will range between these Vled values)

If you have the specs for the LED, you can use them to chose
the current limit, and you'll know Vled for each LED. Otherwise,
20 mA is generally a good value for the limit, and LEDS will light *well* below 20 mA.

In a car, a nominal voltage of 14 for Vsupply can be used to
compute the size resistor you need. With a red LED, that computes
to 610 ohms, and I would recommend using 680 ohms (a standard value)
for a little extra safety margin.  With a white LED, it computes
to 530 ohms. I'd recommend going up to at least 560 ohms, but you
could use the 680 ohm resistor and still get over 15 mA through
the LED.  Probably won't see much change in brightness, either.

Do NOT wire LEDS in parallel. For multiple LEDS,
use series like this, and re-compute R:

Vcc---[R]---[LED1]---[LED2]---[LED3]---Ground

In the case above, for red LEDs, it computes to 430 ohms.
A 470 ohm standard resistor would be fine. With white
LEDs, R computes to 190 ohms, and a standard 220 ohm
resistor would be good.

If you want to use a parallel circuit, do this, and
use the 680 ohm resistor mentioned earlier:

Vcc---+---[R]---[LED1]---+---Ground
      |                  |
      +---[R]---[LED2]---+
      |                  |
      +---[R]---[LED3]---+
      ~                  ~
      +---[R]---[LEDn]---+

That's not the end of the story. The electrical "environment"
in a car is hostile. There's all kinds of electrical transients,
and it may be prudent to protect the LED's with a 15 volt TVS
diode from Vcc to ground.  In addition, the ambient temperature
may be high, and you may want to limit the current to less
than the ~20 mA (or the specs, if you have them) to
compensate for high ambient temperature.

Ed
.

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