## Using l.e.d.s

The following was written for the F.A.Q. section of one of the forums.

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The following is an attempt to anticipate further questions relating to the use of l.e.ds. and, in retrospect, zeners.

The first thing to state is that l.e.ds are current operated devices.

It will be useful first to look at the following, which I will call a “generic diode” curve. You can see that if we apply a voltage supply across it in the forward direction, i.e. positive to anode, negative to cathode, and increase it from zero, very little current flows until the voltage reaches the forward “knee”. Beyond that point the current increases very rapidly for a very small increase in voltage.

This is true for most, if not all, diodes,whatever their type or purpose.

If we were to apply a voltage across an l.e.d. in order to light it, we would have to be quite careful to limit the voltage to a value somewhere on the “knee” so that the current couldn’t increase above a safe figure.

Consequently leds are always used with a resistor in series to limit the current. (In fact, some led indicators are packaged with a built-in resistor.)

To calculate the value of the resistor we need to know two important characteristics of the diode. These are the forward voltage, Vf, (which is a value on the “knee”) and the maximum current, If (max), published by the manufacturers and often included in stockists’ catalogues.

Typical figures for a red l.e.d. are 2V and 20mA.

Suppose we want to operate one from a 12V supply.

The l.e.d. and resistor are in series across the supply, the voltage across the diode will be 2, so the voltage across the resistor will be 10.

If the current through the resistor is to be 20mA with 10V across it, its value must be 500 ohms. The nearest preferred higher value is 510. Using this would allow 19.6mA to flow. It would be more prudent to use 560 ohms so the current would be 17.9mA.

The forward voltage for a blue l.e.d. might be 5, with maximum current also 20mA.

To use one of these from a 12V source, the resistor value would need to be 12 – 5 = 7, divided by 20 = 0.35 (kilohms, because the current is in milliamps), = 350 ohms.

Nearest higher value is 390 which would allow 18mA.

You will see some circuits where l.e.ds are wired in parallel.  It is not a good idea.  The characteristic curves of several of the same type of diode will not be identical.  This means that at any given current the voltage across each may be different, or, conversely, with a certain value of voltage across them the value of current through them may be different.

What this means is that if there are two or more l.e.ds in parallel, since the voltage across them all is the same, one of them may (probably will) take more current than the others.  Its brightness will therefore be higher, and, unless you take care to calculate the shared resistor value so as not to run them at or very near their maximum current, the current through that one l.e.d. may exceed the maximum.

The maximum reverse voltage which an l.e.d. can withstand, i.e. a voltage on the reverse biased "knee" area, is typically 5.

A zener diode's curve is much like the generic one above. It is operated in the reverse biased mode, i.e. on the reverse knee.
The same reasoning about limiting the current applies.