Heathkit IT -3120 - Circuit Description

Page 24
fifii§iiBifj]
I
In other words, for good transistors, lebo will always be less
than lees and lees will always be less than Iceo.
Generally,
you will not obtain any measurable leakage when
testing sil:icon transistors. Most low-power sil icon transistors
have very low leakage with the collector to base leakage
current (lebo) usually less than 1 /lA (one microampere) .
High-power silicon
transistors may indicate an Iceo up to 50
/lA.
Germanium transistors have an lebo that ranges from several
microamperes to as high as 5 rnA. The following chart is a
guide
to help you determine if the leakage is too high for a
germanium transistor. If in doubt, refer to a transistor
manual.
.-
TYPICAL
LEAKAGE
CURRENT
GERMANIUM
lebo Ices
leeo
TRANSISTOR
I
RF-IF-AUDIO
0-5,uA
I
0- 50,uA
I
1 ,uA -
500,uA
AUDIO-
0- 1' 0,uA 1 ,uA-
5,uA -
SWITCHING
100,uA 1 mA
POWER 5,uA -
5,uA -
I
50,uA -
I
100,uA 1 rnA
5mA
NOTES:
1.
Be sure the "B ETA TEST" has been completed before
you perform the following leakage tests.
2. If the leakage cu rrent is greater than 100 ,uA, causing
the meter pointer to deflect off scale, select a higher
current range.
3.
Leakage current will increase in temperature. Even
body heat from holding the transistor in your hand
can increase the leakage current .
As you perform the leakage tests, analyze the result and
determine if the leakages are within acceptable limits.
1. Press the lebo switch.
2.
Press the 100 ,uA switch . Read the leakage current
directlV from the leakage scale.
3. Press the lees switch. Read the leakage current directly
from the leakage scale.
4.
Press the [ceo switch . Read the leakage current
directly from the Ileakage scale.
5. Release the ON-OFF switch to the OFF (out)
position .
6.
Disconnect the transistor from the Tester.
FET TESTING
Drawings C and D of Figure 3 (fold- out from Page 22) show
the schematic symbols for N channel FE T's (field effect
transistors) . Notice that N channel F ET's have a positive
drain-to-source voltage. Drawings E and F show the
schematic symbols for P channel F ET's. These F ET's have a
negative
drain-to-source Voltage.
The following steps measure the Gm (transconductance) of
FET's _ Gm usually ranges between 500 to 10,000 ,umhos.
This characteristic, I ike beta, can even vary between F ET's
of the same type . It is also useful when selecting devices
(matching)
that have the same characteristics.
The foltowing procedure pertains to both in-circuit and
out-of-circuit testing. If you do not obtain the proper results
when the FET is tested in-circuit, remove it from the circuit
and repeat the tests.