Using LEDs

This article is an attempt to explain how to use Light Emitting Diodes (or LEDs), and is aimed at those with little electrical training. However, some simple calculations are unavoidable to obtain proper operation.

LEDs are the modern solid state equivalent of the light bulb. They each generate light, but this is where the similarities end. LEDs are inherently more efficient, more reliable and more rugged than light bulbs.

When first introduced, LEDs were only available in red and had limited brightness, but things have improved significantly and now come in red, orange, yellow, green, blue and white, or even multi-colour, and ultra-high brightness versions are available. They can be used for marker lights, tail lights, signal lights, cab lights, clearance lights and general lighting. Due to their internal construction, they usually emit light towards the front in an approx 60-70 deg angle, so for applications where general lighting is required, buy the type with a diffused (milky) body, or use a diffuser. The body of a LED does not have to be coloured for it to emit its colour, it may be a totally clear package.

Questions commonly asked are "What voltage do LEDs need?", or "What is the voltage rating of a LED?".

This is a relatively non-answerable question. Although a voltage is present across them when operating, they are NOT a voltage dependant component. LEDs do require a current through them to operate, and require a current limiting device, usually a resistor. It is the value of the current through the LED which determines their brightness. Every LED has a maximum permissible current, and if this is exceeded, will damage or destroy it.

In short, the higher the current - the higher the brighness, or the lower the current - the lower the brightness. But that operating current MUST be controlled and limited below the maximum rating.

LED Symbol This is the usual circuit symbol for a LED.
The "A" means Anode (positive or +ve) and the "K" means Cathode (negative or -ve). The arrows represent the light output.

LED Dwg Identifying the connections.
There are 2 legs, the longer lead is the Anode and the shorter one the Cathode. There is also a small flat on the body next to the cathode.
LED Image

Although LEDs are not voltage dependant devices, they do produce a voltage across the device when the operating current flows through them. This voltage varies from approx 1.6V to 4V depending on the type. The table below shows some typical values for common LEDs.

Type Colour IF max VF typ VR max Luminous intensity
 Standard  Red 30mA 1.7V 5V  10mcd @ 10mA
 Standard
 Bright Red
30mA 1.9V 5V  80mcd @ 10mA
 High Intensity   Red 30mA 1.85V 5V  500mcd @ 20mA
 Standard
 Yellow 30mA 2.1V 5V  30mcd @ 10mA
 Standard
 Green 25mA 2.2V 5V  30mcd @ 10mA
 High Intensity   Blue 30mA 3.6V 5V  650mcd @ 20mA
 High Intensity   White 30mA 3.6V 5V  600mcd @ 20mA
 Ultra High Brightness   White 350mA 3.6V 5V  125cd (125,000mcd) @ 350mA
I F max Maximum forward current. 1mA = 1 milliamp = 0.001A
(forward means the LED connected the right way round)
VF typ Typical forward voltage.
(VL in resistor calculation)
VR max Maximum reverse voltage
(reverse means LED connected back to front)
Luminous intensity Brightness at the given current, mcd = millicandela

How to use a LED

IF and VF typ are the 2 LED parameters needed for calculations.

LED cct
Fig. 1

The simplest and usual way of connection. it to just a resistor in series with the LED to limit the current as shown in Fig. 1, and connected across the power supply.

VS - supply voltage (+ve)
Gnd - ground or 0 Volts (-ve)
VL - the LED's typical forward voltage (VF typ from the table above)
I - is the LED operating current and the arrow represents the direction of flow
R - current limiting resistor
(VS - VL) is part of the calculation we need to determine the value of R

Now we must do some simple calculations to determine the value of R. (Example values shown in green)

Step 1

Set some basic values first:- (using a Bright Red LED operating from 6V for this example)

» Select the LED to use, look at the specifications and get the value of VL (VF) (1.9V)
» Choose the operating current (IF) (This must be less than the IF max value, say 10mA = 0.010A = I)
» Choose the supply voltage (VS) (6V)

Step 2

Calculate the other values:-

» Calculate (VS - VL) = (6 - 1.9) = 4.1V
» Calculate the value of R from the formula:-
    R = (VS - VL) / I
       =4.1/0.010 = 410 ohms (Use nearest standard E12 resistor value of 390 or 470 ohms)

Calculations complete! The LED will now be correctly driven within its ratings.

Quick resistor calculator - enter your values:-  
VSupply
(Volts)
V
VLED
(Volts)
V
IF
(milliAmps)
mA
 
Resistor = ?? Ohms
[Use nearest E12 standard value ?? Ohms]
(Power rating ??W or greater)
 

Using Super Bright LEDs

LED cct
Fig. 2

If using an ultra-high brightness 'Super Bright' LED, for say headlights, the current is usually 350mA or 700mA depending on LED type, that is about 20~50 times higher than for a 'normal' LED. This in turn raises the power dissspation in the limiting resistor by 400~2500 times (remember that power is proportional to current squared), from around 0.060W to over 10W! Such a resistor disspates a lot of heat, and gets VERY hot, plus the resistor consumes more than 10x the power used to generate light. Quite inefficient!

The solution is to use an electronic current regulator, which is powered by a power supply that produces a constant current output to drive the LED. The current through the LED stays constant even as the supply voltage varies. Such regulators are available commercially from the LED supplier, and dissipate very little heat.


Notes :-

  • NEVER connect a LED directly across the power supply without a current limiting device (resistor). There is nothing to limit the current though it and will result in destruction of the LED!
  • Values of IF and R are not super critical. Provided the LED is still within its ratings, all that happens is that the LED is a bit too bright or too dim.
  • If the LED is too bright, choose a lower value for IF, say 1/2 the previous value, and calculate the resistor again.
  • It is recommended that the voltage across the resistor be greater than say 1.5V, or 10% of supply voltage (whichever is bigger).
  • Standard resistor values (E12 range) are decade multiples (1x, 10x, 100x ...), or sub-multiples (0.1x, 0.01x), of 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 and are readily available and cheap ($lt;$0.05 ea). Normally 0.25W or 0.5W power rating is satisfactory.
  • Use this resistor colour chart to determine or read the values of small resistors.

Powering multiple LEDs from the same supply

Method 1
LED cct2

Parallel connection with each LED running from the power supply.

Calculate R for each LED separately as above (but usually the same).
Advantage of this method is that if some of the LEDs need to be brighter or dimmer than others, the values of the resistors can be selected individually to match LED brightness. Can also operate from a lower voltage supply.
Disadvantage is a higher current drain from the supply. Note that even 5x LEDs in parallel this way draw typically only 50 mA (0.050A) total compared to a typical small flashlight bulb of 300mA each.

Method 2
LED cct3

Series connection of multiple LEDs from a single supply.

Advantage is that it draws the same current as a single LED and only 1 resistor is needed. IF is the same for all LEDs in the series string.
Disadvantage is that the value of VL is the sum of the VFs of all LEDs in the series string. May prevent operation from lower supply voltages.

In the calculation for the series resistor,
        VL = VF1 + VF2 + VF 3
        where VF is the typical forward voltage of each LED in the string.

Any number of LEDs may be in the string, even different types and colours, the only proviso is that the supply voltage is high enough to feed them.

 

Do Not Connect Like this - Wrong

LED cct4

This type of connection is wrong.

Although LEDs of the same type have the same nominal VF, in practice they are slightly different, and definitely different for different types and brands. One LED (the one with the lowest VF) will take most of the current while the other one will get very little, with consequent big differences in light output and possible damage to the LEDs.

Use one of the configurations above.