Fluke 825A - Polarity Switch

Also, the null detector operates with а common that is
above or below (depending on the setting of the polarity
switch) the potential of the lower input binding post by
the amount set on the voltage readout dials.
3-6. POLARITY SWITCH
Because of the polarity switch S5, upper input post J1
Should never be electrically connected to the 825A chas-
sis, either at the 825A or at the source under measure-
ment. Ав seen in figure 3-2, the polarity switch does
not just reverse the input posts. It reverses the meter
and the internal reference supply. If the instrument
did not contain a polarity switch, the grounded side of
any unknown voltage that is negative with respect to
ground would have to be connected to Ше upper input
binding post. This would ground the upper post and ef-
fectively place C1 (a 0. 47 uf capacitor) across the input
terminals. Even if C1 was disconnected, there would
still be considerable capacitance across the input due
to transformer winding capacitance and stray wiring
capacitance. With this capacitance across the circuit
being measured several problems would arise. The
polarity switch provides equal convenience in measuring
positive or negative voltages without the occurance of
these problems.
3-4
3-7. NULL DETECTOR POWER SUPPLY
a. The B+ supply for the null detector is obtained from
a half-wave rectifier consisting of diode CR201 and а
filter network (C213A, R227, and C213B) that is regu-
lated by an 0A2 tube (V204) and series dropping resistor
R230. Divider resistors R228, R229, R231, and R232
and diode CR202 provide a compensating voltage for the
purpose of adjusting the null detector to zero with R232
when there is no signal input. Diode CR202 keeps one,
side of R232 at approxima tely-0. 6 volts DC with respect
to the null detector common.
b. Тһе filament of V202 and the chopper drive unit
require a stable supply. This is obtained by using a fil-
tered half-wave rectifier (CR203 and C214) that drives a
Darlington connection of Q1 and Q2. А divider string
R234 and R235 uses the B+ supply to obtain approximately
11.8 volts for pin-5 of V202 while pin-4 is maintained
approximately 0 volts through the emitter-base junctions
of Q201 and Q202. Any change in the half-wave rectifier
voltage causes a corresponding change in the voltage
across the emitter-collector junction of Q201 so that the
filament and chopper voltage remain stable. Тһе filament
supply for V203 is obtained from an ordinary 6.3 volt AC
winding.