The Model 887A has three basic operating modes: DC, AC, and TVM. In
the dc mode, the input voltage is connected through the DC Attenuator to the
NULL Detector and Kelvin-Varley Divider. The DC Attenuator attenuates input
voltages above 11 volts to values within the 0- to 11-volt operating range
of the Kelvin-Varley Divider. The Kelvin-Varley Divider is adjusted to make
the output voltage equal to the unknown input voltage. When they are equal,
the Null Detector will indicate zero on the meter.
unknown voltage can then be read from the frontpanel dials which are
mechanically linked to the Kelvin-Varley Divider. In the ac mode, the input
signal is connected to the AC to DC Converter where the ac voltage is
voltage is then compared to the reference voltage as in the dc mode. In the
TVM mode, the Kelvin-Varley Divider and Reference Power Supply are
disconnected and the input signal is connected directly to the Null
In this case the Null Detector functions as a conventional
Figure 5-14 shows the block diagram for the 887A Differential
Voltmeter. As seen in this figure, the circuit is mainly composed of an ac
to dc converter, a dc input attenuator, a dc transistorized voltmeter (TVM),
and an extremely accurate 0- to 11-volt reference. The dc input attenuator
reduces the input voltage by a factor of 100 on the 1000- and 100-volt dc
range. The TVM uses a Null Detector, an attenuator, and a meter to obtain
high sensitivity. The 0- to 11-volt reference uses a range divider and a
Kelvin-Varley attenuator to make the output of two well-regulated zener
diodes adjustable. Refer to the functional schematic for more detail. This
schematic is designed to aid in the understanding of circuit theory and
troubleshooting. The signal flow is from left to right and the components
are laid out in a functionally logical manner.
The overall operation of the voltmeter may be summarized as follows:
To measure the approximate value of a dc voltage between 0 and 11 volts, the
unknown voltage is connected directly across the TVM attenuator.
attenuator is set in such a way that the maximum voltage for each range is
reduced to a signal of 1 millivolt (100 microvolts for the 1-volt range in
the highest null mode). The signal is then applied to the null detector and
causes 100 microamperes to flow through the meter for full-scale deflection.
To accurately measure this dc voltage, the unknown voltage is connected
across the series combination of the TVM and the 0- to 11-volt reference.
The reference voltage is then adjusted with the five voltage readout dials
until it matches the unknown voltage as indicated by the TVM. For voltages
between 11 and 1100 volts, the dc input attenuator divides the unknown
voltage by 100.
The 887A then operates essentially the same as for
measurements from 0 to 11 volts.
All ac measurements are made by first
converting the ac input voltage to a dc voltage by means of the ac to dc
converter. The 887A then operates essentially the same as for approximate
and accurate dc measurements.