Diary of a Trainee Electronics Engineer: January 2017

Image source: www.allaboutcircuits.com

Kick-starting the year with amplifier calculations and analysing protection schemes.

It seems like forever since the New Years celebrations, when in reality it was only just over a month ago. 2017 has only just started and things are already in full swing.

Laser Cutting Induction

Earlier in January I gave an introduction to laser cutting to the members of Hebden Bridge hackerspace, Bridge Rectifier. I really enjoy helping other people to learn new skills, especially considering laser cutting is so versatile and can be used in a wide variety of applications from crafts to engineering. In the induction I included:

• Getting started with Inkscape
• Setting the defaults
• Basic drawing
• Path union, difference, intersection, exclusion and division
• Exporting to DXF
• Using the laser cutter software
• Setting up the laser cutter
• Double-checking your designs
• Laser cutter health and safety

Once I had given the introduction, the members were able to make their own designs and have a go at cutting them. It was lovely to see how fast they picked up using Inkscape for design and using the laser cutter and I was really impressed with the sort of things they were designing, considering it was the first attempt at this for the majority of the members.

A public workshop is due to take place at Bridge Rectifier towards the end of February which I will be helping out with. Once again I’m really excited to take part in this and see how far the members have progressed, since they will now be passing on their knowledge to the workshop attendees.

Amplifier calculations

In my HND across two units we have spent quite a lot of time looking at amplifiers over the past month. We started out by calculating and measuring the gain of both inverting and non inverting amplifiers.

We also looked at how we can use the required gain of an amplifier to determine the resistor values needed the achieve this, like so:

Inverting amplifier calculation
Gain = -Rf/Rin
Given a gain of 7 and assuming a resistor value of 10kO for Rin we now need to determine Rf
7 = -Rf/10kO
Since we have Rin and the gain we need to transpose the original formula to make Rf the subject:
-Rf = gain x Rin
-Rf = 7 x 10kO
-Rf = 70kO resistor value
The minus sign in front of Rf is there to indicate a phase shift of 180 degrees (inverting the signal).

Image source: www.allaboutcircuits.com

Non-inverting amplifier calculation
Gain = Rf/Rin +1
Given a gain of 10 and assuming a resistor value of 10K for Rin we need to determine Rf.
Gain = Rf/10K +1
Transpose to make Rf the subject
Rf = (gain -1) x Rin
Rf = (10 – 1) X 10kO
Rf = 90kO

Image source: www.allaboutcircuits.com

Once the calculated resistor values were put into circuit simulation software, Multisim, we could then compare the calculated and simulated gain against the measured gain of the physical circuit to determine the percentage error.

Multi-stage Transistor Amplifiers

In the next unit we have been looking at multi-stage transistor amplifiers and how each subsequent amplifier affects the overall gain, as well as the various methods of coupling these amplifiers.

Methods of coupling

Direct coupling. In this method of coupling amplifiers, the output of the first transformer is connected directly to the input of the next and so on. This method of coupling is used predominantly in the amplification of very low frequencies. Due to the minimum use of resistors the arrangement of the circuit is very simple and because of the absence of expensive coupling devices this method of amplifier coupling is very cheap. However, it can not be used in high frequency amplification and the operating point is shifted due to variations in temperature.

Image source: http://electroniccktanalysis.blogspot.co.uk

RC Coupling.Here a coupling capacitor is connected to the output of the first stage and to the input of the second stage and so on. This is the most popular method of coupling amplifiers as it is cheap and provides excellent audio fidelity over a wide range of frequencies. RC coupling is predominantly used for voltage amplification.

Image source: http://electroniccktanalysis.blogspot.co.uk

Transformer Coupling.The main reason for the low voltage and power gain of RC coupled amplifier circuits is because the effective load of each stage decreases due to low resistance presented by the input of each stage to the preceding stage. The effective load resistance can be increased, in turn increasing the voltage and power gain by using transformer coupling. Transformer coupling is achieved by altering the turn ratio on the transformer, these are generally used when the load is small and is mainly used for power amplification.

Impedance matching with transformer coupling is excellent, which therefore provides a higher gain – to the extent that if designed properly a single stage of transformer coupling could potentially provide the gain of two stages of RC coupling. Although there are many advantages to transformer coupling there are downsides; the frequency response is poor, it takes up the most space of the three methods of coupling making the overall circuit bulky and these are also fairly expensive.

Image source: electriciantraining.tpub.com

Below is a table comparing the three types of coupling to one and other in terms of frequency response, cost, space, mass, impedance matching and application.

Analysing Protection Schemes in Transmission Systems

In another of the assignments submitted in January I was asked to analyse the protection scheme in a given transmission system, the one used throughout the West Yorkshire region and that is supplied from Drax power station.

Image source: http://www.intechopen.com

Due to the length of electrical power transmission lines and because they run through the open atmosphere they are at much greater probability of developing a fault. Because of this transmission lines require more protective schemes in place, these schemes should have special features such as:

1. During a fault the circuit breaker closest to the fault point should be the one to trip.
2. If for whatever reason this were to fail, then the circuit breaker next closest to the point of fail will trip as a backup.
3. The relay operating time associated with the protection of the line should be as minimum as possible, this is in order to prevent unnecessary tripping or other circuit breakers associated with healthy parts of the system.

These are different from the protection of other equipment in power systems such as transformers. The main three methods for protection of transmission lines are:

• Time graded over current protection
• Differential protection
• Distance protection

My assignment included a breakdown of the three main methods of protection as well as diagrams explaining this. For anyone who would like to know more about this there is a very detailed article available here.

I also included a breakdown of the various types of circuit breaker used in high voltage transmission lines including their advantages and disadvantages, such as:

• Oil type circuit breakers
• Vacuum type circuit breakers
• Air blast circuit breakers
• SF6 type circuit breakers

Diagram shown above is a SF6 circuit breaker.

Image source: http://www.electricalengineeringinfo.com

Stepper Motor Control

In addition to what I’ve been learning on my HND course I have also learned quite a lot in projects this past month. At the beginning of January I began looking at stepper motors and how to control these using an Arduino Uno and the Arduino Motor Shield.

In my first post Arduino Stepper Motor Control, I looked at the basics of stepper motor including the difference between unipolar and bipolar and also what the Arduino Motor Shield is. Following this I managed to get a stepper motor working with an example sketch I found online.

I then went on to writing about making An Arduino Controlled Spooling Machine, in this post I used a different stepper motor to the first post, and learned about implementing a state machine before going on to designing the mechanical fastenings for the hardware.

If I’m starting as I mean to go on then it looks like 2017 is going to be another productive year!