# Hi, 9 leds connected in parallel to one resistor on 12 V, 40 ohm resistor started to smoke!

Discussion in 'Electronic Basics' started by David Harris, Oct 2, 2003.

1. ### David HarrisGuest

Hi, I have 9 leds connected in parallel to one resistor on 12 V in my car,
the resistor is 47 ohms, and the resistor started to get hot and smoke!

I take it that I will need a higher wattage resistor, any suggestions?

The leds are bright blue and they will be running at approx 25ma, and the
total current draw is 9*25 = approx 225ma.

Thankyou!

Scott

2. ### Andrew KirbyGuest

Hi, I have 9 leds connected in parallel to one resistor on 12 V in
Firstly, I'd bear in mind that when the engine is running, you are likely
to see more than +12v on the supply - it's more likely to be 13.5-13.8v.
I don't know what the forward voltage of the particular LED's you have
is, but bear this in mind. In any case, at 225mA, a 47R resistor will
dissipate 2.38 watts.

A secondary suggestion is, given the unstable nature of supply voltages
in cars, it might be possible to protect the LEDs by including a PTC
(positive temperature coefficient) thermistor in the series resistor, so
that if the supply voltage increases, and the current tends to goes up,
so too does the series resistance, thus hopefully preventing damage.

HTH
Andy

3. ### Costas VlachosGuest

You were probably using a 1/2 Watt resistor...

P = I*I*R = 0.225 * 0.225 * 47 = 2.4 Watts. I'd use a 5 Watt resistor to
help dissipate the heat due to its larger body.

But there's another thing to consider with your setup. If one LED burns and
stops drawing current, the remaining 8 LEDs will draw more current, which in
turn may cause another LED to pop, etc., until all LEDs are destroyed. You
could use 9 separate 430 Ohm 1/2 Watt resistors for each LED (space
permitting of course). There was another thread a few months back that dealt
with these issues, do a Google search for it.

cheers,
Costas

4. ### Costas VlachosGuest

To add to this, your LEDs must have *identical* V/I characteristics for your
setup to work (i.e., if one of the LEDs has a slightly lower Vf than the
rest (which is very likely), it will draw most of the current and will
eventually burn (followed by the others shortly after).

Just use separate resistors. Or put them in series like the other poster
suggested (in which case if one burns every other LED in the chain will turn
off).

BTW, when LEDs burn, do they always open? Or can they also short?

cheers,
Costas

5. ### John FieldsGuest

---
Using a single resistor like that is very bad practice because all of
the LED's won't have the same forward voltage and the one with the
lowest forward voltage will hog the current until the voltage drop
across it rises to the point (The temperature coefficient of the forwrd
voltage is something like -3.1mV/°K, so it may never get there!) where
another LED starts to conduct some current, so it might blow up.
---

If you're using LED's with a maximum continuous forward current of 25mA
and your car voltage rises to 13.6V, then for a 4.5V maximum forward
voltage you could put two in series with a current limiting resistor,
like this:

+13.6V----[180 ohms]---[LED>]---[LED]----GND

If you _had_ to run 9 LEDs, then you could use four strings in parallel
and a single LED, like this:

+13.6V--+--[180 ohms]---[LED>]---[LED]--+
| |
+--[180 ohms]---[LED>]---[LED]--+
| |
+--[180 ohms]---[LED>]---[LED]--+
| |
+--[180 ohms]---[LED>]---[LED]--+
| |
+--[360 ohms]---[LED>]----------+
|
GND>------------------------------------+

The 180 ohm resistors will dissipate (13.6V - 9V)*25mA = 115mW, so you
could use 1/4 watt resistors, but the 360 ohm one will
dissipate(13.6V - 4.5V)*25mA = 227mW, so you'd be well advised to use a
1/2 watt resistor there.

If you're worried about gigunta spikes on the 13.6V line(and you should
be!) you could do something like this, where CR1 is a 1N5352B, a 15V 5W
Zener diode.

