How to test germanium transistors

Updated July 13, 2018

Early solid-state electronic equipment made in the 1950s and '60s used germanium transistors. When electronics manufacturers perfected the silicon transistor, it soon took the place of germanium. While the two materials have differences, a transistor's basic job of amplifying electrical currents remains the same. Like the silicon type, a germanium transistor has three leads called base, collector and emitter. Test a germanium transistor's basic functions with a multimeter and a leakage detector. The leakage detector may work on transistors in a circuit, though the multimeter test will work only on loose transistors.

Look closely at the body of the transistor and find the part number. If it is germanium, it will probably begin with a two-letter code starting with "A." Look also for the letters "E," "B" and "C" on the transistor, indicating the emitter, base and collector pins. If the transistor does not have its pins marked, find the part number in the transistor catalogue and determine the pin layout from the part's case diagrams.

Turn the multimeter on and turn its selection knob so it measures resistance. Touch one probe to the base and the other to the collector. Now reverse the probes. Only one of the readings should show a high resistance, and the other should be low. Do the same test for the base and emitter pins. Once again, you should see one low and one high resistance reading. Touch one probe to the collector and the other to the emitter. Note the resistance and then reverse the probes. Both should give high resistance readings. Turn the multimeter off and set it aside.

Turn the leakage detector on and connect its three probes to the transistor, one probe for each pin. Turn the detector's selector knob through the various combinations of pin connections until you find a setting that works. Refer to the entry in the transistor catalogue and note whether you're testing a small-signal transistor or a large-signal one. A small germanium transistor's leakage current should read no more than 50 microamps. A large germanium transistor should read below 2,500 microamps. If the transistor's leakage current reads less than this and the resistance readings from Step 2 checked out, the transistor is probably good. Otherwise, replace it.

Things You'll Need

  • Transistor catalogue
  • Digital multimeter
  • Transistor leakage detector
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About the Author

Chicago native John Papiewski has a physics degree and has been writing since 1991. He has contributed to "Foresight Update," a nanotechnology newsletter from the Foresight Institute. He also contributed to the book, "Nanotechnology: Molecular Speculations on Global Abundance."