Observing Voltage Excursions

TM 11-6625-599-12

are present and the NULL switch is

Note: Set the voltage-divider switches slightly below

the approximate value as indicated in the chart above.

set to .1 or .01, additional filtering

may be required.

(3) Advance voltage-divider switch B un-

(2) If the ac component is of a single fre-

til the VOLTS meter indicator deflects

quency, a twin-T-filter is effective

to the left, and then back one step to

and has the advantage of low total

return the indicator to the right side.

series resistance. If the ac compo-

(4) T r i m voltage-driver s w i t c h e s C

nents are of variable frequency, an

through E as required to obtain 0 in-

ordinary low-pass filter may be used.

dication on the VOLTS meter.

In either case, the capacitors used to

(5) Determine the voltage being meas-

form the filter should have a very

ured from the voltage-divider indica-

high leakage resistance.

tors; be careful to note the location

(3) Since the frequency and magnitude of

of the decimal indication.

the ac voltage component may vary

E x a m p l e : V o l t a g e - d i v i d e r indicators A,

from equipment to equipment, a spe-

B, C, D, and E indicates 3, 8, 6, 5, and 9,

cific filter cannot be specified here.

respectively. I f t h e R A N G E s w i t c h w e r e

Presumably, if a filter is required, it

on 500 and the NULL switch on 10, the

will be specified in the technical man-

d e c i m a l point indicator between C and D

ual covering the equipment under

would be illuminated and the voltage meas-

test. Losses introduced by the filter

ured would be 386,59 volts. However, if

the RANGE switch were on 5, the NULl

should be computed as specified in

switch on .1, and the decimal point indicator

the equipment technical manual.

between voltage-divider indicators A and B

i l l u m i n a t e d , the voltage reading would be

2-8. Observing Voltage Excursions

3.8659.

The vtvm can be used to observe excursions

(6) Turn the NULL switch to VTVM.

of a voltage about a given value, such as the

(7) Turn off and disconnect the voltage

variations in the output voltaqe of a dc power

under measurement from the com-

supply. Determine the approximate value of

mon input terminal and the input ter-

the voltage to be observed by using the vtvm

minal.

as a conventional vacuum-tube voltmeter (para

26 ) and proceed as follows:

c. Ac Component.

a. Set the vtvm to operate as a differential

(1) A low-pass filter is used to reduce

voltmeter (para 27 ).

any ac present on the dc being meas-

b. Observe the excursions on the VOLTS

ured. This filter has an attenuation

meter. The proper scale to use is indicated by

rate of 330 to 1 at 60 cycles per sec-

the position of the NULL switch as shown in

ond (cps). A 0.5-volt ac component

the following chart:

at 60 cps will be reduced to slightly

over 0.001 volt. This may still cause

a reading of as much as 10 percent of

full scale when the NULL switch is

set to .01 and may be negative or

positive. If larger ac components