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Old 01 January 2013, 09:05   #1
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Powering a 10W LED SMD Chip ?

Any electrics enlighten people here on the forum?

Plenty of light from these chips but how to power them from a car battery or equivalent power source producing roughly 12.5 - 13.5V?

Here the specification:
- LED Emitter: 10W

- DC Forward Voltage (VF) : 9v-12 V DC

- DC Forward Currect (IF) : 900mA


As far as i understand they need constant current power source so its not just to plug and play but You also need some kind of a resistor between not to burn the chip? This even if the power source would be exactly 12V? Is this so or am i only making things complicated here...?
For me the the constant current thing is difficult to get, constant voltage is easier to understand. How to calculate the correct resistor (if its needed)

Thanks
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Old 01 January 2013, 09:27   #2
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RIBase
Search for info on PWM Drivers, I have built dive lights with 3w LEDs in the past running of 14.4v nicads.
There are plenty of schematics and some kits out there.
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Old 01 January 2013, 16:17   #3
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Use Ohms law. Current in circuit needs to be 900mA. Voltage worst case 13.5v

R=V/I =13.5/.9 = 15 ohms
Resistor needs to be capable of handling 15 watts and will get warm. May need heat sink. If voltage drops to 12v then LED will be slightly dimmer. Over current will be more likely to damage LED than slight overvoltage.
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Old 02 January 2013, 14:38   #4
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Thanks for advice.

I guess this one will do the job :
DC Converter Constant Current Voltage 5 35V 1 3 30V LED Driver Battery Charger | eBay

But on the other hand, if a simple resistor can do the job, that will be a good option.

I found a resistor calculator over here: Current limiting Resistor calculator for leds

no idea if its calculating correctly but it's suggesting 1.8 ohms/1 W resistor?
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Old 02 January 2013, 14:47   #5
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I am no expert but have gained a lot of information from candle power forums. CandlePowerForums
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Old 03 January 2013, 11:28   #6
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Originally Posted by Peter_C View Post
I am no expert but have gained a lot of information from candle power forums. CandlePowerForums
Thanks, will give it a try !
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Old 03 January 2013, 13:41   #7
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Also not sure if you are familiar with DealExtreme - Cool Gadgets at the Right Price - DX Free Shipping Worldwide I have bought most of my LED's from there, along with a host of other stuff. It takes a little while to receive it, but the prices are right.
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Old 03 January 2013, 13:59   #8
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I just ordered an led torch from them for a project. 6.99 from an amazon seller or 1.80 from DX delivered free from Hong Kong. I'm half expecting it not to turn up - I cannot see how the price covers the postage.

Lots of bits on there to build your own LED projects. - cheaper in my case to buy the whole torch though and strip for bits if not suitable.

Good to hear from someone who has dealt with them. It was verging on the 'too good to be true'.
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Old 03 January 2013, 14:03   #9
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Looks like it will run straight of your power supply direct to me. It will pull a bit more than spec, but will be ok in my opinion. Try one first.

To drop one volt at 1000mA you would put a resistor of one ohm in series. Approx...... I think you need nothing..
But putting a 4 ohm resistor in series will bring the voltage across the led back to spec. Must be a 4 watt resistor.

25 years of electrical engineering I have blown the odd thing...

I have run the domestic LED lights ment for 12 v ac or dc at 24 volt for a while, they are bright but did not blow... Short term..
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Old 03 January 2013, 14:08   #10
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I suspect that you only need a simple ( 12 Volt / 1 Amp ) Voltage Regulator.

Something like......

Buy Linear Voltage Regulators Voltage regulator,MC7812CT 12V 1A ON Semiconductor MC7812CTG online from RS for next day delivery.

also available from Farnell ( Onecall or CPC ).

It would be very good practice to also include a fuse of suitable value ( say 2 amps ) in case anything shorts....
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