24 March 2013

A Beginner's Guide to Driving LEDs - Part 1


LEDs are just great. Nowadays they are cheap, durable and power efficient. You get them in all colours, shapes and sizes.

But along with all the advantages comes a disadvantage, too. LEDs have special demands when it comes to their power supply. In most cases, this is not too bad. But many beginners in electronics have problems to understand those requirements. With this post I would like to help those people to get an intuitive understanding of how to use LEDs. I will keep it simple, but hopefully still useful.

This tutorial is split in four parts and I will post them one by one:
Part 1: LED Characteristics (you are reading this...)
Part 2: LEDs with Serial Resistors
Part 3: Current Sources
Part 4: Transistors as Current Sources
Part 5: LEDs and Microcontrollers 

Good old Ohm's Law
Everybody has heard about it. In short, voltage and current are proportional in certain materials. This means just that if you double the voltage on such a material, the current through it doubles as well. Not really difficult to understand, right? You would get a similar behaviour with water running through a pipe: Double the pressure and the flow of water will double.

Here is what this looks like in a graph:

All as expected...

A Light Bulb
This is an example of a non-ohmic resistor. An "incandescent" bulb uses a very thin piece of wire (the filament). Initially, this wire does behave like a ohmic resistor. The voltage/current diagram (or U/I diagram) starts with a steep, but linear segment. But this is only one part of the story. When the current increases, the power that goes into that component increases also. Consequently, the material heats up and this in turn increases its resistance. The curve flattens for this reason.

Eventually the filament starts to glow and to produce light. The point here is that during normal operation, a change of the operating voltage will result in only a small change of the operating current. So, a light bulb can be operated on a fixed voltage power supply just fine.

The LED's Characteristics
LEDs on the other hand behave quite differently. Whereas light bulbs use heat to produce light, LEDs use quantum mechanical effects. We don't need to know the details. But we do need to know that their U/I diagrams show a behaviour that is almost exactly the opposite to that of a light bulb.

Here is a graph from measurements that I took from an ultra bright red LED. An analogy for this behaviour would be a safety valve: Up to a certain pressure, there is no output. From that level onwards, pressure is released almost unrestrictedly.

Unfortunately, the LED can only tolerate a certain amount of power dissipation. It heats up and once a certain temperature is exceeded, it gets damaged. So it is of vital importance to limit the power for an LED. The problem here is that you would have to set a very specific voltage to keep the LED alive on one hand and to get a decent amount of light on the other hand. Plus, that voltage is different for each individual LED and also depends on ambient temperature.

How not to operate an LED
The above diagram was generated from measurements taken at room temperature (about 22°C). To get my point across I also took measurements at about -16°C (in a freezer) and at about 75°C (in an oven). Here is the result:

It can clearly be seen, that the LED's graph moves right/left by a few tens of a Volt, depending on the ambient temperature. So for a given fixed supply voltage even small changes in temperature will change the operating current significantly.

Imagine you were planning to supply the LED with a fixed voltage. You would design the circuit for use at room temperature. For its nominal current of 20 mA you would have to supply 2.1 Volts. Now what you have to consider is the fact that the LED heats up due to power dissipation - because part of the power consumed by the LED is transformed to heat.

From the diagram we can see that with higher temperatures the current gets substantially higher for a fixed voltage. So the LED would spiral towards its certain death:
  • The LED heats up due to its operating current.
  • Due to the heat, the operating current increases.
  • The LED gets even hotter.
  • The operating current increases.
  • And so on...
  • Until the LED is destroyed.  

There are several practical possibilities to solve this problem. 

Go to part 2 of this tutorial to find out more.

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