Re: Power Source for Backlight Module - Large 4 (product 162

by adafruit_support_mike on Fri Jul 26, 2019 12:49 am

Electrically, the backlight modules are just white LEDs. They light up when you connect a power source that puts out at least 3V, and they consume about 20mA of current.

You can use a 3.7V LiPo as the power source, or a 3xAA battery pack whose average output will be around 4.5V.

Either way, you have to limit the current that reaches the LED, and to do that, you use what's known as a 'current limiting resistor'.

The basic unit of stuff in electronics is called 'charge', which usually means electrons. Voltage is the energy that makes charge go from one place to another, and current is the amount of charge that actually does go somewhere. The unit of charge is the Coulomb (about 6.3 billion-billion electrons). One Volt applies 1 Joule of energy to 1 Coulomb of electrons, and 1 Ampere of current means 1 Coulomb of charge flows past a given point every second.

Electrons can't move freely in solids like metal because they bump into the atoms, and that creates what's known as resistance. Ohm's Law says that 1 Volt provides enough energy to push 1 Amp of current through 1 Ohm of resistance, and is written V=IR.

If we want to find the value of a resistor that will limit current flowing through an LED, we flip the equation around to get R=V/I.

It takes 3V to push electrons through a white LED, so a 3.7V LiPo will only have an extra 0.7V to push current through a resistor. We want 20mA, so 0.7V/20mA=35 Ohms.

If you use a 4.5V 3xAA pack, the battery has 1.5V for the resistor. 1.5V/20mA=75 Ohms.

The only catch is that 35 Ohms and 75 Ohms aren't 'standard values'.

There are an infinite number of possible resistor values, but manufacturers have to choose the sizes they want to make. The industry standard is a set of values called 'E-series', where each value is roughly the same multiple of the previous one.

In the E3 series, each value is about 2.2x the previous one: (1.0, 2.2, 4.7), then you multiply by 10 to get 10, 22, 47, and so on. Each multiple of 10 is called a 'decade'.

In the E6 series, each value is about 50% larger than the previous one: (1.0, 1.5, 2.2, 3.3, 4.7, 6.8). There are six values per decade, and every other E6 value is also an E3 value.

In the E12 series, each value is about 20% larger than the previous one: (1.0. 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2).

There are E24, E48, E96, and E192 series, but they aren't used much because most circuits don't need that kind of precision. On top of that, each new series doubles the number of values you have to keep in inventory.. there are 42 E6 values between 1 Ohm and 10 Megohms (the most common range of resistor values), 84 E12 values, and 164 E24 values. The rule of thumb is to use E6 values if possible, E12 values when the next E6 value is just too far away, and higher-series values when nothing less will do.

For current limiting resistors to an LED, it's usually best to make the resistance a little too high and the current a little too low, so we'd want the next larger numbers above 35 Ohms and 75 Ohms: 39 Ohms and 82 Ohms.

One last detail: LEDs (and all diodes) are polarized: current only flows through them in one direction. If you connect power one way, the LED will light up. If you swap the connections, nothing will happen. Because of that, LEDs have leads of different lengths. The longer one is the positive side (officially the 'anode'), and the shorter one is the negative side (the 'cathode').

It doesn't matter whether you connect a current limiting resistor between an LED's anode and the positive supply voltage, or between the cathode and the negative supply voltage. Both versions behave the same way. Sometimes there are other design reasons to choose one option over the other, but for simply lighting the LED it doesn't make any difference at all.