Calculate the resistor you need for a LED

Stef · 9th December 2006, 20:15

Some people want to play with LED's in a outside of the car.

Just some info on LED's
Red, Green and Yellow needs 1.7V and 20mA (absolute max is 40mA)
Blue and White LED needs 3V / 20mA (max 40mA)
(voltage can change a bit due to make or current)

So 1 blue or white LED needs next resistor:
12V-3V = 9V ( U over resistor) then 9V/0.02A = 450ohm resistor
(470 ohm is standard value).

You can use 3 white LED's in series, leaving a drop of 3x3=9V

Then you need a resistor:
12-9 = 3V > 3V/0.02 = 150ohm ( 180 or 120ohm is a standard value)

Mind also that the car voltage can go up to about 14V.
NEVER EVER CONNECT A LED DIRECTLY TO THE VOLTAGE, YOU ALLWAYS NEED A CURRENT LIMITTING RESISTOR.
Some special LED's have a build in resistor.

czechplastik · 9th December 2006, 22:11

Very technical, cheers for the advice!

neilfs · 2nd March 2007, 23:58

So what happens when you use four white LEDs?

Glad someone knows about this, I've never really understood why LED lights have what looks as if resistors all over them, do they do lots of small circuits in parallel, ie lots of 4 LED chains with a current limiting resistor?

pcr · 3rd March 2007, 01:16

Following Stef's guidelines :-

Stef · 3rd March 2007, 10:30

If you want 4 LEDs in one spot, above is perfect!

neilfs · 3rd March 2007, 11:52

Cool, thanks guys - I'm learning something which I've always never really understood properely. It is actually very basic.

Okay, silly question, just so I fully understand the reason why you always put a resistor in each circuit...

So why would you not just put four leds in series? 4 x 3 = 12V

Or is the purpose of the resistors to protect the LEDs from over voltage if you cannot guarantee a stable 12v supply?

Okay, next silly question.

Of the two examples in the diagram, which one is most battery efficient, draws the least amps, or are they both equal?

Thanks!

neilfs · 3rd March 2007, 11:56

Does anyone know a good cheap suppler of this stuff, other than RS or Maplin?

I have often thought of making my own potted LED light clusters, as all the ones I've bought which are waterproof (they are used on boats) fail as the layer of potting between the PCD and where the wires come out to the outside always breaks down and a small gap forms so the far end of the wire inside the potting corrodes off.

pcr · 6th March 2007, 20:06

Neilfs,

There must always be a resistor the control the current through the led/s. In diagram the resistor values are chosen to give approx 20ma through each led. More current equals more brightness, do not exeed 40ma.

Four leds in series will burn out since nothing is limiting the current.

First circuit approx 40ma, second circuit approx 80ma.

richierich · 8th March 2007, 12:38

If I can remember back to GCSE Physics V=IR (Voltage = Current x Resistance) or Resistance = Voltage/Current.

Stef · 8th March 2007, 22:33

The more LED's you can put in series the more efficient.
Also you are absolutly sure that each LED gets the exact same current.
But for white LED's the voltage drop is 3V or a bit more.
So if you put 4 LED's in series it would result in 12V, leaving no voltage to set the current through the resistor.

neilfs · 13th April 2007, 10:27

Thanks, okay so have as many LED's in series as possible for efficiency but ensure their combined voltage is just below so you can add in a resistor to make up the difference.

That makes sense now - thanks all.

9th December 2006, 20:15	#1 (permalink)
Stef Supporter Triangular Exhaust Join Date: 14th October 2006 Location: Belgium Posts: 407 Thanks: 6 Thanked 7 Times in 5 Posts iTrader: (0)	Calculate the resistor you need for a LED Some people want to play with LED's in a outside of the car. Just some info on LED's Red, Green and Yellow needs 1.7V and 20mA (absolute max is 40mA) Blue and White LED needs 3V / 20mA (max 40mA) (voltage can change a bit due to make or current) So 1 blue or white LED needs next resistor: 12V-3V = 9V ( U over resistor) then 9V/0.02A = 450ohm resistor (470 ohm is standard value). You can use 3 white LED's in series, leaving a drop of 3x3=9V Then you need a resistor: 12-9 = 3V > 3V/0.02 = 150ohm ( 180 or 120ohm is a standard value) Mind also that the car voltage can go up to about 14V. NEVER EVER CONNECT A LED DIRECTLY TO THE VOLTAGE, YOU ALLWAYS NEED A CURRENT LIMITTING RESISTOR. Some special LED's have a build in resistor.

