An ammeter measures electric current. This page illustrates a four-terminal shunt: current flows through a low resistance, while separate sensing leads measure the resulting voltage. The calculator shows how shunt resistance and an ideal analog-to-digital converter determine current per count and input-range usage. It compares ideal resolution, not the accuracy or electrical safety rating of a complete instrument.
Ampere Meter Interactive Calculator
Vary the shunt rating, ADC range, bit depth, and test current to see shunt-meter resolution and ADC counts.
Equation Used
The shunt converts current into a small voltage by Ohm's law. The ADC step size is its input range divided by 2^N, so current resolution is one voltage count divided by the shunt resistance. The count tiles compare the selected test current, half the entered shunt current rating and the full entered rating. They can exceed the ADC’s maximum output code because the tiles do not clip the ideal arithmetic.
- Ideal precision shunt with no temperature drift
- Unipolar ADC range from 0 to Vadc
- The middle comparison is 50% of the entered shunt current rating; it is not a recommended operating point
- Lead resistance and ADC offset are ignored
Count tiles show the ideal, unbounded calculation. The animation separately shows the bounded integer ADC code and flags saturation. The nominal comparison is 50% of the entered current rating, which equals 5 A only for a 10 A shunt.
From current to voltage to a number
The shunt carries the measured current and develops a voltage V = IR. A four-terminal connection separates the main current terminals from voltage-sense terminals. This reduces the contribution of current-carrying lead and connection resistance to the measured shunt voltage.
The animation shows an ideal unipolar ADC reading that voltage. Its displayed code is an integer limited to the available code range. The separate numerical tiles report ideal counts before that clipping. Current above the selected shunt rating and voltage above the selected ADC range are flagged separately.
Signal path
- Current path: passes through the shunt and load.
- Kelvin sense leads: connect to the shunt’s sensing points.
- ADC: converts the sensed voltage into a code.
- Display: represents the ideal current measurement.
Real front ends also need suitable common-mode range, protection, insulation, reference accuracy and noise performance. A small voltage across a shunt does not make its terminals safe: the entire shunt may sit at a hazardous voltage relative to ground.
Different current-measurement methods
Shunts are used in DC and AC measurement systems when their resistance, thermal behavior, bandwidth and signal conditioning suit the task. A current transformer senses changing current magnetically. Hall-effect and other magnetic sensors can measure DC when designed for it.
Clamp form alone does not identify the sensing principle or establish AC/DC capability. Likewise, not every analog ammeter is a moving-coil instrument, and not every digital ammeter measures true RMS. Check the instrument’s specified waveform, bandwidth, range and measurement category for its intended use.
This calculator models a nonnegative test current through a shunt. It does not implement RMS sampling, current-transformer behavior, protection-relay settings or battery charging control.
Ideal current per ADC count
Let Ifs be entered shunt rating in amperes, Vfs its full-scale drop in millivolts, N the ADC bit count, and Vadc its unipolar input span in millivolts. Shunt resistance in mΩ is R = Vfs/Ifs. The number of ideal intervals is 2ᴺ and voltage per count is q = Vadc/2ᴺ in mV.
LSB Current = 1000q/R in mA/count. For test current I in amperes, ideal counts = IR/q. The other count tiles use I = Ifs/2 and I = Ifs respectively. These ideal values are not limited to the ADC output range.
The animation takes the integer floor of ideal test counts and clips it between 0 and 2ᴺ − 1. At or above the entered ADC span it reports saturation. The shunt-current rating is a separate limit: either limit can be reached first.
More bits make the ideal count interval smaller, but do not by themselves improve real accuracy. Shunt tolerance and temperature change, ADC noise, reference and gain errors, offset, common-mode behavior and connections all matter. The calculator excludes those errors and contains no amplifier gain stage.
Shunt heating follows I²R in a separate electrical calculation. Temperature rise and allowable continuous or pulse current need the actual shunt’s construction and ratings; there is no universal overload or thermal margin built into these outputs.
Worked example: a 10 A, 75 mV shunt
Enter Shunt Rating 10 A, Full Scale Drop 75 mV, ADC Bits 16, ADC Range 100 mV and Test Current 5 A. Resistance is 75/10 = 7.5 mΩ. One ideal voltage count is 100/65,536 = 0.001525879 mV, or 1.525879 µV.
LSB Current is 0.2034505 mA/count. At 5 A the shunt drop is 37.5 mV and ideal test counts are exactly 24,576. The 50% rating comparison is also 24,576 counts, while the full 10 A rating corresponds to 49,152 counts.
At Test Current 0.5 A, the voltage is 3.75 mV and ideal counts are 2457.6; the animated integer ADC code is 2457. This fractional ideal count is not a claim of fractional-code hardware or measurement accuracy.
At 10 A, shunt dissipation would be 10² × 0.0075 = 0.75 W. The calculation does not predict its temperature rise or establish that a chosen component may dissipate that power continuously. Reducing ADC Range to 50 mV with Test Current 10 A gives 98,304 ideal counts; the animated code clips to 65,535 and flags ADC overrange.
Choosing a sensing principle
| Method | Measurement principle | Key consideration |
|---|---|---|
| Shunt | Measures resistive voltage drop. | Insertion loss, heating, connections and common-mode voltage. |
| Conventional current transformer | Uses changing magnetic flux. | AC operation, burden, saturation and secondary-circuit requirements. |
| Hall-effect sensor | Senses magnetic field. | DC capability where specified, offset, drift and bandwidth. |
| Fluxgate or compensated transducer | Uses an active magnetic measurement scheme. | Requires its own supply, interface and accuracy specifications. |
No universal price, accuracy or current ceiling follows from the sensing type. For an actual circuit, choose the specified measurement range, isolation and environmental performance.
Ammeter calculation questions
No. It is the ideal current increment corresponding to one ADC voltage count. Real accuracy depends on the shunt, front end, ADC, reference, noise, temperature and calibration.
The tile reports voltage divided by ideal step size without clipping. The animation separately limits its integer code to 0 through 2ᴺ − 1 and flags saturation.
That comparison always uses half the entered shunt current rating. It is independent of the test current and does not recommend operating at that point.
No. The differential drop may be small while both terminals are at a hazardous common-mode voltage. Electrical safety and insulation are not modeled here.
It relies on changing flux and does not provide an ordinary proportional steady-DC output. DC measurement needs a suitable shunt or a sensor specified for DC.
Do not open the secondary of a conventional current-output CT while primary current is flowing; dangerous voltage can develop. Follow the exact equipment’s safe isolation and servicing procedures through qualified personnel.
No. Its bandwidth, crest-factor, range and input limits still apply. The calculator does not sample a waveform or calculate RMS current.
No. Continuous power, pulse energy and overload capability must come from the actual component data. ADC range and current-per-count do not establish those ratings.
References and further reading
Texas Instruments: current-shunt measurement considerations discusses signal level, accuracy and Kelvin connections. TI: shunt selection and layout addresses real component behavior beyond ideal resolution. Schneider Electric: current-transformer secondary hazards explains why conventional CT secondary circuits require specific handling.
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