Marconi Instruments TF868/1 Universal Bridge


  • Range:
    • L - 1H to 100 Henrys
    • C - 1 pF to 100 F
    • R - 0.1 Ohms to 100 Megohms
  • Power: 110/240V AC 50-60Hz
  • Weight: c.10Kg

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Another superbly built piece of kit from the stable of Marconi Instruments - purveyors of fine laboratory grade receivers, transmitters and test gear to Governments everywhere. This device is easily outperformed these days by any cheap DMM - except with a great deal more panache! It is a direct reading instrument which measures values of inductance from 1H to 100 Henrys, capacitance from 1 F to 100 F, and resistance from 0.1 Ohms to 100 Megohms.

The instrument uses the large, single dial for the measurement of inductance, capacitance, and resistance values. Changing the setting of the LCR and RANGE selector switches automatically changes the dial calibration together with the bridge circuit conditions to suit the component under test.

For inductance and capacitance measurements, an RC oscillator-amplifier (switchable to either 1 Kc/s or 10 Kc/s) supplies the voltage. The out of-balance bridge voltage, after amplification and detection, is displayed by the centre-zero meter on the front panel. Measurement of L or C is normally made at 1Kc/s; however, the use of 10 Kc/s is advantageous, for example, when evaluating low-Q inductors.

When measuring resistance, a DC voltage is applied to the bridge. The out-of-balance bridge voltage is interrupted at twice the supply frequency by means of a vibrator (or chopper) before being applied to the amplifier-detector circuits. This system gives a high degree of sensitivity without the use of a high potential across the component.

The 'Phase Balance' control is used to balance out out the resistive component when inductance and capacitance are being measured. This control has two scales which are calibrated in 'Q' and 'Tan' - the appropriate scale being selected by the Q-Tan switch. The Q scale is calibrated from 0.1 to 10, and the Tan scale from 0.001 to 0.1. Q is normally used for inductors and Tan for capacitors.

Q (also known as magnification factor or storage factor) is the ratio of reactance to resistance in a series circuit, or likelihood of conductance in a parallel circuit. Tan (also known as loss tangent or dissipation factor) is the reciprocal of Q.

The test terminals are located on top of the instrument; the flat top provides a useful insulated platform for supporting the component to be tested. This also has printed basic instructions for the device's use

Last updated 4th September 2003