Current Measurement & Probing#

Voltage is easy to see — put a probe on the node and read the result. Current is invisible. You can’t probe current directly; you have to interrupt the circuit or infer current from a voltage drop across a known resistance. This makes current measurement harder than voltage measurement, and the choice of method matters more than it does for voltage.

Why Current Measurement Matters#

Current tells you what’s actually happening:

• Power consumption: Voltage times current equals power. You can’t calculate battery life without knowing current draw.
• Fault detection: A short circuit has normal voltage but excessive current. A failed regulator may output correct voltage but pull abnormal current from its input.
• Efficiency: Input power minus output power is loss. Efficiency calculations require current measurements on both sides.
• Sleep current: An embedded system that should draw 10 µA but actually draws 500 µA has a bug. You won’t find it by measuring voltage.
• Thermal behavior: Components heat up in proportion to I²R. High current through a PCB trace or connector causes heating that voltage measurement doesn’t reveal.

Measurement Methods#

DMM Current Mode#

The DMM interrupts the circuit and measures current through its internal shunt resistor.

Setup: Break the circuit path, insert the meter in series (positive to positive, maintaining current direction).

Pros:

• Simple, available in any DMM
• Reasonable accuracy (±1% or better in mid-range meters)
• Multiple ranges for different current levels

Cons:

• Requires breaking the circuit — not always possible or convenient
• Burden voltage: the meter’s shunt drops voltage (10–200 mV typically), which affects the circuit
• Fuse limits: the 10A range may use an unfused input; the mA range has a small fuse. Overcurrent blows the fuse or damages the meter.
• Average reading: pulsed loads show average current, not peak

Best for: Static or slow-changing current up to a few amps; bench testing where the circuit can be interrupted.

Shunt Resistor + Oscilloscope/DMM#

Insert a low-value precision resistor in series; measure voltage across it and calculate I = V/R.

Setup: Choose shunt resistance for adequate voltage drop at expected current (10–100 mV typical) while keeping burden voltage acceptable. Use 4-wire (Kelvin) connections to avoid lead resistance error.

Pros:

• Works at any frequency (see waveform, not just average)
• No fuse limits — size the shunt for your current
• Can measure DC and AC components simultaneously
• Permanent installation for ongoing monitoring

Cons:

• Requires access to insert the shunt
• Burden voltage affects circuit operation
• Shunt inductance affects high-frequency measurements
• Voltage drop times current equals power dissipation in the shunt — can cause heating

Shunt selection:

Use precision shunts (0.1–1%) for accurate measurements. General resistors drift with temperature and may not handle the power.

Best for: Waveform capture, high-frequency current, continuous monitoring, any situation where you need more than an average reading.

Inline USB Meters#

A specialized device that sits between USB power source and load, displaying voltage and current in real time.

Setup: Plug in series with the USB connection.

Pros:

• No circuit modification needed (for USB-powered devices)
• Continuous real-time display
• Often logs energy (mAh, Wh) over time
• Inexpensive

Cons:

• USB-only — doesn’t help for non-USB circuits
• Moderate accuracy (±1–3% typical)
• Limited current range (usually 3A max for USB-A, 5A for USB-C)
• Adds connector resistance and inductance

Best for: Quick checks of USB-powered devices, battery charging current, USB power bank capacity verification.

Current Clamp (Clamp Meter)#

A clamp that encircles a conductor and measures current through magnetic coupling, without breaking the circuit.

Setup: Open the clamp jaws, insert the conductor, close the jaws. The conductor must carry the full load current — clamping around a power cable that has supply and return together measures zero (the fields cancel).

Pros:

• Non-contact measurement — no circuit interruption
• Safe for high-current measurements
• Works on existing wiring

Cons:

• Resolution and accuracy worse than direct measurement
• Minimum current sensitivity typically 100 mA–1 A
• AC-only for basic clamps (Hall effect clamps measure DC)
• Can’t measure PCB traces or small wires easily
• Position sensitivity — clamp orientation affects reading

Best for: Power wiring, AC mains current, any high-current measurement where breaking the circuit is impractical.

Current Probe (Oscilloscope)#

A specialized probe that outputs a voltage proportional to current, allowing the oscilloscope to display current waveforms.

