If you are an electronic experimenter who cannot get his circuit working, this is for you. I really feel your pain as it brings back my own unpleasant memories – yes, we have all been there. Here are a few tips that will help you to troubleshoot your circuit.
Pay proper respect to Murphy (anything that can go wrong will go wrong…)
Expect problems – do not set your expectations too high. I am not being pessimistic, but realistic – welcome to life! Even with all my experience, I still make lots of mistakes. I once had a technician tell me that his circuits always work the first time. I avoid calling anyone a “liar,” however…
Check list of potential problems (in order of relevance)
Wiring error (includes incorrect pinout, short circuits, and open circuits)
- Note the location of IC pin 1 – pins ALWAYS count counter-clockwise (viewed from top) or clockwise (viewed from bottom). When wiring, I always mark pin 1 with a black dot.
- Note pinout of TO-92 transistors – Most 2Nxxxx devices are EBC (viewing flat, pins down) – most BCxxx devices are just the opposite – most TO-98 devices are ECB. Check the datasheet to make sure -occasionally errors occur in schematics and bills of material in which NPN and PNP devices are accidentally swapped. Spot check by observing polarity – NPN has a positive voltage on the collector and PNP negative – sometimes they are incorrectly drawn upside down.
Incorrect component (includes incorrect resistor or capacitance value, or improper capacitor selection)
- Note that circuits with high inrush current (line capacitor limited power supplies) require a series limiting resistor to keep inrush below approx. 5A. Carbon film resistors often fail in this application, but the old carbon composition and the newer ceramic composition types are very rugged.
- An example of improper capacitor selection would be the use of a polyester capacitor across the power line – such is prone to failure. Another is the use of a leaky electrolytic as a timing capacitor in a long period 555 timer – some may function while others will not.
Defective component (make sure you have spares on hand – use IC sockets – great aid in troubleshooting)
Incorrect shortcut or modification – we cause most of our own problems…
Workmanship problems (includes microscopic solder bridges or cold solder joints etc.)
Defective circuit – Some posted circuits simply do not work – some are of faulty design that may have worked for the author, but with so many variables involved that they may not always function. Read the comments to see if others have had success – if a bad circuit, chock it up for bad experience. Occasionally a comment will link to an updated circuit. If so, go there instead.
Obvious symptoms (AV effects…)
Noise, Smoke, Flash, Sparks, Heat, or Explosion – hopefully you were wearing safety glasses…
This can be an example of a “full house” in which all semiconductor devices fail simultaneously – some ICs may even blow their lids off and you can view the awesome internal lead frame. Electrolytics can vent steam or explode if connected reverse in a power circuit.
When this type of catastrophic failure occurs, it is often the result of incorrect battery or power supply polarity. Make sure that you always install a diode in series with the battery to prevent this kind of damage from an accidental battery reversal – do this even if the schematic does not call for it! Note that even a small 9V transistor radio battery can source 5A into a short circuit, and an IC is essentially a short circuit under this condition, and will fail instantly – transistors generally fare better due to the presence of series resistors. An adjustable lab power supply with a current limit feature is a good way to get a circuit up and running without chancing the destructive power of batteries.
Some circuits due to their nature require everything to function correctly when power is initially applied. An example of this is a high voltage UPS inverter circuit with an off-line power source that provides a bus voltage of 160, 325, or 650VDC – these are very unforgiving. The best way to get one of these off the ground and check for faults is via the use of a Variac (adjustable autotransformer). This type of project is recommended only for the more experienced experimenter.
Observe good component handling – especially with sensitive MOS devices under low humidity conditions. Make sure that your soldering iron (if grounded at all) is grounded via a high resistance resistor. Check out my article on ESD.
Make no assumptions!!!
You may have heard of soldiers who were commanded to “take no enemies,” well this is the troubleshooting equivalent. When we get stuck and cannot proceed, this is virtually always the reason. When you reach this point, you must recheck your work and that is where incorrect assumptions preclude progress. One simply cannot say, “I checked that detail yesterday so that cannot be the problem.” That could be a bad assumption – RECHECK AGAIN! Remember that humans make mistakes, but electrons never make mistakes…
Check schematic against component data sheets – do not even assume the schematic is correct.
Circuit was working, but now it does not…
What was the last thing you did to the circuit? Most likely you caused it by your most recent action.
Use Troubleshooting Tools
DMM (digital multi-meter) – If you do not have one get one and learn how to use it.
Analog VOM (volt-ohmmeter) – also very useful
Oscilloscope – very useful and getting more affordable every day – required for the serious experimenter
Metered power supply – often I have said to myself “it shouldn’t be drawing that much current.”
Divide and conquer
See if the circuit can be divided into sections – small sections or stages are far easier to troubleshoot or eliminate than entire circuits. Transistor stages may often be tested by simply grounding base to emitter to see if the transistor turns off as expected and provide a logical “1” output – or short collector to emitter to provide a logical “0” output.
Obtain a 2nd set of eyes
Yes, get a companion to look at your circuit – he may see things that you do not see.
Let your background processor do its work
I have never read anything about background processing and the human mind, but it surely exists. Set your project aside for a while – sleep on it. If your background processor comes up with something, it will flag your conscious mind. Have you ever lost something and recalled later where it was located (while you were engaged in an unrelated task)?
DO NOT GIVE UP easily
When you find the solution you may be astonished at how dumb you were…
Sometimes you can run into a real sleeper that defies logic. Here are a few that I have seen:
- Reverse banded diode or electrolytic – rare, but confusing
- IC socket fails to make contact, or pin is bent under device so it fails to make contact (symptom: touching pin directly with DMM or scope causes bizarre behavior)
- Unconnected CMOS inputs – yes all CMOS inputs must be connected!
- Protoboard has a bad pin socket – this may be avoided during construction by observing lead insertion force – sprung contacts do happen
- High frequency oscillation that causes bizarre symptoms – requires oscilloscope to troubleshoot.
- Broken lead (inside the insulation)
- Relay does not pick up – biased magnet design requires proper polarity on the coil.
- Contact corrosion – tin plated contacts do not fare well in the presence of vibration.
- Exceeding Vcc or Vss power supply rail voltages on ICs causes unpredictable or bizarre behavior – often in unrelated sections of the same IC (e.g. a negative voltage on an LM339 quad comparator input).
Note that this is in no way intended to be comprehensive. I invite comments so we can add useful information.
Undocumented words and idioms (for our ESL friends)
Murphy – allusion to Murphy’s law that states that anything that can go wrong does go wrong – inverse probability http://www.murphys-laws.com/murphy/murphy-laws.html
Chock it up – idiom – document something as being good or bad – literally “write it on a slate”
Full house – idiom – poker card game term meaning that all five cards in a hand have value. In electronics, it means all semiconductor (or other) devices failed simultaneously – Similar to the expression: “bought the farm”
Sleeper – noun – something that is hidden from normal view, unknown, unexpected. Used in bowling when there is a hidden pin that makes getting a “spare” difficult.