Heathkit IT -3120 User Manual

Page 33
BETA = 00
BETA CAL
The "beta = 00" circuit, as shown in Part A of Figure 6,
primarily compensates for the transistor collector load
resistance, R8,
and in-circuit resistances, Rx.
This
circuit is basically a bridge circuit as shown in Part B of
Figure 6. Resistor Rt represents load resistor R8 plus any
in-circuit resistance, Rx . Because Ax varies widely from one
circuit to another, control R11 (Set Beta = 00) provides a
means of balancing the bridge circuit . A true representation
of beta can now be achieved since any unbalance that occurs
in subsequent tests will be directly associated with the
transistor collector current.
Resistors R 1 through R4 are meter shunt resistances for the
1 A through 1 mA ranges respectively . The 100 flA range
does not use a meter shunt resi stor.
The base of the transistor is now connected into the circuit
through control R12 (Beta Cal) and resistor R10. See Figure
7. Control R12 adjusts the base current of the transistor
until sufficient collector cu rrent flows through the meter to
place the pointer over the CAL X10 mark (full scale) . The
collector current is now equal to the particular current range
you selected.
Because resistor R 1 0 is the same value as the meter
resistance, the meter and resistor A 10 can be interchanged in
the following tests to measure base current without changing
the collector cu rrent.
Notice that a shunt resistor is in parallel with resistor R10 .
This total resistance (R 1 0 and its shunt) is ten times larger
than the resi stance of the meter and its shunt for the same
range . Thus , when the meter is placed in the base circuit, it
will
read 1/ 10 the current measured in the collector circuit.
" "" STOR n "
B l l ~ ) '-../
1 .4
! LlOr A I OnA ImA
SET BE T A, C>Q
T£ ~ ___ ~
I
I
I
~ 4- Rl i
R I R2
,
,
RJ
,
~
R ~
~ R 8
~
I 5 r.
~
l. 5 ()
f 151)
~ 166 I)
I
f
'1
+
I 500 ()
~l: '" l' ~f--+--------<t--!--------+-----<'f-±---
(B * RI2 62 B I
r 25 0KIl I. JV 1.5 V
IA
100 r) A I 01'1 A
I , ') A
,
I
I
R4
RIO
~ R 2 ~ R 3
] OP l
) 1 .5 0
) 1; I)
1 col'
1500 1)
~
..
RIJ \,000 0. 1
IF C O~T ROL IS pu ll ED
BETA CAL
OUT IIA RA GE 0 LYI.
Figure 7

Heathkit IT -3120 Specifications

General

Brand	Heathkit
Model	IT -3120
Category	Test Equipment
Language	English