We start BBox2 by building a simple power supply circuit.
There are numerous types of Integrated circuits and each of them has its own power supply requirements.
The power supply circuit takes an input voltage and provides 3 output voltages – 5V, 3.3V and a third output which is nothing but the input voltage minus the diode forward voltage drop. The recommended input voltage is between 7-24V, and using higher voltages will only cause an increase in the heat being generated by the regulators.
We will be using a 9V battery for BBox2.
Let’s look at the schematic to understand the circuit construction. The input voltage can be applied to either the power jack or screw terminals. This is then passed through a simple 2-pin jumper which acts as a switch. When the jumper is present, the input voltage is fed to a diode, which is used as reverse polarity protection. Remember that a diode only conducts when its anode is positive compared to its cathode so if we connect the input with the wrong polarity, the diode will prevent this incorrect polarity from reaching the rest of the circuit and this will prevent damage.
The output from the diode is called VIN and this is directly passed to one of the output screw terminals. The 7805 is a very common voltage regulator that provides an output voltage of 5V. It belongs to the 78 series of voltage regulators and there are a few such voltage regulators available. The last two digits in the part number denotes the output voltage. So 7805 provides 5V, 7809 would provide 9V while 7812 would provide 12V.
These regulators are called linear voltage regulators and they have something called a voltage drop. In simple words, they take an input voltage, drop some voltage and provide the necessary output voltage.
If you look at the datasheet, the 7805 has a voltage drop of around 2V, which means that the recommended minimum input voltage is 7V. The datasheet also recommends adding a capacitor at the input and output terminals for stability.
C1 & C2 are used for this purpose. They also act as a charge reservoir due to their high values and they can supply the necessary current when there is a sudden increase in the output current being drawn. C9 & C10 have been added to filter out noise. The value of 0.1uF or 100nF is very commonly used to filter out noise and these have been added as a precaution.
The output of 7805 is directly connected to the screw terminal marked 5V. This output voltage is used as the input for the next voltage regulator, the LD1117. The LD1117 provides an output of 3.3V and we use a similar capacitor scheme for filtering. The datasheet lists a maximum voltage drop of 1.3V, so the minimum input voltage should be 3.3V + 1.3V, which is about 4.6V. We are using 5V as the input, which is above the recommended minimum value.
The 3.3V output is connected to the screw terminal and we also add an LED with a current limiting resistor to indicate that the power supply is ON.
We use linear voltage regulators here for their simplicity and while linear regulators work well in most applications, they do suffer for one major drawback: not being very efficient. For linear voltage regulators, the heat generated is directly proportional to the voltage drop. When we apply an input voltage of 9V to the 7805 regulator, it will drop 4V to give us the required output of 5V. This 4V drop will generate some heat, depending on the amount of current being drawn. If we apply 24V to the 7805 regulator, then it will have to drop around 19V to provide the same output voltage of 5V. Again, this voltage drop will give rise to heat depending on the amount of current being drawn. For a given output current, the higher the voltage drop, the higher the heat that will be generated. This is why it is important to try and use a low input voltage whenever possible.
You can also add a heat sink to dissipate the heat generated, depending on the circuit design.
This is what the assembled PCB looks like. Please ensure that the diode, voltage regulators and capacitors are placed with the correct polarity. We connect a 9V battery to the input and close the jumper to switch ON the circuit. The LED will glow to indicate that the outputs are active. We can measure the input and output voltages to make sure everything is as expected.
We will be using this power supply for all future circuits so let’s move on to the next project.