June 18, 1998

Troubleshooting at the component level

Ron Mancini

Understanding component functionality is paramount.

Anybody--including EEs--can troubleshoot the way my mechanic does. He changes parts until the car works. Sometimes he even changes the same part twice. The key to being a good troubleshooter--whether you're a mechanic or an EE--is to in-circuit- test components. Also, understanding component functionality is paramount. In-circuit testing requires detailed component knowledge and original thinking, so few people take the time and effort required to do it well.

A reminder: Before you start testing any circuit, check the power-supply voltages. Now, starting with the bipolar junction transistor (BJT) in a common-emitter configuration, note the collector voltage, then short the base-emitter junction, forcing I_B and I_C to zero. If the collector voltage rises to V_CC, the transistor is good, assuming that the next stage doesn't draw bias current through the collector resistor. Connect a 1-kohm resistor between V_CC and the base; I_C increases in a good transistor, so V_C drops. If the transistor checks out OK, touch your finger or a signal source to the base through a 1-kohm resistor, observe the collector, and observe the amplified noise or signal. A BJT comprises two back-to-back diodes, so you can double-check it with an ohmmeter. These tests are short and sweet, but 99.99% of the bad BJTs fail these tests. Check other discrete semiconductors, such as FETs and diodes, with these tactics and an appropriate substitution of theory.

Troubleshoot an op amp using the same methodology. Measuring the output voltage determines at which rail the op-amp output is. Check the input offset voltage to determine if its polarity jibes with the output voltage. Measure the voltage across the feedback and gain-setting resistors, and calculate the resistor currents. Unequal currents signify too much input current (usually caused by a blown input). Positive inputs connected to ground are difficult to check, but if there is a resistor in series with them, measure the voltage drop, and calculate the input current.

Inject a signal through a resistor (selected to yield unity gain) into the inverting input of nonsaturated op amps. A good op amp inverts the injected signal with little distortion. Odds predict that op amps passing these tests are good, so next consider the input or load circuits. Test other linear circuits, such as multipliers, comparators, and buffers, with this procedure.

You should also test logic circuits using similar methods. Force a NAND-gate input to a low level by loading it to ground (a short works well, but the driving circuit may not like it), and the output goes high. Oh, I know other inputs are connected to the NAND input, but who cares? Forcing inputs high is difficult when low impedances drive them, so trace back along the logic chain until you find gates that respond to a low level, thus forcing both NAND inputs high. Accomplish nondestructive level-forcing by limiting the duty cycle of a pulse generator. You can extend this concept to complex logic, but volume testing justifies a commercial in-circuit logic tester.

My next column discusses how to employ very high temperatures to force early failures.

| EDN Access | Feedback | Table of Contents |