S12 Create fault finding, test and calibration procedures

Posted: October 10, 2010 in Systems and Control

Initial knowledge 1

Current working knowledge 2.5

Having designed and made circuits on a pcb and on prototype boards I certainly have had to do my fair share of fault finding.  The problems have usually come from either poor circuit design or poor construction.  One thing I did not utilise in ED217 were prototype boards.  The first circuit I made worked and as far as I was concerned at the time that was that, of course when it came to rearranging my circuit I started having all sorts of problems and would have benefited from laying it out on a prototype board before I jumped in at the deep end again.  It should be the first point of call when fault finding really checking if the original circuit actually works!

After that its time to move onto the visual checks (after the [power has been turned off).  Although I have improved, in the early stage of the year my soldering left a lot to be desired so that would be the first thing to inspect, checking for dry joints and look for any solder bridging across the tracks.  One thing I didn’t realise was that the track that runs around the PCB can actually lead to a faulty circuit if any solder has managed to work its way onto it.

Whilst checking the tracks it is wise to inspect for any breaks that may have occurred,  this could be quite common if you end up trying to scratch off a large ball of solder that shouldn’t be there!   But it could also be due to over-etching when making the board itself.   Its probably best to use the multimeter at that point to check and see.

Whilst the multimeter’s out its wise to check that the power supply is actually supplying the circuit with the voltage required.  There is probably nothing more frustrating than running a vigorous fault finding procedure only to find out that the batteries are dead.  In fact its probably best to check this first.

Then all the components need checking.  is the chip heating up? can you smell burning? If they are polarised are they in the right way?  Are the values the correct?

When using the prototype boards I have encountered times when I have simply turned on the power and nothing has happened.  This is when fault finding comes into play again.  As there is no soldering involved the visual checks made are different to if I was checking a PCB.  On one of my first attempts I’m not quite sure what I did wrong but the 555 timer got extremely hot so the first check I make now is to see if any part of the circuit is over heating.  I need to be careful here because when I say hot I mean it could actually be enough to burn and there is the possibility that you might actually be able to smell burning.  Fortunately that has been a one off but due to the nature of the fact that it can become a safety hazard it is my first point of call.  Following this it is time to do the visual checks.  First and foremost double check the circuit does actually match the diagram, if not make the alterations and try again.  Next I will make sure that everything is pushed in properly.  I don’t have the smallest of hands and some of the smaller wires can be tricky to get in and when working in a close knit space it is not uncommon to accidently knock out a wire while trying to insert another one.  Whilst doing this it is possible to see if any of the connections have snapped and of course if they have then they need to be replaced.  I am not adverse to putting components in the wrong way round occasionally, so the next thing to do is check that all polarised components such as leds ( long leg positive, smooth side negative) and capacitors are in the right way round, if not turn them round and try again otherwise the circuit will simply not work.  Whilst checking the components it is also important to check the values.  I purchased some transistors from Maplins, I had asked for BC547’S and what I actually received were BC557’S, sometimes components can be exchanged for a similar type but in this case when I checked the data sheet and although numerically similar one is a NPN and the other a PNP so the circuit was never going to work.  Finally one of my personal favorites was putting the IC in the wrong way round or even better just putting the wrong one in, it might sound daft but when concentrating on all the other things it is sometimes the most obvious that stump you.

By using the multimeter it is easier to actually see where the circuit may be faulty.  First check that the power supply is reading at the required voltage before testing components, and then I tend to follow the circuit round testing each individual component to see if they are receiving the correct amount of power.  By doing this you can see if a resistor is offering too much resistance and therefore swap it for a lower value etc

When testing the LED’S on my ambience light I used PIC AXE. By downloadingg a small program specifically for just one of the outputs say to flash on and off it is easy to quickly see if that individual component is working or indeed if that is where the fault lies.

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