16 replies to this topic

[langers2k]

I am thinking of changing my bulbs for LED ones.

I know that some people have problems with them burning out, how common is this? I don't want to spend 20-30 on bulbs and flasher units for them to burn out within 6 months or a year.

Found some nice big LED bulbs rated at 13W each, so hopefully they should be very bright, can car LED's withstand upwards of 14V for prolonged periods of time or not? I was thinking of using voltage regulators on each bulb, found some that should work and only have a .5V voltage drop. So as long as my car has more than 12.5V I will get full power from them!

Any one got any opinions on the reliability of LED bulbs?

These are the bulbs I am looking at using...

[il-barba]

i think you should include a resistor in series with the led's so you reduce the current entering them

[langers2k]

Thought about resistors, decided I may as well use the 12V regulators.

That way I know that my bulbs will always have enough voltage to work properly without risking too much current. If I were to use resistors then I could set them to work properly at 15V (12V for the LED and 3V across the resistor) but it would mean at 12V (9.6V LED, 2.4V resistor) they would be dimmer. Unless the alternator is bad the voltage should never drop below 12.5V so in theory the bulbs would always work at maximum power!

[DaveRob]

LEDS have a different forward voltage drops dependant on colour none I am aware of have 3V forward drops......... you need a resistor to control the current. The expression is all about POWER .... dont mix stuff up I=V/R P=VI in DC ccts. A conventional LED will be rated at about 10-15mA each unit ...go over that and it will die. If you stick a 12V regulator in you will also need resistance in the circuit to controll the current to the LED....The regulator simply fixes the voltage so the current doesnt fluctuate. so at 12V regulation and an LED that draws max 15mA, ( check yours ) 0.015=12/R theirfore R=12/0.015 this is 800R. The power consumption is P=VI so P=12*0.015 so this is 0.18W so you need , in this case, the closest Res to std is 820R and 1/4 watt will be fine.... all of this is rough approximation but these values will be fine.....

Edited by DaveRob, 01 February 2008 - 09:59 AM.

[langers2k]

The problem arises when the voltage is above the rating. As the voltage increases, as the resistance is fixed in the LED it will increase the current flowing through the LED and destroy it.

As the LED's I am looking at are 12V 13W:

I = P/V = 13/12 = 1.1 amps max current for the LED
R = V/I = 12/1.1 = 10.9 ohms resistance across the LED

Assuming a supply of 15V (can't see my car producing this much but you get the idea):

I = V/R = 15/10.9 = 1.4 amps which blows the LED up

We would need to put a resistor in:

R = V/I = 15/1.1 = 13.6 ohms for both the resistor and LED

As they are in series and the LED is 10.9 ohms, we would need a resistor of at least 2.7 ohms

If we put the resistor in series with the LED it would protect it from too much current. However at normal voltages (ie 12V) the resistor would be limiting the current available for the LED.

I = V/R = 12/13.6 = 0.9 amps

P = VI = 12*0.9 = 10.8 watts

For me the idea of LED's is to get as much power out of them as I can to brighten my back end, by adding a resistor I could be limiting the LED to 83% of its power and removing the advantage that I was looking for.

By using a voltage regulator I can make sure no more than 12V is supplied to the LED, if the resistance is fixed in the LED then it cannot pull more than 1.1 amps (therefore 13W) and should never damage itself.

Am I right in thinking the LED's resistance will be fixed or have my calculations messed up somewhere?

[Ethel]

I was under the impression a resister would suffice 'n would have thought one would be built in if the bulb is intended for 12volt auto application.

If you do go with a voltage regulator why not step it down even further, say 5-6v to guarantee the battery will always supply more than the leds need?

