Tag: IV curve

Current and voltage relationships

The non-linear relationship between current and voltage is important but easy to understand.

In this section we explore the following topics.

LED forward voltage and current (IV) curves for IR, red, orange, green, yellow, blue, white and UV LEDs.

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, , the relationship between current and voltage in a resistor is linear. Figure 1 clearly shows that the current increases linearly with increasing … Continue reading "IV curves"
LED equivalent circuit model.

'Resistance' of an LED

LEDs do not have a linear relationship between current and voltage so they cannot be modeled as simply as a resistor using Ohm’s Law, . We can, however, make a simplification and model them over a range of currents as a combination of a resistor and a voltage source. If we look at a typical … Continue reading "‘Resistance’ of an LED"
Loadline 30 mA on a white LED.

Loadline resistance graphic tool

Most LED circuits require a current limiting resistor. The loadline calculator makes resistor selection very simple. Example 1: Chose a resistor to give 20 mA on a yellow LED and 5 V supply. Solution: Choose 20 mA on the If axis. Slide across to the yellow LED curve. Find the nearest loadline: in this case it’s … Continue reading "Loadline resistance graphic tool"
LTST-C170TBKT-Vf-variation.

Variations in Vf and "binning"

LEDs’ forward voltage drop varies from device to device and by more than you might think. This causes a few issues for the user. The Lite-On LTST-C170TBKT InGaN blue LED datasheet, for example, shows the forward voltage varies between 2.8 V and 3.8 V at 20 mA. This is quite a variation. The lower value would, just about, allow … Continue reading "Variations in Vf and “binning”"

 

Ohm’s law – LED resistor calculation

Ohm’s law states that the current through a resistive conductor (a resistor) between two points is directly proportional to the voltage across the two points.

\(I={\frac {V}{R}}\) where I is current (amps or A), V is voltage (volts or V) and R is resistance (ohms or \(\Omega\)).

Example 1 – resistor calculation

Resistor and LED.
Figure 1. Series connection of a resistor and LED. The voltmeters show the voltage drop across each element of the circuit.

I have a 5 V supply and I want to run a red LED at 8 mA.

We can see from the LED’s IV curve that at 8 mA the red LED will have a forward voltage of 1.8 V. That means there’s 5 – 1.8 = 3.2 V across the resistor. Using Ohm’s law we can calculate the resistance.

$$ I = \frac {V}{I} = \frac {3.2}{0.008} = 400~\Omega $$

390 \(\Omega\) is the closest standard value and that will be fine.

Don’t forget to check the power rating of the resistor! See Power calculations.

Notes

Note that the order of the components in this example doesn’t matter. The resistor can go between the +5 V supply and the LED or between the LED and ground. The same current will flow through the circuit and the same voltage drop will occur on the LED.