(7) Set the MODE switch to DC TRANSFER and the SENSITIVITY switch to LOW.
Null the galvanometer using the output controls on the DC voltage standard. Turn
the SENSITIVITY switch to MEDIUM and then to HIGH, nulling the galvanometer as you
The value of the unknown AC voltage can be read out on the DC voltage
h. The theory of operation is explained in this lesson to assist in
understanding the operation and trouble-correcting analysis of the Model 540B. The
figures presented in this section are simplified for easier understanding.
block diagram will be explained first, followed by a detailed description of each
functional section of the instrument. Refer to Figure 2-4 throughout the following
(1) With the MODE switch in the AC SEARCH position, the input signal passes
from the binding posts through the MODE switch, through the first protection relay
(K801) and to the search compensation attenuator. The output of the attenuator is
The signal path is through the MODE switch to the search amplifier where the AC
signal is amplified and then rectified and filtered. R704 is connected across the
input in SEARCH to provide proper input impedance.
The output of the search
amplifier is then connected through the MODE switch to the PERCENT INPUT meter
where the needle deflection is calibrated to read in percentage of input per range
(2) When the MODE switch is in the DC SEARCH position the direct current
path is identical with the AC path except no amplification is required and the MODE
switch provides an alternate path around the search amplifier.
adjustments are provided for the search ranges in the search amplifier and in the
alternate DC path around the search amplifier.
(3) With the MODE switch in the AC or DC TRANSFER positions, the signal
from the input terminals passes through the MODE switch, then through the first
protection relay (K801), and feeds both compensated attenuators.
compensated attenuator output is connected to the protection amplifier and the
The transfer compensated attenuator feeds through the MODE
switch, then through the second projection relay (K701).
From the second
protection relay, the signal feeds the thermocouple.
The output of the
thermocouple is fed to the galvanometer in such a manner that it is opposed or
balanced by the output from the reference supply. The galvanometer indicates the
null or balance between the two.
(4) The protection amplifier is always connected to the output of the
search attenuator, and is designed to function on a pre-set level of AC or DC.
the input from the attenuators and open the circuit to the thermocouple.
protection amplifier also causes relay K301 to close a circuit from the battery
supply which causes the PERCENT INPUT meter needle to deflect into the OVERLOAD
area of the meter scale.
Once the protection amplifier has operated, it must be
reset manually by turning the MODE switch to OFF. The cause of the overload should
be located and removed and the range increased or other remedial action taken
before returning the MODE switch to it's operating position.