Signal Converter
Convert between mA, V and psi process signal ranges.
This tool converts a process value between the standard instrumentation signal spans — 4-20 mA, 0-20 mA, 1-5 V, 0-5 V, 0-10 V and the pneumatic 3-15 psi. Pick source and target span, type the value, and get the equivalent plus the percent of span it represents: 12 mA on a 4-20 loop is 50 %, i.e. 3 V on 1-5 V.
How it works
Every standard span is a linear encoding of 0–100 % of a process variable, so conversion is two steps: normalize the input to percent of its span, then project that percent onto the output span. Spans with a live zero (4-20 mA, 1-5 V, 3-15 psi) keep their fault-detection property through the conversion; spans that start at true zero (0-20 mA, 0-10 V) do not distinguish “measuring zero” from “dead channel”.
The classic hardware bridges mirror this math: a 250 Ω resistor converts 4-20 mA to 1-5 V, and an I/P transducer converts 4-20 mA to 3-15 psi — all straight lines through the same percent scale.
Worked example
→ 1–5 V: 1 + 0.5·4 = 3 V
→ 3–15 psi: 3 + 0.5·12 = 9 psi · → 0–10 V: 5 V
Parameters
| Parameter | Value | Notes |
|---|---|---|
| Current spans | 4–20 mA · 0–20 mA | 4-20 has live zero |
| Voltage spans | 1–5 V · 0–5 V · 0–10 V | 1-5 V = 4-20 mA × 250 Ω |
| Pneumatic | 3–15 psi | live zero at 3 psi |
| Method | percent-of-span mapping | linear both ways |
C implementation
/* Convert between two linear instrument spans via percent of span */
float span_convert(float v, float in_lo, float in_hi,
float out_lo, float out_hi)
{
float pct = (v - in_lo) / (in_hi - in_lo);
return out_lo + pct * (out_hi - out_lo);
}
/* span_convert(12, 4, 20, 1, 5) -> 3.0f (12 mA -> 3 V) */
/* span_convert(12, 4, 20, 3, 15) -> 9.0f (12 mA -> 9 psi) */FAQ
▸Why do 4-20 mA and 1-5 V convert with just a 250 Ω resistor?
Ohm's law: 4 mA × 250 Ω = 1 V and 20 mA × 250 Ω = 5 V. A single precision resistor across the input turns the standard current span into the standard voltage span, which is why 250 Ω sense resistors are everywhere in analog input hardware.
▸What is 3-15 psi and why does it map onto 4-20 mA?
It is the classic pneumatic signal standard that predates electronic loops: 3 psi = 0 %, 15 psi = 100 %. Both conventions use a live zero (3 psi / 4 mA), so they map one-to-one through percent of span — this converter goes through exactly that intermediate step.
▸How does the conversion work mathematically?
Both spans are linear, so the value is first normalized to percent of span — (value − low)/(high − low) — and then de-normalized into the target span. 12 mA in 4-20 mA is 50 %, which is 3 V in 1-5 V or 9 psi in 3-15 psi.
▸What does the out-of-span warning mean?
Your value lies outside the source span (e.g. 3.6 mA in a 4-20 loop). The tool still converts by linear extrapolation, but on real hardware such readings usually mean a fault condition rather than a measurement.
▸Is my data uploaded?
No. All conversion happens locally in your browser.