What are the functions of the universal bridge instrument knobs and switches?

Last Update Time: 2019-01-27 14:53:54

The universal bridge is an instrument made by the principle of bridge circuit balance. It is divided into three types: DC bridge, AC bridge and AC and DC bridge. It can be used to measure resistance value, capacitance, inductance, quality factor, loss factor, Impedance, etc., is suitable for measuring components used in the DC or low frequency range. The measurement accuracy is high.

Take the QS18A universal bridge as an example to introduce the use of the universal bridge.

The QS18A universal bridge is a portable AC and DC bridge. It is powered by a 9V laminated battery and a 6 pcs 1.5V battery. The instrument panel is shown in Figure 1.


Figure1 instrument panel diagram


1-terminal, 2-external socket, 3-toggle switch, 4-range selector switch, 5-loss override switch, 6-indicator, 7-bridge housing ground, 8-bridge sensitivity adjustment knob, 9-reading the input: 10-loss trimming knob, 11-loss balance adjustment, 12-measurement selection switch


1. The role of the knob and switch

(1) Terminal.

It is used to connect the device under test. It is best to connect the device under test directly to the terminal during measurement. If it is not possible to connect,the wire should be connected. The wire should be as short as possible. If necessary, the wire resistance should be removed from the measurement result. When measuring a polar component, "1" is connected to the positive electrode, and "2" is connected to the negative electrode.


(2) External socket.

When using an external audio signal source, the audio signal source can be input from the socket when the band switch is placed “outside”; if the DC offset is applied when measuring the capacitance and the inductance, the socket can be input.


(3) Toggle switch.

When using the 1kHz oscillating signal inside the bridge as the power source, this switch is placed in the “1kHz” position; when using the signal power input from the external socket, the switch is placed in the “outside” position.


(4) Range selection switch.

Select the measurement range, and the indication value of each gear is the maximum value of the gear when measuring.



(5) Loss rate switch.

The range of readings used to extend the loss balance. Normally, when measuring the air core inductor, place it in the “Q×1” position. When measuring high Q inductors and general small loss capacitors, place them in the "D × 0.01" position: When measuring the core inductance and loss of large capacitance, place it in the "D × 1" position.


(6) Indicate meter.

It is used to indicate whether the bridge is balanced. When the bridge sensitivity is maximum, the meter pointer points to zero, indicating that the bridge is balanced.


(7) The bridge housing is connected to the GND terminal.


(8) Bridge sensitivity adjustment knob.

Sensitivity adjustment is realized by adjusting the gain of the amplifier. When the bridge is balanced, the bridge sensitivity is reduced, the meter indication is less than the full scale, and the bridge is gradually increased in sensitivity when the bridge reaches equilibrium, thereby improving the accuracy of the bridge measurement.


(9) Reading disk.

Adjusting the two reading plates balances the bridge. The first reading plate is a step switch. Each reading is 1/10 of the range indication. That is 0.1 unit. The second reading is continuous reading. The full scale is the measuring range. 1/10 of the indication, divided into 50 small cells, each square is 0.002 units.


(10) Loss fine adjustment knob.

The loss when adjusting the balance,should be placed at 0" in general.


(11) Loss balance adjustment.

It can indicate the measured inductance, the loss reading of the capacitor, and the reading of the reading disk is multiplied by the indication of the loss rate switch, which is the loss value.


(12) Measurement selection switch.

When measuring resistance less than 10Ω, it should be placed at R10; when measuring resistance greater than 10Ω, it should be placed at R>10; when measuring capacitance, it should be placed at C; when measuring inductance, it should be placed at L; When the meter is not in use, it should be turned "off" to cut off the internal power supply.


This article is from Allicdata Electronics