How to Measure Transistors

A bipolar transistor

Transistors are electronic devices made from semiconductors such as silicon or germanium. They can amplify electrical signals or function as switches for current flow. Bipolar transistors are the most popular. They have three semiconductor layers with leads. A small current or voltage through the middle layer, called the base, controls current flow through the emitter and collector layers.

The parameter used to represent the amount of current amplification is called the gain. It is symbolized as beta or hfe. To measure the gain out of circuit, use a digital multimeter.

Things You'll Need

• NPN or PNP transistor, such as a 2N3904
• Transistor data sheets
• Digital multimeter

Instructions

1. Identify which leads on the transistor are the base, emitter and collector. Use the data sheet to do this.

2. Turn the multimeter on, and turn the knob to either the NPN or PNP transistor setting. This may be indicated by the transistor type and the letters "hfe".

3. Locate the transistor slots on the multimeter casing. These may be holes labeled with a letter for each of the terminals. For example, the emitter is labeled "E."

4. Gently insert the transistor into the multimeter. The transistor leads must correspond to the proper opening, and the transistor must be in firmly enough to make contact.

5. Record the value of hfe as indicated on the display. This value will be in the range as specified on the data sheet. For example, the range for the 2N3904 is 100 to 300. Your multimeter may read 194, which is within accepted parameters.

Tips & Warnings

• Transistors are very delicate. Be careful when pulling the leads apart to take the measurement, or the component will break.

References

• "Electronic Principles"; Albert Malvino; 1999
• "Getting Started in Electronics"; Forrest Mims III; 2000
• "The Art of Electronics"; Paul Horowitz and Winfield Hill; 1997

Resources

• Bipolar Transistors
• Transistors