Questions for Circuit Debugging

For almost any electronic circuit of non-trivial complexity, debugging is part of the building process. The circuit may have wiring mistakes, a component may be misconnected or the circuit design may have flaws. By asking a general set of questions, you can quickly isolate and solve problems to get the circuit working.

If the schematic from which you built the circuit comes from a trusted source such as a professionally written book, chances are slim that the design is at fault. On the other hand, if you built the circuit from a new design, it pays to analyze the schematic and look for problems there. It might help to bring in an experienced engineer or technician to examine the schematic; the original designer may not catch his own mistakes.

If the circuit you have built has no output, the problem may lie in the connections between it and its power supply or battery pack. If the connections check out, examine major power-consuming components such as transistors and integrated circuits. If they are misconnected, they can draw excessive current, depriving the rest of the circuit of power. Replace a misconnected semiconductor component rather than reconnect it, as it may have suffered damage.

If the circuit has some output but behaves poorly, test the power supply voltage. An inadequate supply voltage causes marginal performance and distorts the circuit’s output. Double-check the values of passive components such as capacitors and resistors; in building the circuit you may have selected parts incorrectly. Check the connections of these components, as incorrect connections can cause distortion.

Disconnect the power supply from the circuit and test it separately. If its voltage is correct, the problem is with the circuit. Connect the power supply to an oscilloscope, set the oscilloscope’s input coupling to AC and adjust the input sensitivity to about .1 volt per division. If you see excessive alternating current ripple or noise on the oscilloscope display, the power supply is defective. Electronic circuits need a clean, well-regulated source of DC from which to operate.

Overheated electronic components have a distinct “hot plastic” smell. If you see a puff of smoke or smell evidence of overheating, disconnect the circuit's power immediately. With the power off, carefully touch transistors, resistors and integrated circuits. If any feel hot, examine the connections to the component. Replace any overheated parts and make sure you connect them correctly.