+13.6V--+--[180 ohms]---[LED>]---[LED>]--+
| |
+--[180 ohms]---[LED>]---[LED>]--+
| |
+--[180 ohms]---[LED>]---[LED>]--+
| |
+--[180 ohms]---[LED>]---[LED>]--+
| |
+--[360 ohms]---[LED>]-----------+
|K |
[CR1] |
| |
GND>----+--------------------------------+

6. ### TerryGuest

Scott:

Ohms Law; Current squared multiplied by the resistance = watts.
(I x I x R = W)

i.e. (0.225 x 0.225) x 47 = 0.051 x 47 = 2.4 watts.
So you should use 'at least' a 3 watt resistor. (And it should
mounted in a manner suitable to its type so that cooling can
occur). Three watts being dissipated by small resistor can get
hot enough to take skin off your fingertips. As you have
discovered!

Another way; suppose that 10.5 volts is being dropped or 'wasted'
across the resistor.
(In order to have, say, 1.5 volts across the LEDs).

Ohms Law again; Voltage squared divided by the resistance =
watts. (V x V)/R = W

i.e. (10.5 x 10.5)/47 = 111/47 = 2.4 watts.
Same result.

There are better ways of 'Dropping the voltage'. However a
current draw, with most of the electricity being wasted as heat,
in this case, of less than one quarter amp, is probably not very
significant in a typical motor vehicle (car).
By the way, if those blue LEDs are outside the vehicle or can be
seen from outside the vehicle, you might want to check what light
displays are legal in your police jurisdictions. Here, blue
lights signify some kind of emergency or road maintenance vehicle
such as snow plough etc.
At one time (back in the 1950s!) there was a custom here, I'm
showing my age by this I know!, to have little blue lamps in the
corners of ones front windshield. They were declared illegal and
then disappeared. But there can be fines for 'illegal' light
displays, especially if they are on while vehicle is driving. The
fewer things you can give the authorities to 'pick on' the
better! have fun.

Couple of thoughts anyway. Good luck.

PS. All the LEDs together are using approximately;
(Using another version of Ohms Law!); Current multiplied by
voltage = Watts (I x V = W)
0.225 x 1.5 = 0.34 watts. However LEDs are much more efficient at
turning electricity into light than lamps.
At the same time 2.4 watts is being dissipated by the resistor!

7. ### TerryGuest

Baph: I was amazed too, with that many diodes (of any kind in
parallel) but am not that familiar with how consistent or
inconsistent such diodes are!
Seemed though, that the original poster needed to have some basic
understanding of V x I = W
And that a resistor is not a resistor is not just a resistor
etc.!
BTW How are those grouped LED rear (brake) light assemblies, on
cars, arranged! Series parallel maybe
Terry.
PS. Maybe of interest to sci.electronics.basics posters is a
story about an almost brand new Volvo Tractor trailer truck that
broke down three times and then it and its loaded trailer had to
be towed due to an electrical problem, eventually discovered,
caused by a temporary replacement driver. Hint! Fuses are
sometimes a better indicator of what and where a fault is
occurring than circuit breakers?

8. ### John FieldsGuest

---
Good luck!

If the LEDs are Silicon Carbide, they've got a typical Vf of about 3.8V,
and a maximum of about 4.5V at 25mA, so even at nominal, four LED's
would drop 15.2V and five would drop about 19V, so they wouldn't be very
bright with 13.6V from the car!^)

9. ### BaphometGuest

I'm amazed that all of the led's work at all. If you want to run them in
parallel, you should seriously consider using a separate current limiting
resistor for each led. Alternatively, you could run 1 string of 5 led's in
series and 1 string 4 led's (each string with its own current limiting
resistor) to save on parts count.

10. ### BaphometGuest

Mea culpa John -

I didn't realize they dropped such a large voltage. I guess it's back to
4 strings of 2 series led's and 1 string of 1 led.

11. ### David HarrisGuest

Thanks all, nice for all the replies!

I've chosen the 47 ohm resistor based on the 14.4V, this value is slightly
above whats needed according to the equation, and at 12V the brightness
shouldnt be too much less. 25ma will be the approx current per led.

Sounds like this could be a potential led death chain reaction waiting to
happen if one led dies! This is a prototype I put together in my bedroom! I
can't access the led legs now because they are set in epoxy!! lol

I'll redesign the next one, what was the details about using a zener diode?

Thankyou all!!