[DaveRob]

hmm.... sounds like you arnt using LEDS on their own as an LED is just a Light Emmiting Diode..... IE its just a PN junction that emmits light when transmitting a current. They dont have a 'resistance' as such. from what you are describing you have a system that already has the additional electronixcs and resistors in place inside the housing. and just need a regulator to prevent the voltage from going too high. If I was doing this Id layout my own cct from scratch, but you look like you have a module you want to use. If your thinkin of using something like a 7812 V reg then its rating is only about 1A supply anyway. If you try to drop down to 5 volts you will draw more current and also the reg will overheat dropping the 7 odd volts....

Just a note... the LED resistance isnt 10R your module might have a percieved impedance of x ohms but I thibnk thats more likly to be with additional cct in the module.

[langers2k]

I was under the impression a resister would suffice 'n would have thought one would be built in if the bulb is intended for 12volt auto application.

If you do go with a voltage regulator why not step it down even further, say 5-6v to guarantee the battery will always supply more than the leds need?

Because at 5-6V the LED's I want to use won't have enough power, might try and find some lower voltage ones and then it would work.

hmm.... sounds like you arnt using LEDS on their own as an LED is just a Light Emmiting Diode..... IE its just a PN junction that emmits light when transmitting a current. They dont have a 'resistance' as such. from what you are describing you have a system that already has the additional electronixcs and resistors in place inside the housing. and just need a regulator to prevent the voltage from going too high. If I was doing this Id layout my own cct from scratch, but you look like you have a module you want to use. If your thinkin of using something like a 7812 V reg then its rating is only about 1A supply anyway. If you try to drop down to 5 volts you will draw more current and also the reg will overheat dropping the 7 odd volts....

Just a note... the LED resistance isnt 10R your module might have a percieved impedance of x ohms but I thibnk thats more likly to be with additional cct in the module.

No the LED's come packaged as an automotive bulb, so you would have thought they would be able to cope with over 12V, but like I say I have seen ones that have burnt themselves out suggesting they can't deal with limiting voltage/current themselves.

The voltage regulator is similar to the 7812 but has a lower voltage drop across it and a higher current. The ID is KA278R12C.

Assuming that I want to use pre-packed LED bulbs, what would you see as the best way to ensure I always give them full power without damaging them?

[DaveRob]

Just use a regulator with more than 1.1A capacity.... and feed the bulb pack with that.... if your battery is running less than 12v you have bigger problems than powering your LED pack

[langers2k]

Sounds good!

Just need to get some regulators sent to the UK now...

Thanks for all your help!

[DaveRob]

You can buy them from Maplin Electronics.... I think their one not far from you

[nurseholliday]

Why don't you just buy LED bulbs that have all the necessary components built into the bulbs and are ready to be fitted as a direct replacement to car bulbs?

[langers2k]

You can buy them from Maplin Electronics.... I think their one not far from you

All the ones I have seen in Maplins had a higher voltage drop across them so I was going to try and get some with a lower voltage drop.

Why don't you just buy LED bulbs that have all the necessary components built into the bulbs and are ready to be fitted as a direct replacement to car bulbs?

I have seen LED bulbs that have come packaged as a direct replacement burn out. Not sure if it is only cheap bulbs or just the bulbs I have seen but if I am getting some I want to make sure they aren't going to go pop!

[jayare]

I've had LED rear side/drake light bulbs in my Mini for getting on for 4 years now & they're fine. Can't remember what brand they were, but they came from my local Motorists Centre. I didn't bother with the indicators though due to needing to fiddle with the flasher unit, etc.

JR

[pedros]

now i might be mad and confused but i find it a little confusing the way you talk about current.

Current is drawn not supplied.
voltage is supplied and it is the voltage that is an issue hence the resistor, all the resistor does is draw additional current which is in effect limited by your fuse which is only there to protect the wiring not the accesories though invariably it does as the cabling should be sized appropiately (except in a 2 fuse setup on a mini it seems).

one of the biggest problems with leds in my experience was always surge on startup which was helped by a small capacitor and resistor to smooth and limit voltage.

if i have got this wrong i apologise but i am fairly sure from my days a hungover electronics student at uni this was the case. but always willing to (re) learn.