Universal Bridge

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HERE has been a marked increase in recent years in the use of bridge methods for the measurement of impedances of all kinds. This increase has come from the greater number of quantities to be measured, from the greater range of their numerical values, and from the greater range of frequency at which the measurements must be made. The values of the magnitude range and frequency range over which measurements are desired are impressive; resistance from a microhm to a megamegolim, inductance from a mil -limicrohenry to a kilohenry, capacitance from a millimicromicrofarad to a millifarad ; not all perhaps, but some of these at frequencies ranging from a cycle per second to a hectomegacycle per second.

A bridge is merely an instrument with which two impedances, known and unknown, may be compared. The known standard is not inherent in the bridge itself. It must be separately provided. The number and variety of these standards is large; for a single standard can rarely cover a range of a thousand to one in either direction from its own value, and that only at low frequency and low accuracy. As these rise the range of ratio drops, approaching unity for an accuracy of .01% and at a frequency of 100 kilocycles per second.

An obvious way by which the number of standards may be decreased and the accuracy of measurement increased is hy the use of the various bridges in which unlike impedances may be compared, as for example, resistance and self and mutual inductance in terms of capacitance, capacitance in terms of resistance and frequency, frequency in terms of inductance and capacitance.

The Type 293-A Universal Bridge has been designed with considerations of this sort in mind. It provides the essentials of a bridge, variable ratio arms and a standard resistance which may be used as an added resistance to satisfy one of the conditions of balance, in such a form that all types of bridges may be constructed. These three resistances are mounted on the panel of the bridge as shown in Figure 1, with their terminals and those of an added

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form the four arms of a simple Wheat-stone bridge when the ten links indicated by the dotted lines are plugged in. For this four-impedance network the arrangement of the connecting links is symmetrical.

The three kinds of bridge networks shown in Figure 3 cover practically all of the bridges used for the comparison of like and unlike impedances. The names of these bridges, together with the kinds of impedances compared on them, are given in the table. All of these bridges may be set up on the

Figure 1. Panel view of a Type 293-A Universal Bridge. Dotted lines were drawn in after the photograph was taken impedance symmetrically disposed. These four pairs of terminals may be connected to each other and to the input and output terminals placed at the upper corners of the panel through six pairs of intermediate binding posts, the actual connections being made by links which plug into the various jack-top binding posts.

The principle on which this terminal board is arranged is shown diagram-matically in Figure 2, in which the full lines represent the permanent connections. The three variable resistances together with the added impedance r RESISTANCE j ! UNDER TEST |

TABLE I

Common Bridges Showing the Type of Network and the Kind of Known and Unknown Elements r RESISTANCE j ! UNDER TEST |

i-io-too-iooo n

Figure 2. Schematic diagram for the measurement of resistance by the Wheatstone Bridge i-io-too-iooo n

TABLE I

Common Bridges Showing the Type of Network and the Kind of Known and Unknown Elements

Rrid^e

Network1

IJ3

S*

Impedance

a

R

R

L

L&R

C

C&R

Grover

a

C

C&R

c

C&R

Schering

a

c

C&R

Maxwell

a

L

C&R

Owen

a

i

C&R

Hay

a

L

C, R&f

Resonance

a

L

C&f

f

L&C

Wien

a

C

R&f

L

R&L

/

C&R

Anderson

h

L

C&R

Anderson-lJay

b

C

C&R

Campbell

c

L

M&R

Carey Foster

c

C

M&R

M

C&R

1Letters refer to the networks of Figure 3.

2£7 represents the unknown quantity that can be measured when the corresponding quantities in the S column are known. L, M. R, C, and f represent respectively self-inductance, mutual inductance, resistance, capacitance, and frequency.

Figure 2. Schematic diagram for the measurement of resistance by the Wheatstone Bridge

1Letters refer to the networks of Figure 3.

2£7 represents the unknown quantity that can be measured when the corresponding quantities in the S column are known. L, M. R, C, and f represent respectively self-inductance, mutual inductance, resistance, capacitance, and frequency.

FlGUKE 3. Practically all bridge networks are elaborations of tbese tliree basic circuits. The letters a, b and c refer to the second column of Table I

Type 293-A Universal Bridge by suitably placing the known and unknown impedances and the interconnecting links. The determination of their positions is facilitated by the procedure illustrated in Figure 4.

Owen's bridge, in which the inductance and resistance of an unknown inductor are measured in terms of two added capacitances and the variable resistances, has been chosen as an example.

The schema tic diagram of the bridge is made and the four arms lettered cyclically A-B-S-U in such a manner that the three variable resistors are used to best advantage. The position of the connecting links may then be drawn on the terminal board diagram, together with the places for connecting the external impedances. Such figures for all the bridges men tioned in Table I are given in the instruction book furnished with each Type 293-A Universal Bridge. Blank diagrams for other bridges are also provided.

The resistors used in this bridge are the new Type 510 Decade-Resistance Units, having the switch contacts below the panel. The three variable resistors are shielded from each other and the whole bridge is placed in a copper-lined cabinet. Double pairs of input and output terminals are provided so that shielded transformers may be used.

The accessories required for the operation of the bridge include a power supply, null detector, and standards of impedance. Suitable instruments for these purposes are described in Catalog F. The Type 508-A Oscillator and the Type 514-A Amplifier

CIRCUIT NO. 5 OWEN

Figure 4

The Type 293-A Universal Bridge set up as an Owen Bridge used in conjunction with head telephones or the Type 488-DM Alternating-Current Meter are particularly recommended as power supply and null detector. The Type 293-P1 and Type 293-P2 Transformers are shielded transformers for isolating electrostatically the power supply from the bridge. The

Type 293-P3 Slide-Wire Resistor provides a continuously variable resistance which bridges the lowest resistance steps in the bridge. It is particularly useful in the measurement of small reactances.

The price of the Type 293-A Universal Bridge is $140.00.

A three-element camera oscillograph with the self-developing feature

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