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Home > Hints and Tips
HINTS AND TIPS ON FAULT FINDING
This page is dedicated to advice on testing and fault finding circuits built by students and the problems that commonly occur when constructing circuits and PCB's. They are faults students tend to make and not aimed at the experienced constructor.
PCB ERRORS

Check for bridging of tracks, especially when many thin tracks run along side one another. This can happen if the PCB hasn't etched the gaps between tracks properly.

Hairline cracks in tracks are difficult to see and can cause a circuit to behave strangely. Use a magnifying glass to inspect the board. A multimeter set on ohms can help test continuity but be sure that you always get a reading of less than 1 ohm. A higher resistance would indicate a break in the track. A common causes for this are scratches on the acetate (if using photo etching), poor printing quality, turn it up to at least 1200dpi or over exposure in the UV box.

SOLDERING PROBLEMS

By far the most common error made by students is the bridging of tracks with solder. Usually this occurs by pads with large balls of solder touching solder on other pads. Try using a desolder pump or braiding to remove excess solder. Take care with desolder pumps as they are quite aggressive and can literally pull the pads and tracks off the board.

Dry joints are fairly common and can be caused by moving the PCB/component before the solder has cooled. Sometimes the joint will appear dull and speckled rather than shiny. Remove solder and re-apply.

The secret to good soldering is clean copper and a well tipped soldering iron. When using photo-etching process, I have found fine grade wet and dry paper works really well, usually 800 grit is a good choice. Try to complete soldering in one session so the copper can be cleaned and soldered as quickly as possible.

The choice of solder can make quite a difference. If you are using an 18w Antex iron which I have found to be most suitable then be sure that the solder you order and use has a melting point suitable. I recommend Hydro-X solder which has a water soluble flux. The flux which discolours with excess heat can be washed off using a toothbrush under a running tap. This process also cleans the tracks again and gives a good finish. A good tip when the circuit has been tested is to spray the copper track side with a laquer to keep them shiny. Both the solder and the laquer are available from Rapid Electronics.

Use of a bulldog clip attached to a small base allows the PCB to be held fairly securly while the solder and soldering iron are applied. This inevitably prevents dry joints as the PCB and components are kept still while the solder cools.

COMPONENT ISSUES

A very common mistake is the orientation of certain components. Diodes and transistors are the main culprits and can cause damage to themselves and other components if incorrectly connected. Transistor connections can be tricky with all the different case types, in particular TO92 types. For example, BC183 and BC183L have different connections. Check in component catalogs for pin orientations or click here for transistor pinouts

When using diodes for protecting transistors driving inductive loads (such as relays), be sure to check orientation carefully. They need to be reverse biased, if not they will conduct when the transistor switches on and usualy destroy itself and the transistor due to the large surge of current.

When inserting components into the PCB, do not force them if the holes are not large enough. Re-drill them to the required size. Most holes will be large enough at 0.9-1mm. For certain components like presets and PCB terminal connectors, a 1.2mm hole is required. Other components like PCB mounted switches and sockets, hole sizes can be up to 2.5mm.

Electrolytic capacitors can be dangerous when connected the wrong way round. If the power supply has the current capability, the electrolyte heats very quickly and the gas produced has enough pressure to propell the metal case many feet accompanied by a bad smell and a mess on the circuit board. This can be dangerous if inspecting the board at the time. TAKE CARE and always check the orientation of electrolytics, in particular the 100uF types.

Integrated citcuits are easy for students to insert incorrectly. Remember the dot identifies pin 1 and a divot in the case identifies the top. These can heat up and crack the case if inserted incorrectly and are almost certainly destroyed if not. It is always good advice to use IC holders.

Not so common but students can make mistakes with the resistor colour code and thus soldering wrong values in place. This is not usually a problem if the value is higher than required but if they are lower then damage to other components is a possibility. Check whatever is in series with the offending component and test/replace if circuit still refuses to work correctly.

If LED's fail to light correctly this can be due to resistor value too high or the LED incorrectly connected. Some of the leg lengths and flat faces that identify polarity can be difficult to see. Test the LED separately with a 330ohm resistor and a 5 volt supply. If an LED is connected without a current limiting resistor, it will almost certainly destroy itself within seconds. Short of this the colour of the LED will be altered and is best replaced.

GENERAL TIPS

Colour code wiring where ever possible when connecting to input and output devices. Very useful for fault finding.

PCB pins are useful for connecting wires to boards. Avoid using solid core wire as it snaps from metal fatigue easily if moved too often.

To help reduce wires breaking when moved a lot, try drilling 2-3mm holes on the edge of the PCB and feeding wires through to act as a form of strain relief.

If you have LED's on your circuit, whenever possible mount them directly onto the PCB. This will require careful positioning of the PCB in its case so the LED can reach the top of the case and show through.

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