Step 33: Build the Controller: Power Terminal and Filtering Capacitors
The cube is complete, now all that remains is a monster circuit to control the thing.
Let's start with the easiest part, the "power supply".
The power supply consists of a screw terminal where you connect the GND and VCC wires, some filtering capacitors, a switch and a an LED to indicate power on.
Initially, we had designed an on-board power supply using an LM7805 step down voltage regulator. However, this turned out to be a big fail.
We used this with a 12V wall wart. But as you may already know, most wall warts output higher voltages than the ones specified on the label. Ours outputted something like 14 volts. The LM7805 isn't a very sophisticated voltage regulator, it just uses resistance to step down the voltage. To get 5 volts output from 14 volts input means that the LM7805 has to drop 9 volts. The excess energy is dispersed as heat. Even with the heat sink that you see in the picture, it became very very hot. Way to hot to touch! In addition to that, the performance wasn't great either. It wasn't able to supply the necessary current to run the cube at full brightness.
The LM7805 was later removed, and a wire was soldered between the input and output pins. Instead we used an external 5V power source, as covered in a previous step.
Why so many capacitors?
The LED cube is going to be switching about 500mA on and off several hundred times per second. The moment the 500mA load is switched on, the voltage is going to drop across the entire circuit. Many things contribute to this. Resistance in the wires leading to the power supply, slowness in the power supply to compensate for the increase in load, and probably some other things that we didn't know about ;)
By adding capacitors, you create a buffer between the circuit and the power supply. When the 500mA load is switched on, the required current can be drawn from the capacitors during the time it takes the power supply to compensate for the increase in load.
Large capacitors can supply larger currents for longer periods of time, whereas smaller capacitors can supply small but quick bursts of energy.
We placed a 1000uF capacitor just after the main power switch. This works as our main power buffer. After that, there is a 100uF capacitor. It is common practice to have a large capacitor at the input pin of an LM7805 and a smaller capacitor at it's output pin. The 100uF capacitor probably isn't necessary, but we think capacitors make your circuit look cooler!
The LED is connected to VCC just after the main power switch, via a resistor.