Types:

• AC current probes: Use transformer coupling. Simple, no power required, AC-only (typically >10 Hz).
• DC/AC current probes (Hall effect): Measure DC and AC. Require power (batteries or probe amplifier). More expensive but essential for DC and low-frequency measurements.

Key specs:

Pros:

• Non-contact — no circuit interruption
• See current waveforms, not just DC averages
• Works on existing wiring
• Can measure very high currents safely

Cons:

• Expensive (especially DC/AC Hall effect probes)
• Bulky — may not fit in tight spaces
• Requires demagnetization (degauss) periodically to maintain accuracy
• Position sensitivity affects readings

Best for: Power electronics debugging, motor drive current, inrush current, any measurement where you need to see current waveform shape without interrupting the circuit.

Choosing the Right Method#

Design for Measurability#

Current measurement is easiest when anticipated during design:

• Add shunt resistor pads: 0Ω or low-value resistors in key current paths (power input, major rails, motor drives) allow easy measurement by substituting a shunt or measuring across existing low-value components.
• Add test points: Even without shunts, test points on both sides of a series component (inductor, fuse, sense resistor) allow voltage measurement for current inference.
• Separate return paths: If ground is common for multiple supplies, measuring individual rail currents is difficult. Separate return paths enable per-rail current measurement.
• Current sense ICs: For production monitoring, dedicated current sense amplifiers (like INA219, INA226) provide digital current readout without external measurement equipment.

Gotchas and Limits#

• Burden voltage affects circuit operation — A 100 mV drop in a 3.3V supply is a 3% reduction. Size shunts or select DMM ranges to keep burden voltage acceptable.
• DMM fuses blow silently — If current measurement reads zero unexpectedly, the meter’s fuse may have blown. Check resistance through the current path before assuming the circuit is open.
• Clamp position matters — Clamping around a power cord with both conductors inside measures zero. The wire carrying current must be isolated.
• Ground loops with shunts — If both scope ground and power supply ground connect to the same circuit, inserting a shunt can create a ground loop or short out the shunt. Use isolated probes or battery-powered analyzers.
• Average vs. peak current — A DMM shows average current. Pulsed loads (transmitting radios, motor PWM) have peak currents much higher than average. The supply and wiring must handle the peak, not just the average.

Tips#

• For µA-level sleep current measurement, use the DMM’s µA range (typically 200 µA or 2 mA full scale) and connect in series with the battery or power input
• When using shunt + scope, set the scope to high-resolution (averaging) mode for cleaner DC measurements
• Clamp meters work best on single conductors — wrap multiple turns of wire through the clamp to multiply sensitivity (10 turns gives 10× the reading; divide by 10 for actual current)
• Demagnetize (degauss) Hall effect current probes before precision measurements — residual magnetization causes DC offset errors
• Log current over time to catch intermittent events — a device that draws high current for 50 ms every 10 seconds won’t show up in spot checks

Caveats#

• Current probe bandwidth matters — A 1 MHz probe won’t capture the high-frequency content of a switching power supply’s current waveform. Match probe bandwidth to the frequencies you need to see.
• Clamp meters are not precision instruments — ±2–3% accuracy is typical; sub-1% accuracy requires calibrated shunts or precision current probes.
• High-side vs. low-side measurement — Shunts in the ground return (low-side) are easier to measure (referenced to ground) but create a ground offset. High-side shunts maintain ground integrity but require differential or isolated measurement.
• Minimum current for clamps — Most clamp meters and probes have a minimum sensitivity around 10–100 mA. Milliamp and microamp currents require direct measurement methods (DMM or shunt).

In Practice#

• A device that runs longer on battery than expected may be measuring current with the DMM burden voltage artificially lowering the supply voltage and reducing clock speed or radio power
• Motor inrush current that trips a fuse but doesn’t show on the ammeter is happening too fast for the meter’s averaging — use a current probe and scope to capture the transient
• A “5A” USB charger that only delivers 3A may have a current limit, or the cable drop may be triggering the load’s undervoltage protection
• Sleep current that varies from board to board is often a pull-up resistor to a floating or mis-configured GPIO — current measurement identifies which specific board is affected, and which rail shows the extra current
• When debugging efficiency, measure input current and output current separately — efficiency = (Vout × Iout) / (Vin × Iin). Low efficiency is either high quiescent current or high switching/conduction losses