Session 4

Power Supplies

Power supplies are an essential part of any electrical circuit. In fact they are absolutely essential. There are many different types of power supply and it would be impossible for us to cover all of them. For today we are going to investigate to common types of dc power supply and then carry out a test to compare their performance. The work you do on this page should also be reviewed for D1 which you will encounter a little later in the unit.

Power Supplies

 

 

 

 

 

By the end of this session the learner will be able to:

  • Describe the function, features and characteristics of an electronics bench power supply
  • Measure the offload voltage, onload voltage and current drawn from a battery when powering a circuit
  • From the measured data, calculate the internal resistance of the battery

BTEC Outcomes Covered

  • Unit 6 Pass 2 (part)


The Battery

We are all familiar with the battery. Recent technological improvements have revolutionized the way a battery is used especially in terms of:

  • Rechargeability
  • Power

Modern communication devices such as tablets and mobile phones would be impossible without the advances made in both battery technology and circuit design.

There are two main types of battery. They are:

  • Primary cells, non rechargeable
  • Secondary cells, rechargeable

How a battery works

For a more in depth look at how batteries work including some of the pioneers of the technology you could check out this excellent article

Fundamentally all batteries are designed using the same principle. Two electrodes (or terminals, separated by a substance known as an electrolyte, in this illustration the ‘thick paste’ is the electrolyte.

Basic battery

The two terminals of the battery have names:

  • The positive terminal + is called the anode
  • The negative terminal – is called the cathode

When each terminal is immersed in electrolyte they produce an electrochemical reaction. The result of this is that the anode releases electrons and thus becomes positively charged and the cathode absorbs the released electrons and becomes negatively charged.

When two terminals are charged differently to each other we have a potential difference which as we have seen before is measured in volts. The typical potential difference created by a battery is 1.5 volts per cell.

If a circuit is connected to the battery, electrons will flow. However the battery will become less efficient over time as the materials within the battery (anode, cathode and electrolyte) get used up. When this happens the internal resistance of the battery increases up to the point where it cannot supply enough power to run your circuit.

Some types of battery are designed so that they can be recharged. Recharging a battery is basically a process of reversing the chemical reaction within the battery itself.

Bench Power Supplies

Batteries, despite the technological advances, suffer from some serious limitations when it coming to testing electronic systems. The most obvious ones are:

  • Finite life, batteries will need to be replaced or recharged which makes them expensive
  • Limited power, to achieve more power the size of the battery generally increases
  • No flexibility, the voltage you get is the voltage you are stuck with
  • Relatively high output impedance or internal resistance
  • Ineffective voltage regulation at higher power settings

A good bench power supply, solves all of these issues. We do not have time in this session to take an in depth look at how a bench power supply works but we can basically illustrate its function with the following diagram.

Power Supply Block

 

This type of illustration is known as a block diagram which is an excellent way of communicating complex systems in a simple fashion. A power supply works like this:

The transformer reduces the incoming ac voltage from your mains supply (240 volts) to a usable level. Usually 24 volts or lower.

AC waves

The rectifier converts the ac voltage to a dc voltage. This video illustrates the difference between ac and dc

Rectifier

 

A filter smooths out the rectified voltage. The filter is usually a very large electrolytic capacitor

Filter

A regulator produces a steady dc output voltage that will maintain its level over a range of electrical loads

Assessed Task 1

Produce an information sheet describing the functions, features and characteristics of the HQ PS1503SB Power supply: Here is an excellent blog resource that goes into some detail about this power supply. This illustration below gives an overview of the most important specifications.

Power Supply Specifications

 

Functions 

What does the power supply do?

Features (examples)

  • Input and output connectors
  • Variable current
  • Variable voltage
  • How is the voltage and current displayed?
  • Is there overcurrent protection?

Characteristics (examples)

  • Input voltage
  • Output voltage ripple (we call it noise)
  • Output impedance
  • Voltage regulation

 

Electrical Resistance

Electrical Resistance.  All electrical components have a resistance to the flow of electrons (electricity). As electrical engineers it is very important to be able to measure the resistance of a component, a section of a circuit or even the entire circuit itself.

Electric Current

Current. The amount of current that flows through a component or circuit is proportional to the resistance. Double the resistance means half the current (for the same voltage applied) conversely halfing the resistance doubles the current. When we measure current it is very important that the meter is connected in series with the circuit so that the current flows through the meter.

Voltage

One way to think of voltage is to think of a water tank. The further we raise the water tank above sea level the more potential energy the water has with respect to sea level and thus if we connect a drain to our water tank, more water will flow i.e. more pressure = more flow.

Voltage is electrical pressure and is the potential (energy) difference between 2 electrical terminals. As with our water analogy, more electrical pressure means more current flow (given the resistance does not change)

Offload Voltage

The image below shows you how to measure the offload voltage of a power source. In this cases the power source is a battery.

Offload simply means that the connected power source is supplying no current or negligible (a tiny amount) of current. We always measure voltage in parallel or across the circuit.

Offload voltage

Make sure you set the range to dc volts and use the voltage and common ports on the multimeter to measure the voltage.

Onload Voltage

Finally we are required to measure the voltage applied to an electronic circuit when it is drawing current (you will be using a resistor of your choice in the assessment) this is called the onload voltage. Below is a photograph showing the connections needed to do this. Note that the voltage supplied by the battery has now dropped a little (more about this in the next section)

Onload voltage

Theory Section

Did you notice the difference between the onload voltage of the battery and the offload voltage? This is because there is a relationship between current, voltage and resistance.

That relationship is described by Ohm’s law  which states:

“The current (I) flowing through a circuit is directly proportional to the voltage (V) applied across the circuit and inversely proportional to the resistance.”

  • Basically this just means that if you double the voltage you double the current (directly proportional)
  • If you double the resistance you  half the current (indirectly proportional)

Ohm’s law gives the following relationships:

  • I = V/R
  • V = IR
  • R = V/I (this is the one we are interested in)

Remember we said that all components, even power supplies have an electrical resistance? Then by Ohm’s law (V = IR), if a current is flowing through that component a volt drop will be produced across it.

This voltage drop is the difference between the offload (no current flowing) and onload (current flowing) voltages. We can use it to figure out the internal resistance of the battery.

Internal Resistance (R) = (Voffload – Vonload)/(Current)

If you use the measurements taken in this session and plug them into this formula you can work out the internal resistance of the battery.

Here is a video showing the process

 

 Congratulations!! You have just applied Ohm’s law to solve an electrical problem.

Complete your report and you have completed this session.

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