Led circuit building
I really want to build custom LED taillights and need a place to learn as much as possible. This might be a great place to centralize knowledge.
LED stands for Light Emitting Diode. LEDs works similarly to other types of diodes, except that they emit light when current is passed through them.
Like other types of diodes, they allow current to flow in one direction but block current flow in the other direction.
When you apply a tiny voltage in the forward direction, at first hardly any current flows and nothing happens (no light). As you gradually increase the voltage, all of a sudden you reach the voltage where current flows and it is like a huge spigot. If you do not limit the current, it will burn up or blow up the LED, literally. This voltage where current will suddenly flow is called the forward voltage or forward voltage drop.
While regular diodes have a forward voltage drop of around 0.7 volts, and schottky diodes only around 0.4 volts, LEDs have a much higher forward voltage drop, typically around 2 to 2.2 volts.
Like regular diodes, LEDs will fail if you apply too much reverse voltage. They will block that reverse voltage up to a point, then they will fail, usually short but sometimes open.
Current limiting can be a simple resistor if you have a known, (relatively) constant voltage. This is fairly inefficient most of the time but is easy to figure out. You just use Ohm's Law to decide what size resistor to use. So let's say you want 20 milliamps (0.02 amps) to flow through your LED. In a 12 volt system, you will drop about 9.8 volts through the resistor (12 minus the 2.2 volt forward drop in the LED). So 9.8/0.02=490 Ohms. 470 Ohms is a standard size that is plenty close, so with a 470 ohm resistor we will get 9.8 volts divided by 470 ohms equals 20.8 milliamps.
To make our LEDs more efficient, we can put several of them in series. In our 12 volt system, we can safely put 4 of them in series and still have some leeway for voltage droop in our supply. If we put 4 in series, that will use around 8.8 volts.
Of course in the real world, our automotive electrical system is not a steady 12 volts. When the motor is running at a decent RPM and there isn't much load, we would typically see around 13.4 to 13.8 volts in the system. With the motor off a healthy battery should supply around 12.5 volts. While cranking the engine over, or winching with the motor off that can easily drop to 10 volts or less. So we need some "headroom" in our LED circuit to allow for these supply voltage variances.
If we were still using a simple resistor, we might see the LED brightness vary way too much once we put 4 of them in series to get some efficiency. So instead, we employ a current regulator that allows a constant current to flow through our LEDs even while the supply voltage varies under different conditions.
Linear makes some real nice LED drivers.
ExpressPCB has free downloadable schematic editor and PCB layout and fabrication.
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