Is Current Draw Expected?#

Total supply current, per-rail current, and spotting overcurrent or undercurrent conditions. Current tells what the circuit is actually doing — voltage tells what the supply is providing.

DMM Series Current Measurement#

Power off, break the supply connection, set DMM to DC Amps (start with highest range if unknown), connect DMM in series: power source → DMM → circuit. Power on and read current.

Compare to datasheet estimates or previous known-good measurements. Higher than expected suggests a fault drawing excess current (short, latch-up, runaway oscillation). Lower than expected suggests something isn’t powering up or a connection is broken.

Current Sense Resistor + Oscilloscope#

When current waveform over time is needed — startup inrush, pulsed loads, sleep/wake transitions — insert a low-value resistor (0.1–1 Ω) in series and measure voltage across it with the scope.

I = V / R_sense

This shows peak vs average current and correlates current with other signals.

Current Clamp Probe#

Non-intrusive current measurement without breaking the circuit. Clamp around one conductor only (supply and return together cancel). Zero the probe before measuring.

DC/AC clamps (Hall sensor) can see DC but have limited bandwidth (< 100 kHz) and DC drift. AC-only clamps (transformer) have better bandwidth but can’t see DC.

Tips#

Always start on the highest current range to avoid blowing the DMM fuse
Verify DMM leads are in the correct jacks — many meters have separate current jacks
Choose sense resistor value that’s measurable but doesn’t significantly affect the circuit (1 Ω at 100 mA drops 100 mV)
Position wire in center of current clamp jaw for best accuracy

Caveats#

DMM current shunt adds resistance (often 0.1–1 Ω on low ranges) — this drops voltage and can affect circuit behavior
Never connect a DMM in current mode across a voltage source — this is a dead short through the meter’s low-impedance shunt and will blow the fuse or damage the meter
Current clamp probes need a single conductor — supply and return in the same cable cancel out
Hall sensor clamps have DC drift — zero frequently
Most clamps are designed for hundreds of mA to amps — sensitivity at microamps is poor
Sense resistor inductance matters at high frequencies — use non-inductive types for fast transients
Ground-referenced scope probes measuring ground-side sense resistor may need differential probe or two-channel subtraction

In Practice#

Current significantly higher than expected indicates possible short, latch-up, or component drawing excess current
Current significantly lower than expected indicates something isn’t powering up or a connection is broken
Current that pulses when it should be steady indicates oscillation or unstable behavior
Startup current spike (inrush) that exceeds supply or fuse rating can cause brownout or fuse blowing — check bulk capacitor inrush
Current waveform that correlates with specific events (transmit, motor motion) helps identify which function draws what
Sleep current that’s higher than datasheet spec indicates something isn’t entering low-power mode properly
Actuators draw current very differently from digital circuits — motors, servos, and steppers produce inductive spikes, high stall currents, and back-EMF; power supply sizing and decoupling for motor circuits deserves separate attention from the logic power rail.
An IC that draws more quiescent current than its datasheet specifies often shows up after the device has experienced an overvoltage or latch-up event — the excess current is flowing through a damaged junction or a parasitic path that was activated by the stress, indicating physical damage propagated downward from an electrical overstress condition.
A circuit that draws the expected total power but has an unexpected thermal distribution is showing, through the energy lens, that current is flowing through a different path than intended — the total energy is correct but its spatial distribution reveals a routing or operating-point error.
An IC that enters thermal shutdown unexpectedly at loads well below its rated capacity commonly appears when the PCB thermal design doesn’t provide adequate heat sinking — the internal thermal protection is responding to actual die temperature, which may be much higher than the package temperature suggests due to thermal resistance from junction to ambient.