Ac-to-Dc Converter

TM 11-6625 -537-14-1

5-5. Ac-to-Dc Converter

terminals. The output of the converter at 400 Hz

(fig. FO-4)

is adjusted by 0.5 VAC GAIN ADJUST R536 at

the converter output. The converter output at 10

a. General. All ac voltages measured by the

kHz is adjusted by 0.5 VAC HF TRIM C519 in

electronic voltmeter are first converted to an

the feedback network.

equivalent dc voltage by the ac-to-dc converter.

d. Converter Operation, 5- Volt Ac Rang{

Levels I and K of AC-de polarity switch S 5

Levels G and H of RANGE switch S2 connect the

connect the converter between the input terminal

input voltage directly to the amplifier. Operation

and the null and range divider. The converter

of the amplifier is explained in b above. The

provides a dc output of 5 volts when full range

n e g a t i v e feedback network selected by S21

voltage is applied to the input terminals in each

consists of R541, R542, and C521. This feedback

range. The dc output voltage is proportional to

network is also used for the 50 and 500 positions

the average value of the applied ac voltage. The

of RANGE switch S2. The output of the con-

output is calibrated to indicate the rms value of a

verter at 400 Hz is adjusted by 5 VAC GAIN

pure sinewave (single frequency ac voltage). The

ADJ R541 in the feedback network, The con-

c o n v e r t e r consists

o f t h r e e resistance-

verter output at 10 kHz is adjusted by 5 VAC HF

capacitance-coupled amplifier stages with high

trim C504 in the grid circuit of V501. The output

negative feedback. Input attenuators and

of the converter is 5 volts dc when 5 volts ac is

predetermined amounts of negative feedback are

applied to the input terminals.

used to obtain a dc output of 5 volts when full

e. Converter Operation, 50- Volt Ac Range.

range voltage is applied to the input terminals in

Level G of RANGE switch S2 connects the input

each range.

terminal to 10:1 attenuator R501, R502, R503,

b. Amplifier Operation. Tubes V501, V502A,

C501, R544, and C502. The attenuator output is

and V502B form three resistance-capacitance-

applied to the amplifier by S2H. The attenuator

c o u p l e d amplifier stages with high negative

output at 400 Hz is adjusted by 50 VAC AT.

feedback. The input is applied to the grid of V501

T E N U A T O R R503. At 10 kHz the output is

by C503, C504, R504, and R505. Cathode bias for

adjusted by 50 VAC HF TRIM C501. The at-

V501 is developed by R513 and C522. The output

tenuator output will be 5 volts ac when 50 volts ac

of V501 is coupled to the grid of V502A by R514,

is applied to the input terminals. Operation of the

C508, R515, C509, R518, and R519. Cathode bias

amplifier is as explained in b and d above to

for V502A is developed by R522, C513, R523, and

obtain a 5 volt dc output from the converter.

C514. The output of V502A is coupled to the grid

f. Converter Operation, 500- Volt Ac Range.

of V502B by R524, C511, R525, C512, R526, and

Level G of RANGE switch S2 connects the input

R 5 2 7 . The grid of V502B is clamped to the

t e r m i n a l to 100:1 attenuator R543, R533, R535,

c o m m o n input terminal by CR503. Negative

R534, C523, C524, and C525. The attenuator

feedback from the plate of V502B is applied to the

output is applied to the amplifier by S2H. The

cathode of V502A by R532. The output of V502B

attenuator output at 400 Hz is adjusted by 500

is coupled through C516. Full-wave rectification

VAC ATTENUATOR R534. At 10 kHz the

is provided by CR501 and CR502 to develop a

output is adjusted by 500 VAC HF TRIM C523.

voltage across R539 which is applied a negative

The attenuator output is 5 volts ac when 100 volts

feedback to the grid of V501. The amount of

ac is applied to the input terminals, Operation of

feedback is selected by level I of RANGE switch

the amplifier is explained in b and d above to

S2. Half-wave rectification by CR501 develops an

obtain a 5-volt dc output from the converter.

output voltage across R536 through R539, C517,

and C518 which is proportional to the average

5-6. Low Voltage Supply

v a l u e of the input ac voltage. The amplifier

(fig. 5-6)

achieves a midband loop gain of approximately 70

a. General. The low voltage supply provides

d e c i b e l s dB with a virtually flat frequency

positive 200 and 150 volts dc to the dc vtvm, and

response from 20 Hz to 10 kHz.

filament voltage for all tubes and the decima,

C . Converter Operation, .5- Volt Ac Range.

point indicator lamps.

Levels G and H of RANGE switch S2 connect the

b. Input Power, T h e p o w e r t r a n s f o r m e r

input voltage directly to the amplifier. Operation

primary contains two windings. These windings

o f the amplifier is explained in b a b o v e . A

are parallel connected for 115-volt operation, and

negative feedback network, consisting of C519,

series connected for 230. volt operation. Terminals

R540, and C520, selected by S21 allows sufficient

on top of the transformer are used to connect the

overall gain so that a 5-volt dc output will be

primary windings. The primary circuit is con-

obtained when 0.5 volt ac is applied to the input

5-10