How to Measure the Quality of a Power Transistor?
2 Answers
Methods for measuring the quality of a power transistor: You can use a multimeter to determine its condition by measuring the inter-electrode resistance. To measure the inter-electrode resistance, set the multimeter to the R×100 or R×1K range and measure the forward resistance between the emitter and collector. If the measured forward resistance between the emitter and collector is very high or the reverse resistance is very low, it indicates that the power transistor is faulty. If the measured forward resistance values of both the emitter junction and collector junction are low, it means the transistor is in good condition. A power switching transistor refers to a transistor that can withstand large currents, has small leakage currents, and exhibits good saturation conduction and cutoff characteristics under certain conditions, without considering its amplification performance. The control electrode (base) current size or direction determines the current flow between the collector and emitter, and the direction depends on whether it is an NPN or PNP transistor.
As a seasoned veteran in the field of electronic repair, measuring the condition of power transistors is routine work. The first step is to ensure the equipment is completely powered off and the car engine is shut down before starting, to avoid accidental electric shocks. I prefer using a digital multimeter set to the diode test mode, then measuring each pin of the power transistor one by one, such as between the base, collector, and emitter. Normally, it should only conduct in a specific direction. If a short circuit or complete open circuit is found, the transistor is definitely faulty. Especially in automotive ECU systems, where power transistors control ignition, it's best to disconnect the connector before testing to avoid interfering with other circuits. In practice, gently touch the pins with the probes—don’t press too hard, as the pins can break easily. Experience tells me that when test results are unstable, the transistor is often aged and needs replacement. Otherwise, more startup issues will arise, and timely replacement saves both money and hassle.
