Forward Voltage Question..

Jed

Say I've got some leds with a forward of 3v. I'm running off of a 5v power supply. How does that work when stringing them in a series? If I put 2 in a series the combined FV is 6v. ??

Mike2545

Yes, but the current stays the same.

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Mike2545

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Mike Green

You need some kind of current limiting, usually a resistor. Any time a voltage greater than the forward voltage of the diode is placed across it, the diode will conduct. Theoretically, there's nothing to limit the current going through the diode at that instant. In practice, power supplies have some internal resistance which limits the amount of current that can be supplied. In most cases, that current is still more than enough to melt the diode or at least one of its internal leads.

Jed

Ok, so the forward voltage is just a voltage limit for the diode? Sorry [noparse]:)[/noparse] I'm still confused. I thought that the forward voltage is the voltage that the diode "used up". In other words, I have a circuit: 5V battery -> 250 ohm resistor -> LED with 3V forward voltage -> ground.
From the way I understood it, the diode will use up 3V from the circuit. Is this incorrect? I guess I'm still confused as to what forward voltage is, and how I need to be worried about it in circuit design. I understand ohms law, but how do I fit the forward voltages into the calculation? If the LED uses up 3 volts, that means only 2 volts is left. If I put another LED in series that will use the remaining 2 volts leaving me with no voltage??

Mike Green

The forward voltage of a diode is a threshold determined by the physics of the materials that make up the diode and, to some extent, the physical structure of the diode. Below that threshold, the diode won't conduct (ideally ... in practice it conducts a little current). Above that threshold, the diode conducts. In real diodes, the forward voltage varies somewhat with the amount of current conducted and there are usually graphs on the datasheet for a given diode that show this. When calculating the voltage drops in a circuit, the forward voltage is subtracted from what's available, so it seems to "use up" some of the voltage available.

Jed

So will the LEDs in a series even light then if they are subtracting all the voltage from the circuit? What if I add a 3rd to the series?

Mike Green

That's not what I said.

If you have two LEDs in series, the voltage available divides across the two LEDs. The LEDs won't conduct unless the voltage across them is equal to or greater than the forward voltage. If you have 3 LEDs in series, the voltage divides across each of the LEDs.

If the forward voltage is 1.7V (typical for red LEDs) and you have two in series, you'll need at least 3.4 Volts across the whole circuit before the LEDs will conduct any significant amount of current. If you have 3 in series, that's a minimum of 5.1V. LEDs have to be conducting to produce light.

Craig Eid

Jed,

The forward voltage rating of an LED is generally listed on a datasheet at a specific forward current with a typical value and a maximum value - see the attached datasheet for a red T 1 3/4 5mm red LED [noparse][[/noparse]http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=67-1074-ND]. The datasheet will also list the wavelength and the intensity (generally in millicandela) and other parameters.

Per the attached·datasheet, the typical forward voltage is 2.0 volts at 20mA and the forward voltage will increase as the forward current increases. Your example of a single LED operating from·a 5V power supply would require a 150 ohm resistor in this application.··Be aware of the maximum forward voltage rating of the·LED as operating above the maximum rating can destroy the LED.



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Craig Eid

www.TriadRD.com

Beau Schwabe

Most Digital Multimeters have a position where you can measure the forward voltage of an LED, usually this is under the diode check. However, some Multimeters may not produce a voltage high enough under the diode test for the LED. In this case, you can configure a 5V regulator (7805) in a constant current mode to determine the forward voltage of an LED. This method also works to measure the forward voltage of a diode.

Note: Since this circuit keeps a constant current of about 10mA(typical) 13mA(max), you can string a few LED's in series as long as you have enough voltage overhead.

For example, if your supply voltage were 12V, then your voltage overhead would be about 7V .... 12V - 5V = 7V ... if the forward voltage of your LED's were 1.7V, then you could place 4 LED's in series using this circuit ... 7V / 1.7V = 4.12 = 4 LED's



Once you determine what the forward voltage is for what current you will be running the LED's at, then using Ohms law, you can calculate the proper current limiting resistor.

Current_Limiting_Resistor =· ( Vsupply - Vled ) / Iled

Where:
Vsupply = the source voltage
Vled = the forward voltage drop of the LED (add the total forward voltage if you are using them in series)
Iled = the LED current







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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 3/6/2009 6:39:55 PM GMT

SRLM

Beau said...
Most Digital Multimeters have a position where you can measure the forward voltage of an LED, usually this is under the diode check.

As soon as I read this, I took a look at my meter and sure enough, it does have a little diode symbol. I didn't get it to work for me, but I'll keep messing it. Thanks for mentioning that. I now know what every switch on my DMM does...