Electrical theory 1.3h - Advanced LED theory.

In this section we’ll look at a LED's constant voltage behaviour in more detail and see the results of an experiment to demo the key points. This will help you understand the use of a current limiting resistor and answer many questions.

Key points:

A LED has roughly a constant voltage behaviour which means:

• they have an absolute turn on voltage (across its terminals) of about 80% typical voltage; the amount of light emitted is bugger all!
• they must be operated at their typical voltage to get plenty of light;
• voltages exceeding 7% of typical voltage causes too much current to flow - this causes LEDs to overheat and prematurely fail! The role of the limiting resistor is to limit the current to safe values for the expected variations in supply voltage.

We’ll look at a typical characteristic for a RED LED again. The graph below shows that the amount of voltage applied to the LED will allow current to flow; hence its brightness. The graph shows there is a definite turn on point (1.65V), current increases linearly above 1.75V and quickly becomes excessive.

Data points:

1. No current at 1.55V, ie LED is unlit;
2. Turn on at 1.65V;
3. 20mA flows at 1.85V
4. 50mA at 1.98V (excessive current)

I’ve created an experiment to illustrate the graph with 4 RED LEDs with various voltages applied. The photo shows the resulting current flowing through the LED and its brightness (located above each DMM) – the camera failed to distinguish the brightness so I created a colour spot grading above the photo. Note each LED had 1 Ohm resistor series connected to it and it was used to measure current using Ohms Law. The current DMM photo was appended as shown.

Experiment results:

• No current at 1.55V

1.  0.73mA at 1.659V
2.  7.8mA at 1.766V
3.  14.0mA at 1.867V
4.  25.9mA at 2.053V (maximum safe current!)

These results for these LED have good similarity to the graph and significant brightness occurs just below 2.0V. This is why the typical voltage should be used to get good brightness. Note: there is a variation between LEDs of same colour because the chemical doping process to make a LED is moderately variable.

END >> RETURN to easy stuff!