Question about LED and resistor

[NealM]

First, thanks to all of you for all the great ideas that are shared on this forum, I’m learning a lot following it.

I’ve got a question about LEDs – I’ve seen them discussed here before but now I have a specific question and can’t figure out what I should do.

I’m panning on using LEDs to light a building. I’ve got some white  LEDs – FW current: 25mA  , FW supply: 3.3V.

I was planning to light them with a 3V power supply. When I checked one of the on-line resistor calculators, it told me that I don’t have enough voltage to light the LED, yet it still lights, it even lights at 1.5V.

As a test I’ve been running the LED for a couple of days at 1.5V - so far so good without a resistor.

Do I need a resistor? If so, what value?

Thanks, 

Neal

[lionelsoni]

You're probably getting much less light than the LED is capable of.  All semiconductor diodes will draw some current right down to zero voltage, but that current drops off rapidly below a voltage that is characteristic of the device type, 3.3 volts apparently in this case.

What kind of 1.5-volt supply are you using?  A dry cell perhaps?  If yours is an AC supply, the actual peak voltage is around 2.1 volts, which makes your getting a usable amount of light more plausible.  Whatever you do, do not subject the LED to more than 5 volts in the reverse direction, which may destroy it.  If you're using an AC supply, that's 3.5 volts RMS.

The way you calculate the resistor size (for operation above the specified forward voltage) is as follows:  Subtract the forward voltage (3.3 in your case) from the available DC supply voltage.  Divide by the desired current in amperes (.025 in your case, for full brightness).  The result is the resistance in ohms to use.  If it is not a standard value, just go up to the next higher value that you can get.

To calculate the resistor's power dissipation, again subtract the forward voltage from the DC supply voltage.  Square this number.  Then divide by the resistance that you are actually using.  The result is the power in watts.  Your resistor's power rating should be no less than this; and a rating of twice what you calculated will keep your resistor reasonable cool.

An example:  Suppose you want to run at full brightness from a 9-volt battery.  Subtract 3.3 from 9 to get 5.7 volts.  Divide by 25 milliamperes to get 228 ohms.  You can probably find a 270 ohm resistor, which is close enough.  Square 5.7 and divide by 270 to get 120 milliwatts.  So use a 1/4-watt 270-ohm resistor.

If you want to get deeper into LED circuits, you might want to find the article I wrote in the September, 2008, issue of CTT.

[NealM]

Bob, 

Thanks for the response. For power I'm using a universal AC/DC adapter set to 1.5 volts. The LED is dimmer than it was at 3 volts, but for the purpose of lighting a model building, it seems like a good level of brightness - it seems less like the building is flooded with fluorescent light.

Thanks for the formula for calculating resistor size. I notice that you qualify it for operation above specified forward voltage. Does that mean that if I use less than specified forward voltage I don't need a resistor?

Neal

[gunrunnerjohn]

If you use less than the forward voltage, the LED won't light.   I recommend you always include current limiting for an LED.