Light bulb

LEDs are replacing incandescent light bulbs in many applications and over the next decade or so we are likely to see very few bulbs in new products. There will always be special cases where certain features of the light bulb make it the best choice – e.g., broad continuous spectrum or both light and heat are required.

Incandescent light bulb

LEDs differ from light bulbs in many respects.

Light bulbs are resistors designed to glow white-hot when connected to the rated voltage. They can heat up to 2,500°C.
They behave like resistors. But like most metallic resistors the resistance increases with temperature. (This means that the temperature coefficient of resistance is positive – a PTC.)
When cold the resistance is much lower than running temperature so the initial surge current can be 15 times that of the steady state current. For example, a 100-watt, 120-volt lamp has a resistance of 144 Ω when lit, but the cold resistance is much lower (about 9.5 Ω). This is why incandescent bulbs are prone to blow on switch-on. A weakened filament will burn
Spectral power distribution of a 25 W incandescent light bulb.

out with the high inrush current.
They can be run on DC or AC.
Bulbs are tolerant of voltage variation of ±10% which is normal on mains supplies.
Bulbs can be made to run directly on a wide range of voltages such as 3 V bicycle lamps to mains voltage lamps of several hundred volts.
Bulbs emit light over a broad spectrum from infra-red through the visible spectrum but heavily weighted to the red end. This is most noticeable in film photos which exhibited a characteristic tungsten red cast. The broad spectrum should not be confused with mixing specific colours or wavelengths to fool the eye into seeing white light.
Bulbs are fragile.

LEDs

LEDs are different in nearly every regard.

LEDs drop in apparent resistance with rising voltage. This is due to the diode’s IV curve. See IV curves.
LEDs have a slight negative temperature coefficient (NTC) value. This means that as they warm up they will pass more current and tend to heat further. If not managed this will lead to over-current failure. See LEDs in parallel – the problem.
LEDs are low voltage. Typical LED forward voltages are 1.2 V (infra-red) to 5 V (white).
LEDs cannot handle negative voltages and require a DC supply. Most LEDs are rated for maximum 5 V reverse voltage.
LEDs are sensitive to electro-static discharge (ESD). (Bulbs are not.)
LEDs’ colour is a function of the semiconductor material and are available in colours from infra-red to ultra-violet. See RGB LED.
LEDs can detect light with a small output current like photodiodes.
LEDs are single sided even with a transparent substrate.

IV curves

A device’s IV curve – current versus voltage curve – is a graph of the current that will flow in the device as a function of the voltage across it.

As suggested by Ohm’s Law, \( V = IR \), the relationship between current and voltage in a resistor is linear. Figure 1 clearly shows that the current increases linearly with increasing voltage and that the rate of change depends on the value of the resistor.

LEDs are rather different:

LEDs are diodes whose P-N junctions behave in a non-linear fashion. Very little current flows until the forward voltage is reached. Above this value the current increases exponentially with increasing voltage.
LEDs like all diodes conduct in one direction and do not (until the reverse breakdown voltage is reached) conduct in the opposite direction.
As LEDs’ colour is determined by the band gap¹ of the semiconductor and the forward voltage, \(V_F\), also varies with the material.

LED forward voltage and current (IV) curves for IR, red, orange, green, yellow, blue, white and UV LEDs. — Figure 2. Typical IV curves for various colours of LEDs.

The IV curves are useful for estimating the current that will flow at particular voltages, etc., and for calculating resistor values.

There are a few points worth noting from the curves:

The forward voltage matches the band gap which increases from red to violet.
It should be clear that trying to power LEDs in parallel – not recommended normally – is particularly bad idea when the colours are mixed. e.g. Connecting a red, green and blue LED in parallel on a 2.0 V supply would result in:
- Red: 44 mA.
- Green: 12 mA.
- Blue: 3 mA.

Red, green and blue currents at fixed voltage supply. — Figure 3. The currents that would flow through a red, green and blue LED connected directly to a 2 V supply.

- - The red would hog the majority of the current.
  - IR (infra-red) LEDs have the lowest forward voltage.
  Note that in this chart the curves have been extended up to 100 mA. Most small indicator LEDs cannot take this current. See LED current rating.

Plotting an IV curve

This video demonstrates measurement of the current through a green LED as voltage is adjusted. The results are plotted to generate the IV curve for the LED.

It is also possible to observe some shimmer on the multiplexed display while humming. Humming vibrates the eyes in their sockets which can create stroboscopic conditions within the eye. By humming at about 70 to 72 Hz (near music note C2) I was able to observe shimmer on the LED display.

Measuring LED forward voltage

The video clip above demonstrates measurement of LED forward voltage using a multi-function component tester. Results were:

Colour	\(V_F\)
Infrared	1.14 V
Red	1.76 V
Yellow	1.85 V
Green	1.96 V

RGB LED

LEDs give off monochromatic light at a frequency determined by the semiconductor material. An RGB LED (Red-Green-Blue) combines LEDs of the three primary colours to give a full colour spectrum.