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How do I use 3v LED's without a special converter

post #1 of 7
Thread Starter 
I want to use my 3v orange leds. I don't feel the need to pay $30 just to power some 3v led's. How can I use these ?

If I wired 4 of them in a series, would they be able to run off a 12v psu line?
post #2 of 7
yes
in series each led would get the same ammount of current flow.
but orange leds are not nessisarily ~3v but very close to that. I think most oranges are speced for a lot lower voltage. more like 2.5-2.8 , so add a bit of resistance.

It depends on:
1) the actual gate voltage (Vf). which depending on the actual frequency of orange, it should be lower voltage than that.
2) The current handling capability of the led, be it a small 5mm or 10mm one or a big fat 1-3W type high powered led. The more current it can handle the more current you can cram into it, without shortening its life.
3) any built in resistance or diode packages that exist between the gate and power, that is already in the design.

So you need to know some of its specs first. or even a loose idea of its current handling.
because with 4 orange in series with 12V a small ammount of added resistance is likely to be needed for longevity.
The current through package being the most important thing, and cooling for the big fat high powered ones.

so add in a bit of resistance, or a adjustable resistance and measure.
measure the total current flow through the curcuit , that will be the current flow through Each item in series in that curcuit. Adjust the resistance until you get to a usable current flow, be that spec, or max spec. Piece of cake, and then it will last many many years.
Edited by Psycogeek - 8/15/11 at 2:35am
post #3 of 7
Yeah you will need some specs. I blew a 3mm green LED up today (on purpose of course) at 6v (Peaked at 4v i think). I just use a '12v' resistor (not sure what it is in OHMs) for 1 LED, you would imagine 4 LEDs in series would be ok, but I can't gurantee it.

EDIT: Now that i think about it, you would probably need a resistor for the sake of their lifetime.
Edited by Uncle Dolans - 8/15/11 at 3:56am
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post #4 of 7
If the LEDs are indeed rated for 3V (listed as the forward voltage on the datasheet), then you can wire 4 in series and run them off of the 12V rail.


BTW, the color of the LED does not dictate the voltage or current draw of the LED, I've seen plenty of red LEDs rated for 2.1V while others are rated for 3.3V TTL logic levels.

Note: if you have multiple LEDs in series, then they should all be rated for the same current draw (forward current). Also for series LEDs; naturally you don't want the voltage drop across the LEDs to be higher than the supply voltage, if the voltage drop is equal to the supply voltage then a resistor isn't necessary, a resistor should only be used when the supply voltage is higher than the voltage drop across the LEDs.


There are some online wizards (e.g. LEDlinear) for calculating series/parallel LED arrays with the necessary resistor(s), but I personally just do the calculations myself and have found the wizards can be slightly off (perhaps rounding errors).

The calculations for resistor sizing (using Ohm's law and the power relationship) are actually fairly simple and often faster than plugging stuff into a wizard.
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post #5 of 7
Thread Starter 
Quote:
Originally Posted by radodrill View Post
If the LEDs are indeed rated for 3V (listed as the forward voltage on the datasheet), then you can wire 4 in series and run them off of the 12V rail.


BTW, the color of the LED does not dictate the voltage or current draw of the LED, I've seen plenty of red LEDs rated for 2.1V while others are rated for 3.3V TTL logic levels.

Note: if you have multiple LEDs in series, then they should all be rated for the same current draw (forward current). Also for series LEDs; naturally you don't want the voltage drop across the LEDs to be higher than the supply voltage, if the voltage drop is equal to the supply voltage then a resistor isn't necessary, a resistor should only be used when the supply voltage is higher than the voltage drop across the LEDs.


There are some online wizards (e.g. LEDlinear) for calculating series/parallel LED arrays with the necessary resistor(s), but I personally just do the calculations myself and have found the wizards can be slightly off (perhaps rounding errors).

The calculations for resistor sizing (using Ohm's law and the power relationship) are actually fairly simple and often faster than plugging stuff into a wizard.
I know about Ohms law, and electrical physics. I just wanted to know about the longevity and possibility of just soldering the LED's together.
post #6 of 7
Quote:
Originally Posted by noak View Post
I know about Ohms law, and electrical physics. I just wanted to know about the longevity and possibility of just soldering the LED's together.
As I said before, you only need a resistor in the series string if the sum of the LED voltage drops is lower than the supply voltage; if it's equal to the supply voltage, you don't want a resistor.

The life of the LEDs is adversely effected by over-volting them; not by supplying exactly the voltage they're rated for.
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post #7 of 7
http://en.wikipedia.org/wiki/Light-emitting_diode
specifically refer to the voltage of the gate items itself on non-phosphor based led items.
as to that applying to logic level outputs when instead being driven by a PSU itself, apples and orangatangs .

Any led can have built into it now amazing curcuitry, there exists a 3mm led that had 3 colored gates (rgb) and built in curcuitry to cycle through the colors. the curcitry smaller than the head of a pin.
they are also working on various colored leds that the colors are phosphor based, not the original gate frequency. so that would also make a world of difference.
but when it comes to the original gate frequency on commonly use leds, the voltage is commonly known.
So
basically any led or led curcuit could be set up for an array of voltages, but the semiconductor gate itself on common leds that were getting for cheap, and today,is pretty specific.

so back to the spec sheet, because with any led PACKAGE , the par gate voltage would not mean a lot. if they have something else stuffed into the package.
Many of the things i see marking a Orange as 3V , are just loose specs tossed out to sell stuff, the real actual voltage will be shown when an expert shows the graph, or a user fures up a meter and sees what they are really holding.

the reality spec is when you shove the voltage in and reach the current it is speced for, that will be the voltage you would want to run it at. 1 (mili)ampmeter, 1 adjustable resister, and 2 minutes of time, and long term happyness.
Edited by Psycogeek - 8/17/11 at 7:29am
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