Combination Bridges [BRIDGES and other NULL DEVICES (1967)]

For reasons of economy, convenience, and versatility, many users prefer bridges which measure more than one quantity. Because of this demand, a number of bridges are in current use which measure resistance and capacitance ; or resistance, capacitance, and inductance.

This Section describes representative combination bridges which perform these joint functions, and calls attention to the special features of some of these instruments.

5.1 UNIVERSAL BRIDGE

The universal bridge has been mentioned earlier (Sections 3.4 and 4.4). This instrument was the forerunner of all combination bridges, having appeared some time ago and offering measurements of resistance, capacitance, and inductance.

Fig. 5- 1 shows the circuit of the universal bridge (also called general-purpose bridge ). This arrangement is seen to be a skeleton circuit which supplies the ratio arms R 1 and Ra, the latter collectively referring to resistors R2 and R6, balancing arm R7, unknown terminals X-X, standard terminals S-S, detector terminals DET, generator terminals GEN, and changeover switch S2. When generator, detector, unknown, and standard are properly connected to this skeleton, a bridge of the desired sort results. Typical uses are described below.

Resistance:

1. Connect the unknown resistance (Rx) to terminals X-X.

2. Connect a jumper between terminals S-S.

Fig. 5-1 . Universal bridge,

3. Throw switch S2 to position A; this connects R7 as the lower right arm of the bridge, where it acts as the resistance standard.

4. For d-c resistance, connect a battery to the GEN terminals, and a center-zero d-c meter to the DET terminals.

5. For a-c resistance, connect an audio oscillator to the GEN terminals, and an oscilloscope, a-c vtvm, or high-resistance headphones to the DET terminals.

6. Balance the bridge by adjustment of R1, R7, and S1. At null : 5-1 where,

Ra is the resistor (R2 to R6 ) selected by SI. Capacitance

1. Connect the unknown capacitance (Cx ) to terminals X-X.

2. Connect a standard capacitor (Cs) to terminals S-S.

3. Set switch S2 to position A. This places R 7 in series with the standard capacitor, as the power-factor balance.

4. Connect an audio oscillator to the GEN terminals.

5. Connect an oscilloscope, a-c vtvm, or high-resistance head

phones to the DET terminals.

6. Balance the bridge by means of R1 and S1 .

7. Improve the null by adjusting R7, readjusting R1 if necessary.

If no improvement results and null is imperfect, throw switch S2 to position B, thus placing R 7 in series with the unknown capacitor, and readjust R7. At null : where,

Cx = Cs (Ra/R1) 5-2 Ra is the resistor (R2 to R6 ) selected by SI.

Inductance

1. Connect the unknown inductor (L,J to terminals X-X.

2. Connect a standard inductor (L.) to terminals S-S.

3. Set switch S2 to position A. This places R 7 in series with the standard inductor, as the Q balance.

4. Connect an audio oscillator to the GEN terminals.

5. Connect an oscilloscope, a-c vtvm, or high-resistance head

phones to the DET terminals.

6. Balance the bridge by means of R1 and Sl.

7. Improve the null by adjusting R 7. If no improvement results and null is imperfect, throw switch S2 to position B, thus placing R7 in series with the unknown inductor, and readjust R7.

At null : 5-3 where, Ra is the resistor (R2 to R6) selected by S 1.

The plug-in nature of the universal bridge gives this instrument its great flexibility. Many circuits can be set up with it. The elimination of selector switches for changing the values of standards eliminates much of the trouble resulting from the stray reactance introduced by such switches. Perhaps the single disadvantage of this bridge is its lack of compactness : a reasonable supply of standards must be stored.

