Current sensing is easy to underestimate. An ADC reads voltage, not current. To measure current, the system must first convert current into something measurable.
The first model is: current must be transformed into voltage, magnetic field, or induced current before electronics can measure it.
Shunt resistor: current -> voltage drop
Hall sensor: current -> magnetic field -> Hall voltage
Current transformer: AC current -> magnetic flux -> induced current
These are not three interchangeable implementations of the same idea. They have different physics and tradeoffs.
How Current Becomes Measurable
Current is charge flow. A microcontroller or ADC does not directly accept “current” as an input variable. The circuit must first make current create another measurable effect.
Common conversions are:
- A known resistor turns current into voltage
- A magnetic sensor observes the field produced by current
- A transformer observes changing magnetic flux from AC current
This is why current sensing is not only about the sensor IC. The conductor, isolation boundary, thermal path, common-mode voltage, bandwidth, and layout all belong to the measurement chain.
Shunt Resistor
A shunt resistor converts current into voltage:
V = I * R
Then:
I = V / R
This is simple, accurate, cheap, and works for DC and AC. The cost is voltage drop and power dissipation:
P = I^2 * R
For small currents, the voltage is tiny and needs low-noise amplification. For large currents, heat becomes a serious design issue.
Shunts can be placed on the low side or high side.
High-Side and Low-Side Sensing
Low-side sensing is easier but shifts the load ground. High-side sensing preserves the load ground but requires an amplifier that can handle high common-mode voltage.
Low side: load -> shunt -> ground
High side: supply -> shunt -> load
Low-side sensing is simple because the measurement is near ground, but the load ground is no longer exactly system ground. That can disturb communication, protection circuits, or analog references.
High-side sensing keeps the load ground clean and can detect short-to-ground conditions more naturally. The cost is a current-sense amplifier that survives and rejects high common-mode voltage.
Four-terminal Kelvin connection is important. Otherwise PCB trace resistance and solder joints become part of the measurement.
Shunt Error Sources
The most common shunt problems are:
- Resistance tolerance
- Temperature coefficient
- Self-heating
- PCB copper resistance
- Solder joint resistance
- Amplifier offset
- Common-mode rejection
- ADC resolution and noise
Because shunt power is I^2 * R, high current heats the resistor. Heating changes resistance unless the shunt has a low temperature coefficient and proper thermal design.
For small currents, the main enemy is often offset. A few microvolts of amplifier offset can become a large current error when the shunt voltage is also tiny.
Hall Current Sensor
Current flowing through a conductor creates a magnetic field. A Hall sensor measures that field.
Current
-> magnetic field
-> Hall element response
-> current estimate
With a magnetic core, the field can be concentrated near the Hall element. This provides isolation between the high-current path and measurement electronics.
Hall sensing can measure DC and AC. That makes it useful for batteries, motors, inverters, and power systems.
Its limits include:
- Zero offset and drift
- Temperature drift
- External magnetic fields
- Core remanence
- Bandwidth limits
- Saturation
- Lower small-current accuracy than a good shunt in many cases
Open-loop Hall sensors directly measure field. Closed-loop Hall sensors drive a compensation winding to cancel flux and infer current from the compensation current. Closed-loop designs are usually more accurate and linear, but cost and power are higher.
Open-Loop and Closed-Loop Hall
Open-loop Hall sensing directly measures the magnetic field caused by the primary current:
Open loop: measure how large the magnetic field is
It is simpler, cheaper, and lower power, but linearity, zero drift, and temperature performance are limited by the Hall element and magnetic core.
Closed-loop Hall sensing drives a secondary winding to cancel the primary flux:
Closed loop: cancel the field, then measure the compensation current
Because the magnetic core operates near zero flux, closed-loop designs usually improve accuracy, linearity, and dynamic response. They also consume more power and cost more.
Current Transformer
A current transformer, or CT, works through electromagnetic induction. The primary current creates changing magnetic flux, and the secondary winding produces current.
AC current
-> changing magnetic flux
-> induced secondary current
-> burden resistor voltage
CTs provide good isolation and low insertion loss. They are common for AC mains measurement.
But they cannot measure DC. A constant DC current produces no changing flux and no continuous secondary output.
CTs also need a burden resistor and safe open-circuit handling. Leaving the secondary open can create high voltage.
What Isolation Solves
Isolation is not just about avoiding electric shock. It also solves measurement reference problems.
In high-voltage or high-current systems, the measured conductor may sit at a dangerous or noisy potential. Isolation lets the sensing electronics stay on a safe low-voltage side while current information crosses through magnetic coupling or an isolation barrier.
Isolation matters for:
- User safety
- High common-mode voltage
- Ground-loop avoidance
- Protection coordination
- Compliance with creepage and clearance rules
But isolation does not automatically mean high accuracy. Isolated sensors still have drift, bandwidth, offset, and calibration limits.
Bandwidth, Isolation, and Accuracy
Choosing a current sensing method starts with the application:
- DC or AC?
- Need isolation?
- Current range?
- Acceptable voltage drop and power loss?
- Bandwidth and transient response?
- Common-mode voltage?
- Accuracy and temperature range?
- Safety and certification requirements?
Bandwidth and dynamic range are often underestimated.
Motor drives, inverters, switch-mode power supplies, and short-circuit protection need fast current information. Energy metering and slow battery monitoring can tolerate lower bandwidth but need stability and calibration.
If the sensor bandwidth is too low, current spikes may be averaged away. If the range is too small, startup or fault current saturates the measurement. If the range is too large, small-current resolution may be poor.
Practical Selection
A practical decision path is:
- Need DC measurement and best low-cost precision? Start with a shunt.
- Need isolation and DC? Consider Hall.
- Need isolated AC only? Consider a CT.
- Need very low loss? Avoid large shunts.
- Need very small-current accuracy? Be careful with Hall offsets.
- Need fast protection? Check sensor and amplifier bandwidth.
- Need mains or high voltage? Treat safety standards as part of the sensor choice.
A shunt is often best for precision and simplicity at moderate currents. Hall is often best when DC isolation and large current are needed. CTs are strong for isolated AC measurement.
Engineering Takeaway
Current sensing is not one technology.
Shunt: current becomes voltage, accurate but lossy
Hall: current becomes magnetic field, isolated and DC-capable
CT: AC current becomes induced current, isolated but AC-only
The key sentence is:
The sensor does not read current directly.
It reads a voltage, magnetic field, or induced signal created by current.