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Old 03 January 2013, 14:25   #11
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You don't say what your application is. You imply you'd use a 12V Lead Acid type battery if it was that easy. it is but you'll need an in series resistor.

You don't need a voltage regulator if you are using a car battery. LEDs aren't THAT fussy - so 9-13.5V would be fine. What fries LEDs is current. Basically if you put the LED across the two battery terminals it will try and shove as much current as possible through the LED and will explode.

While voltage regulators may also regulate current they aren't really designed to. Instead they are designed to be able to regulate voltage and provide a certain amount of current. Most when asked for more current would just shutdown or provide less voltage. Putting an LED across the regulator will just behave like a short.

So go back to what Donald says on post 3. Ohms Law:

V= I R so you want I < 900mA, R = V / I So to get I < 900mA, R > V / I or R>13.5/0.9 therefore R>15ohms.

You then need a power rating for the resistor:
P= I V then 0.9 * 13.5 = 12.15W

If you consider a drained batter as 9V with the same resistor I becomes 9/15 = 600mA, and P = 5.4W so Donald has worked through your worst case, I could be persauded that worst case might be 15V or 18V as I've seen alternators and chargers chuck out some silly power occasionally.

If you need more than one I'd look at:
SBCHE1515RJ - TE CONNECTIVITY / CGS - RESISTOR, 17W 5% 15R | Farnell United Kingdom
(pack of 5)

If just 1 needed look at:
WH25 16R JI - WELWYN - RESISTOR, 25W 5% 16R | Farnell United Kingdom

Bear in mind though that the resistor will heat up and waste power. So if you have a 15W resistor in there you need to decide if thats better than putting in a 15W conventional bulb. How much light are you getting? How much light do you need.

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Old 03 January 2013, 14:32   #12
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Originally Posted by C-NUMB View Post

no idea if its calculating correctly but it's suggesting 1.8 ohms/1 W resistor?
It is correct, not sure you have used the right variables. If you put a supply voltage of 13.5, a LED voltage of 12V, and 900mA with one LED you'll get the kind of numbers we got. If you drop it to a much lower voltage on the LED it will need a lower resistance and power. But thats not what the spec implied.

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Old 03 January 2013, 15:10   #13
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You will not need a massive 15 watt resistor, just a 4 ohm 4 watt. But best is this

10W LED Driver DC8-26V AC/DC 12v For 10 Watt LED Light

On eBay for 3... USA I think....
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Old 03 January 2013, 15:21   #14
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ShinyShoe , you are quite right that C-numb doesn't state the appplication , but he does say it's 10W .

Surely your typical 0.2" LED only wants about 15mA normally , only 1mA if it's low current .

We can't be talking about a single led here can we ?
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Old 03 January 2013, 16:01   #15
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They are 10 watt LED array chips running around 10 volts at 1 amp, they give a lot of light out, think Audi front head light.. Almost.......
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Old 05 January 2013, 10:48   #16
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A lot of good, useful comments, thanks!

Mainly have been purchasing "Plug & Play" leds, first in flash light applications, but later also the house lighting has been converted to LED light, guess my home lighting is now 90 % converted to led. The winter here is long and dark, and the kids seams to make a sport of leaving all the lamps permanently on needed or not, home or converting to leds is one way to reduce the electric bill(but at a cost off course).

Branded ones (Philips, Osram ..)are extremely expensive but on the other hand the quality of some of the stuff from is pretty bad. The tint of the led is pretty important(for home applications I use only warm white), also using correct Wattage(hig enough) for the each spot is critical for a comfortable nice light. Most halogen transformers will not work well as a LED PSU, need to swap them for specific led drivers so that is also one thing to consider.

Recently have found good led's on both and For time been, SMD leds has the best price/lumen ratio, however they works best in applications where indirect light can be used.

The idea of playing whit led chips come after a purchase of a 10W SMD LED Floodlight. Plenty of light, no heat and a very low power consumption at a reasonable cost. Planning to install 6 of those in my garage as working light.

Yes, single leds was the initial topic, almost like like these:

1p 10W Warm White High Power Bright 900LM LED Lamp SMD Bulb Chip 9-12V DC AR | eBay

The high power smd chips are priced reasonable when sold as bulk, so it does not matter even if will destroy a few...Now there is a new type called COB leds, I am waiting for first delivery. For the home made stuff don't yet have such specific applications, will test and see what comes up.

DC, Volt, Amps, Watt stuff is somehow within my comfort level but regarding ohms, diodes I am on thinner ice so will be in troubles regarding anything that does not run on constant voltage. But hope to learn more and get help from various forums!
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Old 06 January 2013, 11:12   #17
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Semiconductors are weird, as the conductance (or impedance if you want to look at it that way) varies over different circumstances. In other words, as you light an LED, it will drop, say 2.7V. Drive it harder (pump more current through it) and the voltage drop does not necessarily track the increase in current, and may, for 3 times the current, drop only 2.9V.

I'm with Simsy on this one: An LED driver is designed to supply a constant current through whatever's across its load pins (rather than a constant voltage, as a voltage regulator does.) Makes it less likely to fall out of a safe (for the device) operating range.
You can also find drivers with output adjustment leads in case you want a dimmable setup.

That array most likely consists of 3 to 5 LED junctions, as they typically take 2 to 3 volts or so each. The high power LEDs drop a lot more than the normal .2" low power ones.


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Old 06 January 2013, 11:55   #18
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I agree also. White LEDs are much more fickle about current than the old red, green, yellow LEDs that could be driven by a simple resister circuit.

This is one of the reasons why even cheap LED devices such as pocket torches have a drive circuit, although for single 1.5v battery devices this will also act to boost voltage to LED operating range.

Fwiw iirc that the correct formula for driving an LED is

R [ohms] = v(batt [volts]) - v(LED forward [volts]) / i(LED forward [ma])

This takes into account the voltage drop across the LED where as simple ohms law does not. Iirc that there was a discrepancy in resister value above and this probably explains why.
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Old 07 January 2013, 09:11   #19
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If you have a soldering iron, this seems a cheap solution.

Super simple high power LED driver
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Old 10 January 2013, 00:30   #20
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You need a proper LED driver. If you start your engine, voltage goes to 14.5V, will shorten LED life. (maybe to 1 second...)
A simple resistor will work, if you take a generous safety margin. But realize that that resistor will get very hot and that it will need cooling air but you don't want it to corrode.

Also, make sure that you have a good thermal sink on the LED. I have played with them and driven at full capacity they get too hot within 10 seconds. It really is an enourmous amount of light!
A good idea is to use the LED at 500 mA. Light goes down about 15%, but heat production will be lowered at least 50%.
All my orders from have arrived, about 15 so far. Quality is usually " good enough ", with a couple of exeptions both ways.

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