5.2 RESISTANCE-CAPACITANCE BRIDGE

Among radio-tv servicemen and electronics maintenance technicians, a bridge for resistance and capacitance measurements is popular. Figs. 5-2 and 5-3 show an inexpensive bridge of this type (Knight KG-670 ) which is available either as a factory-built instrument or in kit form. This bridge covers the resistance range from 100 ohms to 5 megohms in two steps : 100-50,000 ohms, and 10,000 ohms-5 megohms ; and it covers the capacitance range from 10 pf to 1000 mfd in four steps : 10 pf-0.005 mfd, 0.00 1-0.5 mfd, 0. 1-50 mfd, and 20-1 000 mfd. Additionally, it checks the leakage of capacitors at any of the following d-c voltages : 50, 150, 250, 350, or 450. Fig. 5-3 shows the bridge circuit. This .is a potentiometer-type slide-wire circuit in which the balance potentiometer (R4) supplies the two ratio arms of the bridge. The third arm of the bridge contains the unknown; and the fourth arm contains a standard capacitor (C6, C7, or C8) for capacitance measurements, or a standard resistor (R7 or R8 ) for resistance measurements. The power-factor balance rheostat (R6) is always in series with the standard capacitor, and its dial reads directly in power factor from 0 to 50 percent. The bridge signal is a 60-Hz voltage delivered by a special secondary winding on the power transformer, T 1. The detector is a 6E5 magic-eye tube (V2). D-c voltage for the leakage test and for the magic-eye tube is supplied by voltage-doubler rectifier tube VI, and the step-type voltage divider (resistors R12-R17 and selector switch S2C). During the leakage test, the eye tube acts as a simple current indicator.

Courtesy Allied Radio Corp.

Fig. 5-2. Resistance-capacitance bridge.

Fig. 5-3

An unknown capacitor is connected to terminals 11 and 12. An unknown resistor is connected to 12 and 13 (12 is the common terminal for resistance and capacitance and is the positive terminal for the leakage test ). 11 is negative. Switch SI (lower left corner of the front panel in Fig. 5-2 ) selects either the resistance-capacitance or leakage function of the bridge. The three-section selector switch, S2a-S2b-S2c, in the upper right corner in Fig. 5-2 selects the various resistance, capacitance, and leakage-voltage ranges.

5.3 COMBINATION INDUCTANCE BRIDGE

Fig. 5-4 shows the circuit of an inductance bridge which is either a Maxwell or a Hay bridge, depending on the setting of a dpdt changeover switch, S3. An inductance range of 10 uh to 100 hy is covered in four steps : 10 uh-1 mh, 1-100 mh, 0. 1-10 hy, and 1-100 hy. Only two standard capacitors are required for the four inductance ranges, C1 being used for the two lower ranges, and C2 for the two higher ones. The resistance in the No. 2 arm of the bridge is switched along with the standard capacitance. However, only three resistors ( R1, R2, R3 ) are required, R2 being used on ranges B and C. When switch S3 is set to its MAX position, the Q-balance rheostat (R5) is connected in parallel with the standard capacitor (C 1 or C2 ) , and the circuit is that of the Maxwell bridge (for comparison, see Fig. 4- 10, Section 4) . This arrangement is suitable for Q's higher than 10. When switch S3 is set to its HAY position, the Q-balance rheostat is in series with the standard capacitor, and the circuit is that of the Hay bridge (for comparison, see Fig. 4-9, Section 4) . This arrangement is suitable for Q's up to 10.

Fig. 5-4. Combination inductance bridge.

5.4 IMPEDANCE BRIDGE

An impedance bridge is a general-purpose instrument for measuring capacitance, inductance, and a-c and d-c resistance. This all-around instrument accomplishes its several purposes by means of a switching arrangement which, by properly rearranging the components, sets up the resistance-, capacitance-, and inductance measuring circuits.

Fig. 5-5. Gener.1 Radio Type 1650-A impedance bridge.

(A) Resistance.

(B) Series capacitance.

(C) Parallel capacitance.

(D) Series inductance. (E) Parallel inductance. (F) O-c bridge.

(G) A-c bridge.

Fig_ 5-6. Setups of the impedance bridge.

