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04-16-2012, 08:39 AM   #16
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Quote:
 From my understanding here we don't need a resistor because since our power supply only deliver 1.8V, there is no way the LED would draw more than her normal voltage, and no higher voltage means no higher current so the current it will received will be a little less than its specification here and then the light output will be a little dimmer. Or am I wrong and the LED will drawn 1.8V and will be hit by the 500mA and die right away ?
The LED doesn't "draw" voltage from the power supply. In this example, the LED will never be forward biased because the power supply output is too low and therefore the LED will not allow any current flow.

Let's assume you have a variable output power supply that goes from 1 volt to 3 volts and can deliver 500 ma of current.
Hook your 20 ma rated LED up to this PS and start adjusting the voltage upward. There will be no current flow until the LED forward biases. At 2 volts the LED will forward bias and burn up because there will be 500 ma flowing through it.

04-16-2012, 11:18 AM   #17
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Quote:
 Originally Posted by a7ecorsair The LED doesn't "draw" voltage from the power supply. In this example, the LED will never be forward biased because the power supply output is too low and therefore the LED will not allow any current flow. Let's assume you have a variable output power supply that goes from 1 volt to 3 volts and can deliver 500 ma of current. Hook your 20 ma rated LED up to this PS and start adjusting the voltage upward. There will be no current flow until the LED forward biases. At 2 volts the LED will forward bias and burn up because there will be 500 ma flowing through it.

Thanks I understand
I need a small precision in a similar matter but with a different power supply. If I take 2 LR6 1.5V 2600mah battery and hook them up and take a 3V LED 20mA, in this case the LED will be forward biased but what current will it get and will it work ?

04-16-2012, 12:01 PM   #18
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Quote:
 Originally Posted by joed It is designed to be used on a 2 volt supply. A true bare LED has a voltage of .7 volts. This one has been designed for 2 volts. They also make them for other voltage. I have used the 12 volt ones often. They don't need any resistors. Now if you want to use the 2 volt LED on a different voltage than 2 volts then a resistor is going to be needed.
Typical LED's have operating voltages from about 1.8 to 3.5V. Voltage is determined directly by the color of the LED. See my earlier post on this, or the wikipedia article on LED's for a decent explanation. A 0.7V LED would be very dark red or infrared and is unusual. A 2V LED would probably be green.

All LED's have a specified operating voltage (discussed in detail in my earlier post), but this does not mean they are intended to be used with a constant voltage supply and thus don't require a resistor. Here's an LED data sheet for a green LED with a rated Vf of 2.1V for example: http://media.digikey.com/pdf/Data%20...ST-C191GKT.pdf

04-16-2012, 02:27 PM   #19
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Quote:
 Originally Posted by Khivar Thanks I understand I need a small precision in a similar matter but with a different power supply. If I take 2 LR6 1.5V 2600mah battery and hook them up and take a 3V LED 20mA, in this case the LED will be forward biased but what current will it get and will it work ?
Get yourself a variable resistor to work with and use a 12V power supply. Using a 3V supply to illuminate a 3V LED is a poor design.

04-16-2012, 06:15 PM   #20
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You can make a solid state series resistor that will act as a constant current source using an LM117 voltage regulator and a resistor. For 20mA select R to be approximately 62 Ohms. 56 Ohms is a standard value and that will yield 22mA across a widely varying input voltage up to 30 volts. That might take some of the guess work out of it.

 The Following User Says Thank You to curiousB For This Useful Post: mpoulton (04-17-2012)

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