Measuring Current with a Multimeter
Current measurement is the function most beginners avoid — because it requires breaking the circuit, because the probes must go in different jacks, and because getting it wrong blows an internal fuse. Once you understand the procedure, in-line current measurement becomes straightforward, and it is invaluable for verifying that a circuit is drawing the expected current, finding short circuits, and characterising battery drain in portable equipment.
The Golden Rule — Series Connection
An ammeter measures the current flowing through it. For this to work, all of the circuit's current must pass through the meter. This means the circuit must be broken and the meter inserted in the gap — connected in series with the load. The meter ideally has zero resistance so it does not disturb the current it is measuring.
Current measurement requires breaking the circuit and inserting the meter in series. Red probe connects to the higher-potential side (toward positive supply); black probe connects onward to the load.
View LargerThis is the opposite of voltage measurement (parallel). Confusing the two is the most common cause of blown meter fuses: connecting an ammeter in parallel with a voltage source places the meter's near-zero shunt resistance directly across the supply and draws enormous current — instantly blowing the internal fuse.
Current Input Jacks and Fusing
Most DMMs have three input jacks: COM (common/negative), V/Ω (voltage and resistance), and A or mA (current). Some meters have separate jacks for high current (up to 10 A or 20 A) and low current (up to 200–500 mA). This matters:
| Jack label | Typical maximum | Internal fuse | Use for |
|---|---|---|---|
| mA or µA | 200–500 mA | Yes — usually 500 mA or 1 A quick-blow | Low current measurements — LED current, transistor bias current |
| A or 10 A | 10–20 A | Yes — usually 10–15 A slow-blow, or none on some meters | High current — power supply output current, battery charging current |
Step-by-Step Procedure
- Estimate the current first. Look at the circuit: with a 9 V battery and 470 Ω resistor, I = V/R = 9/470 ≈ 19 mA. Use the mA jack. If in doubt, use the highest range (A jack) and switch down.
- Power the circuit off before breaking the circuit to insert the meter.
- Move the red probe to the correct current jack (mA or A as appropriate). Black probe stays in COM.
- Break the circuit at a suitable point — remove a jumper wire, disconnect one end of a component, or open a test point.
- Insert the meter in the gap. Red probe connects to the side toward the positive supply; black probe connects to the side toward the load (continuing the current path).
- Power on and read the current.
- Power off, remove the meter, reconnect the circuit before removing the probes from the current jacks.
- Move the red probe back to V/Ω. This prevents accidentally connecting the current jack to a voltage source on the next measurement.
Burden Voltage
A real ammeter is not zero resistance — it has a small shunt resistor inside across which it measures the voltage drop. This resistance is called the shunt resistance and the voltage across it at the measured current is called the burden voltage.
In most practical circuits the burden voltage is small enough to ignore. A 0.1 Ω shunt carrying 100 mA drops only 10 mV — negligible in a 9 V circuit. However, in very low-voltage circuits (1–2 V logic supplies or millivolt-level signals), even 10–50 mV of burden voltage can represent a significant fraction of the supply and will change the circuit's operating point slightly. Always compare the burden voltage to the circuit's supply voltage and judge whether it matters.
Clamp Meters
A clamp meter (also called a clamp-on ammeter or current clamp) measures current non-invasively — without breaking the circuit. It works by measuring the magnetic field produced by the current in a conductor. The clamp's jaws surround the conductor and the Hall-effect sensor or Rogowski coil inside measures the field. The meter computes the current from the field strength.
Clamp meter advantages:
- No circuit disruption — clamp around an existing wire
- Safe for high-current measurements — rated for line current (100 A+)
- No risk of blowing internal fuses
Clamp meter limitations:
- Basic clamp meters only measure AC current (50/60 Hz) — AC clamps will read zero on DC
- DC-capable clamp meters (Hall-effect type) are available but more expensive
- Less accurate than in-line shunt measurement at low currents (<1 A)
- Cannot measure inside a bundle of wires — you must separate the single conductor you want to measure
For ham radio use, a DC-capable clamp meter is very practical for checking 12 V supply current to a transceiver — just clamp around the positive lead without disconnecting anything.
Ham Radio Current Measurements
- Power supply current draw: Measure the current a transceiver draws in receive versus transmit mode. A healthy 100 W transceiver draws roughly 2 A on receive and 18–22 A at full TX power. Unexpectedly high receive current often indicates a shorted transistor in the PA.
- LED indicators: Verify that current-limiting resistors give the correct LED current (typically 5–20 mA). Excessive current shortens LED life; insufficient current gives dim indication.
- Battery discharge rate: Measure the total current drawn by the station to calculate battery run-time (hours = Ah capacity ÷ current in amps).
- Keyer and accessory current: Check that accessories draw expected current. High unexplained current in a shack wiring run usually indicates an insulation fault or a faulty component in the accessory.
Hands-On Experiment
⚖ Experiment: Measure LED Current and Verify Ohm's Law
Build a simple LED circuit and use the multimeter in series to measure the actual current. Then compare the measured current to the value predicted by Ohm's Law using the voltage you measured in the previous lesson's experiment. This links voltage measurement, current measurement and the circuit law together in one practical exercise.
- 9 V battery with clip connector
- 470 Ω resistor
- LED (red — forward voltage approximately 2.0 V)
- Breadboard and connecting wires
- Digital multimeter
- Build the circuit on the breadboard: battery positive → 470 Ω resistor → LED anode → LED cathode → battery negative.
- First measure the voltage across the 470 Ω resistor using DC voltage mode (probes in V/Ω jack). Record the voltage (VR).
- Power off. Move the red probe to the mA jack. Break the circuit between the positive battery terminal and the resistor. Insert the meter in this gap: red probe to battery positive, black probe to the resistor end.
- Power on. Read the current (I).
- Power off. Restore the circuit and move the probe back to V/Ω.
- Calculate I from Ohm's Law: I = VR / 470 Ω. Compare this to the directly measured current.
The measured current should be approximately 14–15 mA for a red LED with a 9 V supply (voltage across R ≈ 6.9 V, I = 6.9/470 ≈ 14.7 mA). The Ohm's Law calculation from the voltage measurement should match the direct current measurement within the meter's accuracy specification — confirming both measurements are correct and consistent with the theory.
Frequently Asked Questions
I blew the fuse in my meter. How do I replace it?
Open the battery compartment on the back of the meter (usually a sliding panel or two screws). The fuse(s) are typically glass cartridge fuses in holders next to the battery. Note the rating printed on the blown fuse (e.g. F500mA 250V or F1A 250V) and replace with an identical rating and type — never uprate the fuse. Some meters use ceramic or subminiature fuses in the main PCB accessible only by fully opening the case. Check the meter's manual for the correct fuse specification.
Why does my 10 A current jack have no fuse on some meters?
High-current shunts can carry 10–20 A indefinitely and the current path has such low resistance that a conventional fuse would need to be rated so high as to offer no useful protection. The burden is on the user to connect to an appropriate current source. Some meters include a 10 A slow-blow fuse; others rely on the circuit's own fuse or breaker for overcurrent protection. Always note the time rating — most unfused 10 A jacks are only rated for 10–30 seconds before the shunt overheats.
Can I measure the supply current to my transceiver without disconnecting anything?
Yes — use a DC-capable clamp meter. Clamp around the positive 12 V supply lead to the transceiver (not in a bundled cable). The clamp reads DC current non-invasively. Alternatively, install a permanent in-line ammeter shunt in the supply lead and use the meter in voltage mode to read the shunt voltage, converting with I = V / Rshunt.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.