Meter Usage - Continued
We must also take separate account of transistors designed to handle more power than those
already considered. Those used in the audio-output and power-supply applications may fall into the
medium-power and high-power class. However, circuit application is not a reliable guide. The
manufacturers ratings should be consulted when there is any doubt. Transistors with collector
dissipation rated below 250 milliwatts may be considered low-power types. Where the rating
exceeds 3 watts, we are dealing with high-power types.
To understand why power rating will affect resistance readings, we once more consider
semiconductor diodes. A diode designed for use as an video or audio detector passes little current,
whereas one intended for use as a power-supply rectifier will handle a current many times greater.
This difference is reflected in both forward and reverse resistance readings. The detector diode will
measure much higher than the power supply rectifier. Similarly transistors designed to handle
greater power will pass more current and show lower resistance values.
Tables 8-3 and 8-4 give ohmmeter connections and typical resistances measured respectively on p-
n-p and n-p-n medium-power types. Readings for high-power transistors are given in tables 8-5 and
8-6. Note how forward resistance of either diode sections, drops off from 120-140 to 80 and then 50
ohms as power rating increases. The same pattern is observed in reverse resistance readings.
However, a helpful fact can be noted. Although there may be uncertainty concerning exact
resistance readings obtained in any single measurement, there is a very clear relationship between
forward and reverse resistance, with the latter being a few hundred times greater than the former. If
this does not show up on a test, the transistor may be considered defective.
Figure 8-4. Transistor Shown as a Two-Diode Element