Question on testing transistor TIP120 NPN using multimeter

Question

Could someone please help to confirm if these values for a TIP120 NPN transistor based on the diode setting of my multimeter, indicate any potential fault?

• Base (Positive) with Emitter (Negative): 0.732v

• Base (Positive) with Collector (Negative): 0.631v

• Collector (Positive) with Emitter (Negative): OL

• Collector (Positive) with Base (Negative): OL

• Emitter (Positive) with Collector (Negative): 0.553v

• Emitter (Positive) with Base (Negative): 1.608v

My concern is with the Emitter(positive)-Collector(negative) and Emitter(positive)-Base(negative) as based on this Youtube (https://www.youtube.com/watch?v=lB7oUD1UVEw) the NPN type should for these connections should show OL instead, although i'm not sure if there should be a difference for different types of NPN.

Answer 1

When you put a DMM in diode mode it uses a voltage high enough so forward biased diodes will conduct. Diodes conduct from Anode to Cathode (see labels on D2, note that Cathode is K since C is already taken). Unfortunately, there is no standard for exactly how a DMM works in this mode.

A TIP120 is not a pure Darlington, it has an extra 2 resistors and an extra diode.

This TIP120 appears to be good. To understand the last measurement, you will need to know the exact specifications of the meter when it is in diode mode.

I indicated the diode path(s) in your measurements below:

Base (Positive) with Emitter (Negative): 0.732v: D4 + D5

Base (Positive) with Collector (Negative): 0.631v: D7

Collector (Positive) with Emitter (Negative): OL

Collector (Positive) with Base (Negative): OL

Emitter (Positive) with Collector (Negative): 0.553v: D6

Emitter (Positive) with Base (Negative): 1.608v: ***

*** There are no forward biased diodes in this path, but there are resistors.

^{simulate this circuit – Schematic created using CircuitLab}