Scott

PS the leds were nice and bright, and that was in daylight, and no heat was
being thrown out either!
Decided on a WELL overspecced 47 ohm 7W wire wound resistor, or maybe a 10W.

12. ### Watson A.Name - Watt SunGuest

Blue LEDs have about 3.6V aross them, so that leaves about 10V. At 21
mA per LED, that means you should have a 470 ohm resistor for each
LED. Quarter watt resistors would be okay, but it would be better to
use half watters. Use one resistor in series for each LED, do not
connect the LEDs in parallel.

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13. ### Watson A.Name - Watt SunGuest

That's really no help at all. You can't find a PTC in any store, it
would have to be special ordered. And why bother when a simple
circuit like the following would do the same thing.

One way to do it would be to use three groups of three LEDs in series,
each group with the following circuit. For 25 mA, change the 33 ohm
resistor to 24 ohms, or easier, put two 47 ohm resistors in parallel.
Three blue LEDs in series in the circuit need 11V to get current
regulation. Below that, the current will drop off to a few mA at 9V.

+--------------------+------- Positive
| | Supply V.
| |
| --- LED
10k \ \ / =====>
to 47k / ===
ohms \ |
/ |
| / Q1
| | / Gen'l
| | Purp NPN
+----------------| 2N3904 or
| | 2N4401
| | \ E
Q2 \ \
Gen'l \ | |
Purp NPN | 470 ohms |
2N3904 or |------/\/\/\----+
2N4401 | |
E / | |
/ \
| 33 ohms /
| for \
| 20 mA /
| |
| |
| |
+---------------------+-------
Negative Supply V.

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14. ### Watson A.Name - Watt SunGuest

Didja catch your boo-boo? Should be 9 separte resistors, one for each
LED, instead of 9 separate resistors for each LED.

And indeed, 81 resistors would definitely need some space. ;-)

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15. ### Costas VlachosGuest

Yeah, that's what I meant: 9 separate 430 Ohm resistors, *one* for each LED.
It wouldn't work otherwise anyway. Thanks for pointing it out!

cheers,
Costas

16. ### Watson A.Name - Watt SunGuest

[snip]
Blue lights signify that you bought the car at K-Mart, so take that
Blue Light Special back and get your money back!
Around here in So Cal, a mod that kids make to cars is to add blue
LEDs to the windshield washer squirters on the hood. I've seen blue
LEDs in other places too, sometimes flashing. It's very unwise as he
said to have these on when on the streets, because the cops may think
you're trying to pretend your car is an emergency vehicle.

Another fad around here in La-La Land is to replace the headlight
lamps with bluish-white ones. Light looks like it's coming from a
Beemer, Boxster or Benz. Then you get closer and see it's just a
little old Honda with a six inch exhaust pipe that sounds like it's
farting continuously. :-O
I can't figure out where you got the 1.5V from above. The V drop for
red LEDs is about 2V, and for blue ones in this case, it's 3.6V.

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17. ### Watson A.Name - Watt SunGuest

No, no, NO. Blue LEDs drop at least 3.3V, so 5 LEDs would add up to
more than 16V, and four would be 13.2V; both strings would barely
light if at all.

I put 3 blue LEDs in series, and it took over 10V to get the current
up to 20 mA.

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18. ### Andrew KirbyGuest

A secondary suggestion is, given the unstable nature of supply
Huh? You might not find PTC's in radio shack, but they are readily
available, and offer protection using a simple series implementation, which
the OP can definitely manage.

19. ### Watson A.Name - Watt SunGuest

Like I showed in the (missing) schematic, it only takes two ten cent
transistors and 3 resistors to make a current limiter, and it does a
better job. View with Courier font.

+--------------------+------- Positive
| | Supply V.
| |
| --- LED
10k \ \ / =====>
to 47k / ===
ohms \ |
/ |
| / Q1
| | / Gen'l
| | Purp NPN
+----------------| 2N3904 or
| | 2N2222A
| | \ E
Q2 \ \
Gen'l \ | |
Purp NPN | 470 ohms |
2N3904 or |------/\/\/\----+
2N2222A | |
E / | |
/ \
| 30 ohms /
| for \
| 20 mA /
| |
| |
| |
+---------------------+-------
Negative Supply V.

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Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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