Fig. 5-5 shows one such bridge (General Radio Type 1650-A) which provides the following ranges of measurement selected by the two switches in the upper left and right corners of the front panel : capacitance, 1 pf-1100 mfd (seven steps ); inductance, 1 UH-1100 hy (seven steps ); and resistance, 1 milliohm-11 megohms (eight steps ). The lower right (CRL) dial reads direct capacitance, resistance, and inductance, and its reading is multiplied by the upper-right selector switch (CRL MULTIPLIER). The lower left (DQ) dial reads the dissipation factor (D) of capacitors or Q of inductors, de

pending on the setting of the upper-left selector switch (CRL SELECTOR). In capacitor tests, dissipation factor at 1 kHz may be measured between 0.00 1 and I when the resistance-balance rheostat is switched in series with the standard capacitor, and be

tween I and 50 when this rheostat is in parallel with the standard capacitor. In inductor tests, storage factor (Q at 1 kHz) may be measured between 0.02 and 10 when the resistance-balance rheostat is switched in parallel with the standard capacitor (Maxwell bridge ), and between 1 and 1000 when this rheostat is in series with the standard capacitor (Hay bridge ).

Fig. 5·7. Portable impedance bridge with automatic DO balance.

Figs. 5-6A to E show the five bridge circuits which are obtained through the selector switch. Figs. 5-6F and 5-6G show how the bridge is switched from the d-c Wheatstone type (Fig. 5-6F) for d-c resistance measurements to the a-c type (Fig. 5-6G) for capacitance, inductance, and a-c resistance measurements. In the d-c bridge, an internal battery (B) acts as the generator, and a center-zero d-c meter as the detector. An external battery may be connected to the EXT GEN terminals for higher voltage than the internal six volts.

(Normally, the EXT GEN and BIAS terminals are short-circuited with jumpers. ) In the a-c bridge (Fig. 5-6G), an internal, transistorized, I-kHz oscillator acts as the generator, and a transistorized amplifier and rectifier (DET) driving the d-c meter acts as the

Courtesy Electro Scientific Industries

Fig. 5-8. Wide-range, high-accuracy portable impedance bridge.

detector. (The amplifier may be peaked at 1 kHz or operated broad. ) An external generator may be connected to the EXT GEN terminals for a different frequency, and a d-c supply may be connected to the BIAS terminals for d-c polarization of a capacitor or inductor.

(Normally, the EXT GEN terminals are left open, and the BIAS terminals short-circuited with a jumper. ) An external a-c detector (null indicator) may be connected to the DET OUTPUT terminals.

In the a-c bridge, internal battery B powers both the oscillator and amplifier.

The circuits shown in Figs. 5-6A to E are those obtained by selector switching in all impedance bridges. Individual instruments differ in ranges of measurement, type of internal generator and detector, type of CRL and DO readout, and whether an internal battery or external power line is used.

Fig. 5-7 shows an impedance bridge (Hewlett-Packard Model 4260A) which, in addition to the usual circuits (Wheatstone, capacitance comparison, Maxwell, Hay), embodies the following features :

1. A counter-type digital CRL readout (upper right indicator ) with automatic placing of the decimal point by the range switch (center panel ) and accordingly with elimination of calculations.

2. Selectable, all-electronic, automatic adjustment of the DQ balance in proper step with the manually operated CRL adjustment for rapid initial balancing for either C or L.

Either an internal or external a-c and d-c generator may be employed, as well as an internal or external detector. This bridge provides the following ranges : resistance, 10 mill iohms-10 megohms (seven steps ); capacitance, 1 pf-1000 mfd (seven steps ); inductance, 1uH 1000 hy (seven steps ); dissipation factor, 0.00 1-0. 12 (series capacitance ) and 0.05-50 (parallel capacitance ); and Q, 0.02-20 (series inductance ) and 8-1000 (parallel inductance ).

The impedance bridge shown in Fig. 5-8 (Electro Scientific Model 250DE) provides the following ranges : resistance, 0-12 meg (eight steps ); capacitance, 0-1200 mfd (seven steps ); inductance, 0-1200 hy (seven steps ); dissipation factor, 0-1.05 (two steps ); and Q, 0-10.5 (series inductance ) and 10-1000 (parallel inductance ). The accuracy of measurement is superior to that of most portable impedance bridges: resistance, 0.1 %; capacitance, 0.2%; and inductance, 0.3%. A solid-state ac-dc generator and detector are employed internally. The main-balance control is a special arrangement of fixed-decade steps and variable resistance, adjusted by means of the concentric CRL dial system in the lower right comer of the front panel.