Voltage-to-Current Converter
Calculate output current and sense resistor power for V-to-I converter circuits.
Component Values
Results
Iout = Vin / Rsense
Voltage-to-Current Converters Explained
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.
V-to-I Conversion
Iout = Vin / RsensePower in Rsense
P = Iout² × RsenseKey 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
Practical Examples
Industrial 4–20 mA loop from a 0–5 V sensor output. The 4 mA offset is used to detect broken wire (zero current = fault).
Span R = 5 V / 16 mA = 312.5 Ω; offset circuit adds 4 mA bias
Drive a high-power LED at exactly 350 mA regardless of Vf variation using an op-amp + transistor V-to-I converter.
R_sense = Vref / I = 1.25 V / 0.35 A = 3.57 Ω (use 3.6 Ω)
Did you know? The 4–20 mA current loop standard (IEC 60381-1) was chosen because 4 mA baseline (not 0 mA) allows detection of broken wire faults — a 0 mA reading means "fault," not "zero value." Industrial sensors using 4–20 mA can drive signals over cable runs exceeding 1 km without voltage drop errors.