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2nd Year Electronics Lab

The second year electronics lab is part of the module PX271. You can download the labscript, mark scheme and aims and objectives at:-

http://www2.warwick.ac.uk/fac/sci/physics/teach/module_home/px271

 

Most of the information here will already be in your lab scripts, but I have tried to pick out a few key points.

Make sure that you read both your lab scripts and the accompanying guide, read it before you go to the session and do the preparatory work before you turn up for the lab.


The most important factor in this lab is time

You are unlikely to finish the lab if:-
  • You don't read the script before attending the lab.
  • You don't do some work on the lab script outside 'lab time'.
  • You take long breaks during the lab - including turning up late and leaving early.
  • You don't do some plans for the circuits.

How to test the circuits

Do not allow the earth lead of the scope probe to touch anything that is not at Earth - the same Earth connection runs from the power supply through the mains to the scopes.

Calibrate your probe lead with the facility on the scope - ask if you are not sure how to do this.

The best way to test a circuit is usually to use the scope lead and always initially set the scope to DC - this way you see the DC and the AC parts of the waveform. In some cases you are asked to use a multimeter - this is usually when using the scope is not possible.

When testing ICs (chips) you will often need to put the probe onto a leg of an IC - take off the hook attachment from the probe and use the needle like part to carefully measure the voltages on each pin.

Test each bit of the circuit as it is made when possible - most of the circuits can be thought of as separate blocks and can be tested individually, even when connected to other parts.


Common faults and how to test for them

The most common faults are due to poor construction on the tracks.

When putting in a chip make sure to cut the tracks underneath that connect different pins together. Be bold in this and make sure there are no small whiskers of track remaining.

When soldering do not use excessive solder and make sure that no small whiskers make connections between tracks. Run a sharp edge between the tracks to make sure there are no unintentional connections.

Dry joints - these are components that appear soldered but are not actually in electrical connection to the board. Can be caused by bad soldering technique - a similar fault can occur when the track lifts off from the board - check for electrical continuity, preferably using the scope.

Circuit diagram

Make sure that you understand the circuit diagram - connections are often made in error by students, where the circuit diagram actually shows one wire 'going over' another. If one wire goes over another and there is no dot then it is not connected.

AC rectifier

You will need to enlarge the holes to fit the diodes into the board. If you are not careful you can make the holes too big and introduce breaks into the track - check for track continuity - are parts of the track that should be at the same voltage actually at the same voltage - preferably use the scope when safe to do so.

Transistors & IC's

This causes the most problems in terms of debug time - you can save loads of time by firstly thinking about what you are going to test for. Usually start your tests with the scope set to DC and the probe with its pin exposed.

The simplest fault is using the wrong device - make sure that you have the correct one before you solder it in.

Power connections are a common problem - does the device have the power on the correct pins. Remember that the ICs need power connections and sometimes they are not shown connected to power explicitly. Not all ICs have the same pin configuration - check what should be connected to each pin for every IC.

Do not connect unused inputs on the op-amp to anything at all - not even to ground, as it will destroy them. Leave any unused pins "floating" and not connected to anything at all.

Does the device work as you would expect it to - for example does an op-amp try to make the inputs the same value, or where a fraction of the output is fed into an input is the value of the voltage what you would expect. You need to know how the device should work in general and understand what its trying to do in the circuit.


Answering questions in the lab script

Remember that questions should be answered by you and not copied or edited from someone else's answer - it is easy to spot where this occurs and you will be penalised.

Do not expect the demonstrators to simply give you the answer - they are there to help you think for yourself. This lab is just as much about getting you to think about how you should tackle problems as the specifics of electronics.

There are some great resources in these data storage devices made from paper - they are called books. You will need to refer to books to get some of the answers.

It is a bit of a cliché but you need to understand the question in order to come up with the answer. If unsure ask a demonstrator.