Op-Amp Voltage-to-Current Converter Calculator
Output current, load compliance and sense resistor for a V-to-I (transconductance) stage, e.g. 4–20 mA.
What it does
A voltage-to-current converter forces a current set by the sense resistor through the load: I = Vin / Rsense, as long as the op-amp can supply enough voltage (compliance).
Where it's used
Driving 4–20 mA industrial current loops and constant-current sources.
How to use it
- Choose whether to find the current or the sense resistor.
- Enter the control voltage and Rsense (or the full-scale current).
- Enter the load resistance and the supply.
- Check that the compliance result is within the output swing.
The formula explained
- I
- Vin / Rsense — load current
- Rsense
- resistor that sets the current
- Compliance
- I × (R_L + Rsense) must stay below the output swing
Worked example
A 20 mA full-scale loop from a 2.5 V control signal into 500 Ω.
Inputs:
- Calculate: Rsense for a full-scale current
- Control voltage Vin (max): 2.5 V
- Full-scale current: 20m A
- Load resistance: 500 Ω
- Op-amp supply / output swing: 24 V
The calculator gives:
- Output current: 20 mA
- Sense resistor: 125 Ω
- Transconductance: 8 mA/V
Tips and common mistakes
- If the load is too large the current stops being accurate — check compliance.
- Use a precision, low-tempco sense resistor.
- Real 4–20 mA transmitters add an offset so that 0 input gives 4 mA.
Questions people ask
How is the sense resistor chosen?
R_sense = V_in(max) / I(full scale). A 2.5 V control signal for 20 mA needs 125 Ω, which dissipates 50 mW at full scale.
What is compliance voltage?
The voltage the op-amp must produce to push the current through the load and sense resistor: 20 mA × (500 Ω + 125 Ω) = 12.5 V in the example. Add the op-amp’s own headroom; if the supply can’t provide that, the current falls short.
Why do current loops use 4–20 mA instead of 0–20 mA?
The “live zero” at 4 mA lets the receiver tell a reading of zero from a broken wire (0 mA), and leaves enough current to power a two-wire transmitter from the loop itself.
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Enter your values to get the result with the formula, substituted values and an explanation of each step.