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This thread is about: Calculate the resistor you need for a LED, it's in How To at the Honda Civic forum Civinfo; Some people want to play with LED's in a outside of the car. Just some info on LED's Red, Green and Yellow needs 1.7V and ...

9th December 2006, 22:11	#2 (permalink)
czechplastik King of the rodeo Civinfo guru Join Date: 24th October 2006 Location: Belfast Posts: 4,734 Thanks: 0 Thanked 3 Times in 3 Posts iTrader: (0)	Very technical, cheers for the advice!

2nd March 2007, 23:58	#3 (permalink)
neilfs Supporter Wheelnut Join Date: 18th November 2006 Location: Hindhead, Surrey Posts: 68 Thanks: 2 Thanked 0 Times in 0 Posts iTrader: (0)	So what happens when you use four white LEDs? Glad someone knows about this, I've never really understood why LED lights have what looks as if resistors all over them, do they do lots of small circuits in parallel, ie lots of 4 LED chains with a current limiting resistor?

3rd March 2007, 10:30	#5 (permalink)
Stef Supporter Triangular Exhaust Join Date: 14th October 2006 Location: Belgium Posts: 407 Thanks: 6 Thanked 7 Times in 5 Posts iTrader: (0)	If you want 4 LEDs in one spot, above is perfect!

3rd March 2007, 11:52	#6 (permalink)
neilfs Supporter Wheelnut Join Date: 18th November 2006 Location: Hindhead, Surrey Posts: 68 Thanks: 2 Thanked 0 Times in 0 Posts iTrader: (0)	Cool, thanks guys - I'm learning something which I've always never really understood properely. It is actually very basic. Okay, silly question, just so I fully understand the reason why you always put a resistor in each circuit... So why would you not just put four leds in series? 4 x 3 = 12V Or is the purpose of the resistors to protect the LEDs from over voltage if you cannot guarantee a stable 12v supply? Okay, next silly question. Of the two examples in the diagram, which one is most battery efficient, draws the least amps, or are they both equal? Thanks!

3rd March 2007, 11:56	#7 (permalink)
neilfs Supporter Wheelnut Join Date: 18th November 2006 Location: Hindhead, Surrey Posts: 68 Thanks: 2 Thanked 0 Times in 0 Posts iTrader: (0)	Does anyone know a good cheap suppler of this stuff, other than RS or Maplin? I have often thought of making my own potted LED light clusters, as all the ones I've bought which are waterproof (they are used on boats) fail as the layer of potting between the PCD and where the wires come out to the outside always breaks down and a small gap forms so the far end of the wire inside the potting corrodes off.

6th March 2007, 20:06	#8 (permalink)
pcr Magic Rear Seat Join Date: 5th September 2006 Location: Shropshire Posts: 851 Thanks: 0 Thanked 2 Times in 2 Posts iTrader: (0)	Neilfs, There must always be a resistor the control the current through the led/s. In diagram the resistor values are chosen to give approx 20ma through each led. More current equals more brightness, do not exeed 40ma. Four leds in series will burn out since nothing is limiting the current. First circuit approx 40ma, second circuit approx 80ma.

8th March 2007, 12:38	#9 (permalink)
richierich Smell My Cheese! Rocketship door handle Join Date: 17th May 2006 Location: Bicester, Oxfordshire Posts: 1,847 Thanks: 0 Thanked 1 Time in 1 Post iTrader: (0)	If I can remember back to GCSE Physics V=IR (Voltage = Current x Resistance) or Resistance = Voltage/Current.

8th March 2007, 22:33	#10 (permalink)
Stef Supporter Triangular Exhaust Join Date: 14th October 2006 Location: Belgium Posts: 407 Thanks: 6 Thanked 7 Times in 5 Posts iTrader: (0)	The more LED's you can put in series the more efficient. Also you are absolutly sure that each LED gets the exact same current. But for white LED's the voltage drop is 3V or a bit more. So if you put 4 LED's in series it would result in 12V, leaving no voltage to set the current through the resistor.

13th April 2007, 10:27	#11 (permalink)
neilfs Supporter Wheelnut Join Date: 18th November 2006 Location: Hindhead, Surrey Posts: 68 Thanks: 2 Thanked 0 Times in 0 Posts iTrader: (0)	Thanks, okay so have as many LED's in series as possible for efficiency but ensure their combined voltage is just below so you can add in a resistor to make up the difference. That makes sense now - thanks all.