Section Il. OPERATION UNDER USUAL CONDITIONS

TM 11-6625-537-14-1

Control, indicator

Function

or connector

VOLTS meter . . . . . . . . . . . . . . . . . . . . . . .. . .

Indicates the voltage being measured in volts when used as a vtvm, Indicates

the difference in volts between the unknown voltage and the internal

reference voltage when used as a differential voltmeter. Upper scale 500-0-

500 is used when the NULL switch is set to VTVM, At all other times,

lower scale 1-0-1 is used.

Sets meter to zero mechanically. This adjustment should be used only after the

M e c h a n i c a l zero control . . . . . .

voltmeter has been turned off for at least 3 minutes, or when the internal

meter terminals have been shorted.

Provided for attaching a recorder to monitor voltage excursions. Upper

RECORDER OUTPUT terminals (on rear

panel, fig. 2-3)

terminal wiII be positive when VOLTS meter needle deflects to the left.

Varies the voltage level at the RECORDER OUTPUT terminals form O to

Gain ADJ RI control (on rear panel) . . . . . . .

approximately 20-millivolt full-scale deflection.

Section Il. OPERATION UNDER USUAL CONDITIONS

connect the shorting link between the lower input

3-3. Types of Operation

terminal and the chassis ground terminal when

Electronic Voltmeter ME-202(*)/U may be used

the system being measured is grounded.

as a conventional vacuum tube voltmeter or as a

d. Use of Shorting Link. A 0.47 micro farad

differential voltmeter. It may be used to measure

capacitor (C1) is connected from the middle input

positive or negative dc voltages, to measure ac

terminal to the chassis ground (lower) terminal. It

voltages, to observe excursions of a voltage about

is possible in some cases for C1 to acquire a

a nominal value, or to measure high resistances.

charge and cause inaccurate indications. F o r

3-4. Operation Precautions

example, C1 will become charged through leakage

a. General. The precautions below must be

resistance over a period of time if there is no

observed to insure maximum accuracy before

external connection to the input terminals and the

performing the following procedures,

c o n t r o l s are set as follows: RANGE to 500,

(1) Preliminary starting procedure (para 3-5).

NULL to any position, AC-de polarity to +

(2) Procedure for desired type of operation

(positive), and voltage readout dials to several

(para 3-7).

hundred volts. This condition may cause an error

b. Tilting Electronic Voltmeter. The voltage of

on measurements under 5 volts. Momentarily

the internal standard cell will temporarily change

connecting the shorting link from the chassis

if the electronic voltmeter has been inverted. The

ground terminal to the lower input terminal will

electronic voltmeter will not be able to measure

discharge C1 and prevent any inaccurate in-

voltages within specifications until the standard

dications.

cell voltage returns to its normal value. Whenever

e. Recommended NULL Control Settings for

the electronic voltmeter has been tipped more

D i f f e r e n t i a l DC Measurements. The following

than 450, allow it to remain in a normal upright

characteristics of the electronic voltmeter must be

position for at least 30 minutes before performing

c o n s i d e r e d if the NULL and RANGE switch

any differential voltage measurements.

s e t t i n g s recommended in table 3-1 for dif-

NOTE

ferentially measured dc voltages are not used.

T i l t i n g will not affect accuracy of the

When the RANG E switch is set to 500, the last

e l e c t r o n i c voltmeter when used as a

voltage readout dial (E) changes the reference

conventional vtvm to measure ac or dc

v o l t a g e in steps of 0.01 volt. The unknown

voltages.

voltage would therefore have to be an exact

multiple of 0.01 volt to obtain a null when the

c. Ground Loop Precautions. Potential dif-

N U L L switch is set to .1 or .01. Also, it is

ferences are often found between different points

u n l i k e l y that an unknown voltage of a few

on power system grounds. The potentials may

hundred volts will be stable within 0.01 volt. A

cause current to flow from the power system

b a d l y fluctuating line voltage may cause the

ground, through the electronic voltmeter and the

electronic voltmeter to monitor the regulation of

equipment under test and back to the power

its o w n 500-volt dc reference supply. For

system ground. These ground loop currents may

example, when differentially measuring 500 volts,

affect the accuracy of the electronic voltmeter. To

a 10 percent change in line voltage may cause the

a v o i d power system ground currents, do not

3-4