Inductance Bridges

AN INDUCTANCE BRIDGE

An inductance bridge is a device used to measure the inductance of a component. 

Inductance is the property of a circuit that opposes changes in current. It is caused by the magnetic field that is generated when current flows through a conductor.

Inductance bridges work by comparing the inductance of the component under test to that of a standard inductor. The two inductors are connected in opposite arms of a bridge circuit.

An AC signal voltage is applied to the input of the bridge.

The bridge is adjusted to give zero output and a reading is made.

Inductance bridges can be used to measure a wide range of inductance values, from microhenries to henries.

BASIC SLIDE-WIRE INDUCTANCE BRIDGE 

Fig. 1 shows the most rudimentary inductance bridge. In this arrangement, the variable balancing resistor is a single strand of resistance wire (the slide wire) tautly stretched between points A and B (or wound around a form having a circular cross section) and provided with a sliding contact (the slider). The resistance is directly proportional to its length. As the slider is moved along the wire, it divides the latter into two parts: D1; the length from point A to the slider, which has a resistance of R1; and D2, the length from point B to the slider, which has aresistance of R2. 

The bridge is composed of unknown inductor Lx (connected to terminals X-X), a standard inductor Ls, and the two sections R1, R2 of the slide-wire variable resistor, which provide the ratio arms. 

The bridge is balanced by moving the slider along the wire until the detector response is minimum. At this null point, 

Lx/Ls= R1/R2, 

and from this relationship the unknown inductance may be determined in terms of the standard:

 Lx = Ls( R1/R2) 

The total resistance of the slide wire is unimportant to the calculation. So also are the actual resistance values on each side of the slider. In fact, distances D1 and D2 may be measured in inches, centimeters, or arbitrary linear units and used in the calculation in place of resistances: 

Lx = Ls( D1/D2) 

For this reason, the basic slide-wire bridge is convenient for emergency measurements of inductance, since it requires only a standard inductor and a length of bare resistance wire stretched along a meter stick or yardstick, in addition to a generator (e.g., an audio oscillator) and a detector (e.g., AC voltmeter, oscilloscope, or high-resistance headphones).

THE NEED FOR Q BALANCE 

A complete null is impossible with a simple slide-wire bridge unless the losses in the unknown and standard inductors match. The reason for this is the complex nature of the practical inductor as an impedance network (inductance and resistance in combination, the resistive component representing the losses).

It is convenient to express the relationship between losses (resistive component) and inductance (reactive component) as the figure of merit or Q of the inductor. 

A complete null is obtained only when the Q of the unknown inductor equals that of the standard inductor. When the Q’s are unequal, as they almost always are in practice, enough resistance must be added in series (sometimes in parallel) with the lower-loss inductor to reduce its Q to that of the higher-loss inductor. Addition of resistance to one or the other of the inductors will complete the balance of the bridge.