Voltage-controlled current source

The source, based on a Type TL084 quadruple opamp, is intended to convert an input signal of 0-5 V into a current of 0-20 mA. This type of circuit is used, for instance, to transfer measurands (quantities being measured) over long leads. Since the resistance of the leads is part of the current loop, it is of no consequence and can not affect the measurement.

Opamp ICia is a straightforward input amplifier. Opamp IC lb adjusts the direct voltage component of the amplified input signal: the operating point may be shifted – with P2. It is, for example, possible to arrange an output current of 4 mA for an input voltage of 0 V. The output current range is then 4-20 mA.

Opamp IC1 and T1 convert the output of IC1b to a signal of 15 V. This makes it possible for ICid and T2 to function as a voltage-to-current converter. The output current flows to earth via load resistance RL.Varying the values of R2 and P1 allows the amplification to be altered as required. The circuit may also be used as a temperature-to-current converter by making the potential divider at the input consist of a fixed resistance and one with a negative temperature coefficient.

When the requirements are exacting, the two zener diodes should be temperature compensated.

Voltage-controlled current source Schematic diagram

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