Hello mates!

 

I want to use a high power CREE LED at my car but I don't know how to wire it.They run at 3V and 1500mA.My friend said to use a 1k resistor and that's it

So how do i convert the 12V to power a 3V led?

 

 

This LED is quite expensive,and very powerful 500Lm

 

 

 

 

 

it won't light with a 1k resistor. if it needs 1500mA or 1.5A then ohms law says V=IR. therefore 12/1.5= 8 ohms for the whole circuit. The LED presents 3v/1.5A= 2 ohms therefore you need 6ohm resistor at 1.5 amps which is a big resistor. eg

 

http://lcled.en.alibaba.com/product/478840864-210792006/50W_6ohm_Load_Resistor.html

 

Possibly more elegant finding a 3v regulator and wiring it like this:

 

http://www.duckd.cz/en/voltage-regulator-5v-33v

Thank you very much.

This is the technical chart of the LED                  http://www.zexstar.com/files/fisa%20tehnica%20cree%20xp-g2.pdf

 

I like the first choice with a resistor is much simple for me.

Actually that resister I posted to earlier is a lot over kill for the power (i was in a hurry). the 6 ohm resister is dropping 9v at 1.5 amps so give or take 13.5 watts.

 

This one below is a bit under spec'd but as its 6.8 ohm so the current will be slightly less and the LED will last longer, this resistor will get quite warm though it will need to dissipate 12 watts or so, a 15 or 20 watt resistor at 6 ohms would be ideal.

 

http://www.maplin.co.uk/p/wirewound-10-watt-68-ohm-resistor-h6r8

can you please explain one more time how do you  calculate the rezistor you need.

I'm a novice in electronics.

You would be much better off with a DC-DC converter. They are 95% efficient and so they generate very little heat unlike a resistor.

This is the first one I found on ebay http://www.ebay.co.uk/itm/DC-DC-Converter-12V-Step-down-to-3-3V-3A-15W-Power-Supply-Module-waterproof-/121233814228

A DC to DC converter is a regulator as posted above, but as they come pre assembled in a little box that might / would be an easy option. 

 

 

To calculate the resistor:

Volts= amps x ohms (or V=IR)

12v = 1.5A x R ohms  therefore R=8 for the whole circuit

 

The LED uses 3v at 1.5A. 

so

3v = 1.5A x 2 ohms

 

however we alreadly calculated 8 ohms for the whole circuit  so we need another 6 ohms which is the resistor. The resistor is dropping 9v

(12v minus 3v for the Led = 9v)

 

The power disipated by the resistor is P=IV (power = Amps x Volts)

 

1.5A x 9v = 13.5 watts resistor required. (Slightly less for the 6.8 ohm as there will be slightly less current)

Edited 14 December, 201312 yr by paddypaws

thanks,I'll print your post

...A DC to DC converter is a regulator as posted above...

Actually they are a switch mode power supply, and unlike a resistor or regulator, they are 95% efficient, so produce little heat. They are also cheaper that most 15W resistors.

Some LED modules are sensitive to voltage so you need to use at least 13.8V in your calculations, if you are using a resistor as the supply is 'unregulated'. If you stick a meter on the supply, it is rarely as low as 12V.

As per the LED data sheet it can handle up to 3.5 volts so not that sensitive even at 13.8 volts it'll be fine. Resistors aren't expensive at all eg http://uk.rs-online.com/web/p/panel-mount-fixed-resistors/7204880/

Edited 14 December, 201312 yr by paddypaws

13.8V is the minimum you should be using for your calculations. Output range for the alternator should be considered to be 13.8V to 14.4V

 

For an expensive LED, a proper regulator is a much better option. A regulator and two electrolytic capacitors is pretty cheap for peace of mind.

 

In fact, probably less expensive than a 15W resistor, which are usually aluminium heat sink housed.

Edited 14 December, 201312 yr by demonufo

Bit of confusion about how leds and other diodes work. The forward volt drop of a diode should be thought of as a voltage threshold that must be reached before any current flows (a simplification but you will see how this helps). [The datasheet values of foward voltage are a result of the current  flowing and NOT a specification of what it will handle as implied in one post] 

 

The data sheet for this CREE device states that 1500 mA is the Maximum current that it will handle and must be de-rated further for high ambient temperatures and poor heat-sinking used to take away the heat generated. Note also that it is a surface mount device that is normally used on a substrate that has been sized to dissipate the about 3 watts being generated. My suggestion is to set the operating point at 1000 mA. This also coincides with a 3 V forward drop and 3 W dissipation in the LED. But don't just connect wires to it - it needs a heatsink. Take a look at the new CREE torches on the market this Christmas. Their aluminium case is like a heatsink or radiator in order to get rid of the heat generated by the led. Take a look at the datasheet for how to solder to a pc. board.

 

Using a dropper resistor is not ideal in a car because the voltage is unstable. However you can calculate the voltage across the resistor by first subtracting the diode fwd voltdrop from the maximum supply voltage. So (14.4V - 3V) = 11.4 V. With a required set-point current of 1A [1000 mA], this needs a resistor of (11.4/1) ohm or 11.4 ohm.

This resistor has to dissipates (11.4V x 1A) watts which is 11.4 Watt. You would normally use a wirewound resistor of twice this power rating or more to keep its surface temperature low.

 

You can also see from this calculation that the resistor is dissipating nearly three times the energy of the led. In total you must loose around 14 W somewhere!

 

As mentioned earlier, the efficient way is to use a 12V to 3 V converter designed for car applications - such as the one in the link. This is also regulates against the varying input voltage of a car battery. Efficiency is 95-99% so now the main loss is just the 3 W in the led and the light output is constant against fluctuating battery voltage.

 

Just as an side, the "puddle light leds" in another thread appear to have Led Drivers (read regulators) built into the strip for every three leds. 

 

A long answer but I hope this gives people an insight into the subject.

 

pikpilots numbers look correct.to the data sheet. I'd taken the o/p s numbers, supply 12v with an led running at 3v / 1500ma.