Sencore TF151 - Checking Diodes and Rectifiers Out of Circuit; Out-of-Circuit Diode Test Procedure; Zener Diode Testing; Varicap Diode Testing

CHECKING DIODE AND RECTIFIERS OUT OF CIRCUIT
A more accurate check out of the circuit may be made on a diode or rectifier wi th
the leakage position on the TFl51. The actual reverse current or leakage can be
measured and compared with the forward current to find the actual front to back
ratio of the diode. To connect the diode for the out of circuit test, see figure 9 and
use the following steps:
1. Connect the red lead to the anode of the diode.
2. Connect the yellow lead to the cathode or positive lead of the · diode.
3. Set the function switch to LEAKAGE position and the Type switch to NPN.
The meter should indicate forward current and read at or near full scale.
4. Switch the Type switch to the PNP position and read the reverse current of
the diode on the meter on the MICROAMP scale of the meter.
The two readings may now be compared to find the actual front to back ratio of the
diode. In normal practice, the forward current should be at least 10 times as large
as the reverse current or leakage for good diode action. A germanium diode will
have a reverse leakage range around 10 to 50 microamps whereas a silicon diode
may not register on the TFl51 meter.
BLACK LEAD
E
RED LEAD
C
Figure 8. Connecting Diode Or
Rectifier For In- Circuit Testing
ZENER DIODES
YELLOW LEAD
B
RED LEAD
C
Figure 9. Connecting Diode Or
Rectifier For Out-Of-Circuit Testing
Zener diodes may be checked with the TF151, but the results will not indicate the
regulating point of the diode . Zener diodes above eight volts will exhibit no leakage
and will show good forward conduction on the out of circuit test. A Zener diode of
around six volts will have a reverse leakage of better than three quarters full scale
or arou·nd 2000 microamps and a zener diode of less than five volts will read the
same in both directions, due to the zener effect. Therefore, a check on a zener
diode may be misleading unless you are aware of this effect.
VARICAP DIODES
A varicap diode is just like a regular silicon diode. The only difference between
the two is that the varicap diode has a controlled capacity effect. The varicap diode,
when checked on the TF151 will appear the same as a regular silicon diode. The
reverse current or leakage will be one of the most important checks on this type of
diode. Excessive reverse leakage current will cause the Q of the varicap to be lower
than normal which will reduce the sensirivity of a receiver or reduce the output of
an oscillator circuit.
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