Total newbie simple led questions!

[rob.m]

Hi everyone,first thanks for taking the time to read this and reply to me

My experiance in electronics does not go far im afraid and i dont mind admitting that! i have some from school ten years ago but thats about it

So im here because im trying to design my own taillights for my car,as there are not many aftermarket ones for me that i like.Now using the web for most of the afternoon and this site **broken link removed** for my calculations i can pretty much do what i need to do,but i want to know a little more that i cant seem to find through google so hope you people can help me

Below is a standard spec of a super bright led!The question i have is how would i go about changing the brightness of the led ? would i alter what my forward voltage operates at-ie 3.2v=bright, 3.6= brighter? or do i change the continuous forward current?ie continuous forward current being at 30ma and to get it brighter go up to 50ma continuous or would either way do what i want?

Absolute maximum ratings: Ta = 25°C
Power dissipation: 108mW
Peak forward current: 100mA
Continuous forward current: 30mA
Derating factor: 0.4mA/°C
Reverse voltage: 5V
Operating temperature: -25 to +85°C
Storage temperature: -35 to +100°C
Soldering temperature: 260°C / 5secs
Absolute optical characteristics:
Ta = 25°C, If = 20mA, Vr = 5V
Symbol Min Typ Max
Forward voltage: Vf 3.2V 3.6V
Luminous Intensity Iv 6000mcd 7000mcd

Thanks for any of your input guys i know from reading other posts on here im way below your 'normal'standard of questions but at least you wont have to work you brain on these! Cheers rob

 

[whiz115]

Give it a go at the maximum it can handle and it will light up as much as it is possible you have 108mW Power dissipation (as the specs say..) with
3.6V/30mA.

Don't calculate your circuit for "100mA" as the peak forward voltage indicates..because it will get burned.

 

[ronsimpson]

LEDs convert current to light. To change the brightness, change the current. The voltage is not something you have much control over. Do no do “3.2v=bright, 3.6v=brighter”! You should ‘15mA=bright, 30mA=brighter’. In this case you will probably get the 3.2 and 3.6 volts but as temperature changes and from one batch of LEDs to the next batch the voltage will vary too much.

As the temperature changes the voltage changes. You must regulate the current.

I get LEDs in that vary form 3.0 to 3.6 volts depending on when they were made. If I regulated on voltage the current would vary over a 3:1 range.

 

[audioguru]

The absolute max allowed continuous current is 30mA at room temperature and is less out in the sunshine and when they are crammed together. The current must be reduced 0.4mA for each degree C above room temperature.

The spec's don't say the angle they can be seen so it is probably very low (focussed) to make the LEDs "appear" brighter on-axis. Then they are illegal for cars because the viewing angle is too narrow.

 

[Boncuk]

bright LED

Hi Rob,

it's already said that LEDs don't change brightness with voltage. No matter what the forward voltage of the LED is the current is the criterium for maximum brightness. The forward voltages differ a lot even with LEDs of the same type.

Here is a cheap constant current regulator circuit for your LED. In the example given the forward voltage drop is approx. 2.2V for a green LED. Using a red LED it will decrease to approx. 1.8V leaving the current unchanged.

Changing R2 to 42 Ohms the resultant current will be 29.9mA. Use a 50Ohm adjustable resistor for accurate 30mA output. Start at the highest value observing the current.

Starting at zero Ohms means overkill and all you'll obtain is one short flash (if at all)

Regards

Hans

P.S. The LM317L is very small (TO92 package)

 

[rob.m]

Ok you've given me more questions than answers now! so how critical is the temperature change? as these are led's in a rear light of a car,temperature is going to be a changing varibal from outside conditions, the running leds are going to be used the most and possibly the fog led's so these would also create heat rise withing a sealed light, the reversing and indicator leds will only be on for short times

so all this is going to affect the life of the led as designing the circuits is based on that,if i dont factor in the temperature changes how dramitally is this going to affect the life of the led?even if i only get say 5000 hours out of an led that is some years on a car!

Also to get around the changing voltage of a car instead of using a voltage stabiliser/regulator could i not just do my calulations based on what the highest voltage would be,then when the alternator is not working voltage would just be circa 12 volts and all that would happen is the leds will be slightly dimmer?or do i really need to make sure of a constant 12 volts through a regulator?

 

[audioguru]

If your LEDs have a narrow viewing angle then the cops will give you an expensive ticket or another car won't see you and Crash!
Their max allowed current is 30mA so they will be fine at 20mA if they are bright enough.
Two LEDs in series and in series with a current-limiting resistor will make one string. Make many strings like that.

 

[ronsimpson]

I would do a set of numbers basted on 3.2, 3.4 and 3.6 volts for Vf. Because the Vf will be anywhere in that range. That is basted on 25C. In automotive design the temperature range is huge. I made a product that failed at –40C and had problems.

I am looking at a data sheet for a very similar LED. There is a “forward current derating curve”. It shows that the LED will work at 30mA at 30C and below. At 50C it can only handle 20mA. I believe in the summer, with the bulb on, you can easy hit 50C. I would probably use 20mA not 30mA.

A car battery may be “12 volts” but when the engine is on it probably runs more like 15 volts. My truck runs 16 volts.

I have seen people who series 4 LEDs to get a “12 volts” LED. Apply it directly across the battery and it ‘works’. Four ‘center’ 3.4 volts LEDs will Draw:
3mA @ 12 volts
10mA @ 13 volts
20mA @ 14 volts
30mA @ 15 volts
40mA @ 16 volts
But four 3.2 volt LEDs will draw 50mA at 16 volts. Four 3.6 volt LEDs will draw only 0.8mA at 12 volts.

If I was doing this I would series up two LEDs to make a 6.8 volt LED pair. Add a resistor.
Vbat=16 volts Vr=9.2 volts Vled=6.8 I=20mA Engine is on.
Vbat=12 volts Vr=5.3 volts Vled=6.7 I=12mA Engine in off.
Vbat=10 volts Vr=3.3 volts Vled=6.5 I=7mA Bad battery.

If you want constant brightness then you will need to build a current regulator. (not a voltage regulator) Automotive light bulbs vary brightness with voltage.

 

[Norlin]

as for viewing angle, since there's going to be a plastic shield over the LEDs, couldn't he make that into some sort of diffuser and overcome the angle problem?

 

[audioguru]

If you diffuse the light from a narrow-angle LED then you end up with a dim LED like the LED manufacurer started with.
You need bright wide-angle LEDs that are made to be bright over a wide angle.

Your LED's datasheet does not say its viewing angle so maybe it is wide but I doubt it.

 

[rob.m]

Hi guys thanks for all your input so far, as for the viewing angle of the led in the exmple above it was just an example of an led and not one i was actually gonna use sorry should of made that clearer in the first post,

Do you think that if i made a string of 3 leds with a forward voltage of 3.4 and with a resistor of 220 ohms im pushing the led to far or should i be safe (british weater dont usualy get that cold or hot!) i have based calculations of the car running with alternator on at 14.4 volts?

 

[Diver300]

For the tail light of a car there is no point in using white LEDs.

Red LEDs are much cheaper and have lower voltage for the same brightness.

Constant current drivers are a very good idea as if you get a battery voltage reduction, if you turn off the engine, you don't want the tail lights to be the first thing that fails.