Voltage-to-Current Converter

A voltage-to-current (V-to-I) converter produces an output current directly proportional to an input voltage: Iout = Vin / Rsense. The sense resistor sets the conversion gain. This circuit is essential anywhere you need to control current from a voltage source.

The Howland current pump is the most common precision V-to-I converter design. It uses a single op-amp with four matched resistors to create a true floating current source — one that can source and sink current into any load impedance. When all four resistors are equal, Iout = Vin / Rsense.

Industrial process control uses the 4-20 mA standard: 4 mA represents zero and 20 mA represents full scale. This current loop is immune to cable resistance and common-mode noise, making it reliable over hundreds of meters of wiring. A V-to-I converter at the transmitter and a sense resistor at the receiver are all that is needed.

Key Points

• Iout = Vin / Rsense — independent of load impedance
• Sense resistor must handle power P = Iout² × Rsense
• 4-20 mA: industrial standard for current-loop signal transmission
• Howland pump: floating current source (bidirectional)

Applications

• 4-20 mA industrial process control transmitters
• Constant-current LED drivers
• Electrochemical measurement (potentiostat)
• Motor current control circuits