Heathkit IB 5281 Manual

Page 1

HEATHKIT
MANUAL

HEATH COMPANY - BENTON HARBOR, MICHIGAN

Page 41

HEATHKITE

Part 2 of Pictorial 7-1 shows the configuration for the
RLC Bridge. It is basically the same as that of the
Wheatstone Bridge. A fixed, internal component
standard (2,) rather than a calibrated potentiometer
(as used in the Wheatstone Bridge), is used in con-
junction with the Range switch, SW2. The Range
switch selects the multiplication factor and a certain
frequency for the type of component being tested. The
variable function is provided by a single control,
which is divided to act as two arms of the bridge, at R1
and R2. Changing both of these values on the bridge
provides a much greater range than a single control
would provide.

When you test an unknown component value [ij , the
known component (ZS) must be the same type as the
unknown component [ZX]. The balance control at R1
and R2 is actually matching the ratio of the unknown
component to the known standard The range switch
is marked in R, L, and C values to simplify the read-
out.

The RLC Bridge circuit uses an AC source rather than
a DC type since capacitors and inductors cannot be
tested with DC. The AC required to operate the bridge
is generated by a Wien bridge oscillator. This oscil-
lator consists of transistors Q1 through 05. It provides
a low impedance output to drive bridge transformer
T1. Level control R24 adjusts the oscillator output
level to keep meter M1 on scale. The oscillator output
voltage is rectified by diode D3 and is used as a con-
trol voltage at the gate of transistor 02. Q2 acts as a
variable source resistance for transistor 01 and con-
trols its gain. The oscillator provides three output
frequencies, which are selected by the Range switch
for the particular type of component being tested. The
frequency used for each range is as follows:

RANGE FREQUENCY
ZS [external standard] 1000Hz
RXl 1000 HZ
RX100 1000 HZ
RX10l( 1000 HZ
L><1 [1. 100 kHz
Lx.1 mI-I 10 kHz
LX.01 H 1000 Hz
Cx1 pF 100 kHz
CXF .0001 F 10 kHz
Cx.01 [LF 1000 Hz

Page 39

PART 1

+
DC -
SOURCE .-

PART 2

AC
SOURCE

PICTORIAL 7-1

Meter M1, which indicates a null (or balanced bridge
condition), is powered by amplifier stages QB
through Q10. DC feedback is provided via R31 while
AC feedback is provided through the meter circuit via
capacitors C14 and C15. Both types of feedback en-
hance circuit stabilization.

Power is supplied by two 9-Volt batteries (see Intro-
duction" for the type] and/or the Heath Model IPA-
5280-1 Power Supply.