1. A Simple Power Supply Circuit

Objectives: 

• • To build a suitable power supply circuit that can supply our digital circuits.

  • To understand voltage regulators and decoupling capacitors.

  • To build a simple LED circuit that indicates when our circuit is powered.

Note: The full set of videos is presented on the YouTube channel: DerekMolloyDCU

Introduction:

One of the first things we need to do is create a power supply circuit. We are going to be using a 9V battery to drive our circuits. This experiment sets up a basic power supply circuit with a simple LED power indicator.

Video

Additional Notes:

Voltage Regulator

A voltage regulator is a fairly complex but easy to use device that takes a varied input voltage and outputs a constant voltage at a lower level than the input voltage. For example, in our circuits we will be using a 9V battery and our 5V regulator is chosen to output a constant 5 Volts. Figure 1 displays the LM7805 Voltage Regulator that we will use - The datasheet for the LM78XX is attached to this document. There are also 12V (LM7812) and  15V (LM7815) versions, hence the LM78XX notation. 

As can be seen in figure 1 (and the attached datasheet) the pin on the LHS (left-hand side) is the Voltage Supply In (at 500mA it will accept from approx. 8V to 20V) and will output on the RHS in the range 4.8V to 5.2V, so as close to 5V as would be required for our circuits. The middle pin should be connected to your ground rail. The aluminium plate at the back of the voltage regulator is there to dissipate heat. The hole allows us to bolt it to a heat sink, which would allow us to output greater currents, up to 1A with a suitable heatsink attached. This regulator will give us a nice and clean steady 5V supply, perfect for driving the TTL chips used in this module.

Figure 1. A LM7805 Voltage Regulator

If our battery supply degrades, it is possible that our 9V battery is not meeting the minimum ~8V required to drive the LM7805 voltage regulator. If this was to occur, we could look at using a Low-Dropout (LDO) voltage regulator, which can require as little as 6V to operate a 5V regulator. 

Your kit contains a LDO Voltage Regulator - The Fairchild KA7805ETU 5V 1A LDO Voltage Regulator (See the datasheet at the bottom of this page).

Decoupling Capacitors

Coupling is often an undesirable relationship that occurs between two parts of a circuit due to the sharing of power supply connections. This relationship means that if there is a sudden high current demand by one part of the circuit that the other part of the circuit is affected by noise. A small capacitor, known as a decoupling capacitor, acts as a store of energy that removes AC-like (ripple) signals that may be present on our DC source.

Figure 2. A 100nF Capacitor - it will have 104 written on the surface

Figure 2, shows a picture of a 100nF (0.1uF) ceramic capacitor that is fine for our circuit. This value can be used a general rule of thumb, but for commercial designs you would have to choose this value carefully.

The numbering code for capacitors is reasonably straightforward. Unfortunately, the numbering on ceramic capacitors can be very small. The numbering is as follows:

1. The first number is the 1st digit of the capacitor value

2. The second number is the 2nd digit of the capacitor value

3. The third number is the number of zeros, where the capacitor value is in pF (pico-Farads)

4. Further letters can be ignored, but represent the tolerance and voltage rating of the capacitor.

So, for example: 

104 = 100,000pF = 100nF

102 = 1000pF = 1nF

472 = 4700pf = 4.7nF 

We are using a 9V battery and our circuits will work fine without the decoupling capacitor; however, it is good practice to have it there in case that you plan to drive your circuit from a plug-in mains transformer. While your multimeter will read a constant approx. 5V for our supply from the plug-in transformer through the voltage regulator, it averages over very many samples. If you were to examine the waveform with an oscilloscope you will see significant noise present, which would have strange effects on the ICs in the circuits that we are to examine.

Light-Emitting Diode (LED)

A Light-emitting Diode (LED) is a semiconductor-based light source that was traditionally used as a state indication light in all types of devices. Today, high-powered LEDs are being used in car lights, back lights for monitors and even in place of filament lights for general purpose lighting (e.g. home lighting, traffic lights etc.) due to their extremely high efficiency. We are going to be using very low power LEDs in our circuits, usually to indicate if a state is true or false. You kit contains several different LED colours, but they all have similar electrical properties. 

Figure 3(a) The symbol for an LED, and (b) An actual LED noting the polarity

The symbol for an LED is illustrated in figure 3(a). The anode(+) is usually connected to a more positive source than the cathode (-). Figure 3(b) shows an LED, which has one leg longer than the other. The longer leg is the anode(+) and the shorter leg is the cathode (-). The plastic surrounding of the led often has a flat edge, which indicates the -ve leg of the LED.

Our LEDs have certain requirements. They require a maximum voltage of 3V and a working current of 20mA. A LED does not have a significant resistance, so if we were to connect the LED across our 5V supply we are exceeding the voltage and the current requirements (it should be okay for short periods of time). However, to do this properly we need a resistor. So, if our supply is 5V and we wish to have a drop of 3V across the LED, we would like 2V to drop across our series resistor. We would also like to limit the current to 20mA, so we need a resistor of value:

    R = V/I    (as V = IR, Ohm's law)

    R = 2V/0.02A = 100 ohms

So, our a circuit to light an LED would look like Figure 4. Here we place our resistor in series with the LED. The resistor forces a current of 20mA through the LED and has a voltage drop of 2V, thereby limiting the voltage across our LED to be 3V - meeting the specification requirements.

Figure 4. An LED circuit designed to meet the properties of our LEDs

Finally, a demonstration of the use of a Transistor

In this simple circuit we want to demonstrate the use of a transistor. This circuit uses a high gain NPN transistor with the gate input floating in the air. Please change this circuit around until you are confident with the use of a transistor.

Advanced: 

While not explicitly part of this experiment you might also be interested in how we might connect a mains PSU to our breadboard and the extra circuitry that can be used to ensure that we obtain a quality supply: