led questions

Discussion in 'Electronic Basics' started by redls1bird, Aug 19, 2005.

1. redls1birdGuest

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

2. RodneyGuest

For standard LED's you only want about 3 volts and about 20 milliamps so you
need to connect a resistor in series with the LED for a voltage drop of 11
volts since automotive voltage us about 14 volts(13.8) when the vehicle is
running. Using Ohms Law, R = V/I so R = 11/20e-3 which is 550 ohms. A 470
ohm resistor would be a good enough standard value since the LED will take
more than 20 milliamps. I know someone who usually uses 470 ohms for
automotive purposes. I used 220 ohms for my six volt motorbike.
You need 470 ohms for each LED you use. Connect the LED's in parallel with
one another.

Rod

3. ehsjrGuest

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

4. JazzManGuest

JazzMan
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5. James T. WhiteGuest

Ed's is one of the best I've seen for beginners.

The only thing I would add is to make sure that you calculate the power the
resistor will have to dissipate (Current * Current * Resistance) and select a
resistor with a power rating at least that large. Good practice is to use a
resistor with 2X the required rating.

Good luck.

6. RodneyGuest

I can't argue with you, it just would depend on the application. I used a
white LED to replace a high beam indicator on my motorbike and it works
fine. It does happen to be in parallel with the other lights. Just had to
solder a 220 ohm resistor to it. If I use LED's for the other lights I'll
do the same. I realise you could hook up multiple LED's in series with a
single resistor but if one LED fails you will have trouble knowing which one
is dead when the whole string goes out. Just like Christmas lights eh?
LED's don't go out too often but they can. They may be less reliable in
automotive apps.

7. Kitchen ManGuest

Right, I think that's the source of the confusion in the discussion.
You are wiring resistor-LED-pair series circuits in parallel with
other devices using the same source, and I think the ehj-someone is
implying that non-current limited LED's should not be paralleled.

In pictures:

+V--[R]--[LED]--Rtn
|---[circuit]---|

and not:

+V--[R]---[LED]--Rtn
|-[LED]---|
|-[LED]---|

which would probably be ok *if* R is chosen for the current used by
the parallel group, and the LED's are closely matched for voltage
drop.

8. Kitchen ManGuest

That one is really quite nifty. Thanks.

10. RodneyGuest

Maybe I should explain that in the old days they used to make Christmas
lights in series. When one light went out you had to test every bulb to see
which one was out. Today they are making strings of LED Christmas lights
which I believe are all connected in parallel.

Rod

11. Michael A. TerrellGuest

No, they are not all in parallel, because LEDs don't play well
together in that configuration, and it would waste a lot of current.

--
Link to my "Computers for disabled Veterans" project website deleted
after threats were telephoned to my church.

Michael A. Terrell
Central Florida

12. redbellyGuest

Rodney wrote:
Today they are making strings of LED Christmas lights
That makes sense only if each LED has it's own separate resistor to set
the current:

| |
|--/\/\/\/--|>|--|
| |
|--/\/\/\/--|>|--|
| |
|--/\/\/\/--|>|--|
| |
|--/\/\/\/--|>|--|
| |

where -/\/\/\/- is a resistor and -|>|- is an LED.

Mark

13. John FieldsGuest

---
Frayed knot.

1. Each resistor will have to dissipate about 2.4 watts

2. What'll happen when the mains polarity reverses and puts the
cathodes of the LEDs at 170V positive WRT the anodes?

14. Kitchen ManGuest

Thanks for the clarification.

15. redbellyGuest

I guess I could have added that low-voltage DC is being used, but I
thought that was obvious.

Mark

16. John FieldsGuest

---
I tend to think of a string of Christmas tree lights as working off
the mains, so to get that low-voltage DC they'd have to have a
power supply of some sort. Maybe a wall-wart? I don't know, I
haven't bought a string of lights in over ten years. Is that how
they do it?

17. redbellyGuest

I don't know either. I was originally responding to somebody who
said they THINK that Christmas lights are available as LED's wired
in parallel. My point was that IF that is the case, they would
each need a separate series resistor. And of course run off a
low voltage DC source.

When you think about it, the possible savings in efficiency are
likely offset by the inconvenience of using a wall-wart, the
power drain of the resistors, and the fact that they would only
be used for 3 or 4 weeks out of the year. But I honestly don't
know what is available.

Regards,

Mark

18. Tom MacIntyreGuest

I'm pretty sure that is the case. The new ones can have a few bulbs
fail and the string still works.

Tom

19. redls1birdGuest

thank you all very much for the info, this is exactly the kind o
stuff i was looking for. i posted ona few other lighting specifi
forums and got absolutely nothing. appreciate all the help an

20. redls1birdGuest

well, here is my thoughts. i have a set of non functional vents o
the quarter panel of my car. asthetically they are nice, but i woul
like to make them have a function. my thought was to set up a smal
led array behind each one (orange in color) and use them as a sid
parking/turn signal indicator. looking at the links i got some grea
info, including how i could create just a park light, but how could
turn it on and off with the blinkers? optimaly, i would like t
change the intensity of the lights to indicate a turn. i read in
few places about 555 chips, but i dont know what they are or how t
use them. any more ideas or links? like i said i appreciate all o
the help