4-23
CONTROLS
Figure 4-18 is a panel view of the model 250DE bridge switches, dials, controls, and connections.
Refer to the figure as we briefly discuss some of the switches and dials below.
The FUNCTION switch selects the type of bridge circuit that will measure resistance,
capacitance, or inductance.
The RANGE switch selects the multiplier for each function.
L-R-C decade dials are a DEKASTAT decade resistor that is the main balancing element of the
bridge. The setting of the dials after the bridge is balanced indicates the value of inductance,
resistance, or capacitance.
The D-Q dial is used to balance the phase of the capacitance or inductance of the bridge. The
setting of the dial after the bridge is balanced indicates the value of dissipation factor (D) or
storage factor (Q).
The GEN-DET switch selects bridge generator and detector connections, ac or dc, internal or
external generator. The switch also connects the internal batteries to the battery test circuit.
The DET GAIN control adjusts the sensitivity of the ac-dc detector and turns on power to the
generator.
CONNECTIONS
L, R, and C terminals 1, 2, and 3 are used to connect unknown resistors, inductors, and capacitors to
the bridge. Resistors and inductors are connected between terminals 1 and 2, and capacitors are connected
between terminals 2 and 3. EXT BIAS terminals are normally connected with a shorting lug. They allow
insertion of a dc voltage or current to bias capacitors or inductors. EXT DET connector is a BNC coaxial
socket that allows an external detector to be used with the instrument. It is connected to the bridge at ALL
TIMES.
EXT D-Q terminals are normally connected with a shorting lug. They allow an external rheostat to
extend the range of the D-Q dial. EXT GEN terminals provide a connection to the bridge for an external
generator. When the GEN-DET switch is in the AC EXT GEN position, the terminals connect an isolation
transformer so that a grounded external generator can be used. When the GEN-DET switch is in the DC
EXT GEN position, the terminals are connected directly to the bridge.
BATTERY
The model 250DE bridge has a battery supply consisting of four 1.5 V dc batteries with an expected
life of 500 hours. The battery power supply should be checked before each day’s operation. Turn DET
GAIN control to 1 and set GEN-DET switch to BATT. TEST (battery test). If the meter deflects beyond
the BAT OK mark, the battery is good.
RESISTANCE MEASUREMENTS
Resistance is usually measured with direct current for maximum accuracy. The model 250DE bridge
can be used to measure resistance with alternating current, but external reactance compensation is usually
required. On high-resistance ranges, care should be taken to avoid leakage across a resistor under test.
Insulation with a resistance of 109 ohms, which is adequate for most purposes, will cause a measurement
