The ideal resistance of an ammeter is exactly zero ohms, but in the real world, every digital multimeter (DMM) contains a physical shunt resistor that introduces a small internal resistance. Depending on your meter and the selected range, the internal resistance of an ammeter typically falls between 0.01 Ω and 10 Ω. Measuring this value is a critical bench technique for verifying the health of your internal fuses, checking for degraded shunt resistors, and calculating the burden voltage your meter will inject into low-voltage circuits.
Why the Internal Resistance of an Ammeter Matters
To measure current, an ammeter must be placed in series with the load. The current flows through an internal precision resistor (the shunt), and the meter measures the voltage drop across it using Ohm's Law. Because this shunt has physical resistance, it creates a burden voltage that subtracts from the voltage available to your circuit.
On a high-quality bench meter like the Keysight U1232A or a Fluke 87V, the 10A range uses a massive, low-resistance shunt (typically around 0.01 Ω). If you draw 5A, the burden voltage is only 50 mV ($V = 5A \times 0.01\Omega$). However, if you switch to the mA/µA range, the internal shunt resistance jumps significantly—often to 1 Ω, 10 Ω, or even higher on cheaper meters.
Bench War Story: Measuring a 150 mA load on a 3.3V ESP32 circuit using the mA jack on a budget DMM with a 10 Ω internal shunt drops a full 1.5V across the meter. The ESP32 sees only 1.8V, triggering a brownout reset loop. The meter reads erratic current, and the maker assumes the microcontroller is faulty. Always calculate your burden voltage before measuring low-voltage, moderate-current embedded circuits.
For a deeper look at how shunt networks are designed inside these tools, refer to the All About Circuits guide on ammeter design.
Meter Setup and Safety Requirements
To measure the resistance of an ammeter, you need a second, known-good multimeter (the measuring DMM) to test the target multimeter (the DUT).
Meter Setup Block (Measuring DMM)
- Dial Position: Resistance (Ω) or Continuity. If manual ranging, select the 200 Ω or 20 Ω range for the highest resolution.
- Lead Jacks: Red probe in V/Ω, Black probe in COM.
- Range: Auto-range, or manually set to the lowest ohms range to resolve milliohm values.
Probe Placement on Target Ammeter
Insert the probe tips directly into the center contacts of the target ammeter's jacks. To measure the 10A shunt, place one probe into the target's COM jack and the other into the 10A jack. To measure the mA shunt, place the probes into the COM and mA/µA jacks. Press firmly to bypass surface oxidation on the jack sleeves.
Step-by-Step: Measuring Your Ammeter's Internal Resistance
- Isolate the Target Meter: Power off the target multimeter and disconnect all test leads from it.
- Null the Test Leads: Short the probes of your measuring DMM together. Note the lead resistance (usually 0.1 Ω to 0.4 Ω). If your meter has a Relative (REL) or Null mode, activate it to subtract this baseline.
- Set the Target Meter: Turn the target meter's dial to the current measurement function (e.g., 10A or mA). Note: On meters with active FET-switching for auto-ranging, you may need to power the target meter ON to close the internal relays. Consult your service manual. For standard hardwired jacks, leave it OFF.
- Make the Connection: Insert the measuring probes into the target meter's COM and 10A jacks.
- Record and Subtract: Read the value on the measuring DMM. If you did not use REL mode, subtract your nulled lead resistance from the displayed value to find the true internal shunt resistance.
- Repeat for mA/µA: Move the red probe to the mA/µA jack and repeat the process.
Expected Readings: Good vs. Blown Fuses and Degraded Shunts
Use the table below to interpret your measurements. These values represent typical high-quality DMMs (like the Fluke 87V or Brymen BM235). Budget meters may have slightly higher baseline shunt resistances.
| Target Jack / Range | Expected Good Resistance | Blown Fuse Reading | Degraded / Overheated Shunt | Typical Fuse Spec |
|---|---|---|---|---|
| 10A / 20A Jack | 0.01 Ω to 0.05 Ω | OL (Open Loop) | > 0.10 Ω | 11A HBC Ceramic (e.g., 100kA IR) |
| mA / µA Jack | 1.0 Ω to 10.0 Ω | OL (Open Loop) | > 15.0 Ω or drifting | 400mA to 1A Fast-Acting Glass/Ceramic |
If your 10A jack reads exactly 0.00 Ω, your measuring leads are likely making poor contact, or your meter has a very high-resolution micro-ohmmeter function that needs recalibration. If it reads OL, your high-rupturing-capacity (HBC) fuse is blown—a common result of accidentally measuring current in parallel with a voltage source.
Common Mistakes That Yield Misleading Readings
- Forgetting to Null Lead Resistance: Standard silicone test leads have 0.2 Ω to 0.5 Ω of resistance. If you measure a 10A shunt (0.01 Ω) without nulling the leads, your meter will display 0.3 Ω, leading you to falsely conclude the shunt is degraded or the fuse is partially blown.
- Measuring Active Circuits: Attempting to measure the resistance of an ammeter while it is still wired into a powered circuit will result in wildly inaccurate readings and can destroy the measuring DMM's ohmmeter circuitry. Always isolate the target meter.
- Parallax and Contact Errors: Resting probe tips lightly on the plastic rim of the jack rather than pushing them fully into the metal sleeve introduces contact resistance. Wiggle the probes slightly; if the reading jumps by more than 0.05 Ω, your probe tips are worn or the jack springs are fatigued.
- Ignoring Thermal EMF: If you are using a 4-wire Kelvin micro-ohmmeter for precision bench calibration, temperature gradients between the probe tips and the jack metals can generate microvolt-level thermal EMFs, skewing sub-milliohm readings. Allow the meter to acclimate to room temperature.
Frequently Asked Questions
What is the ideal resistance of an ammeter in a theoretical circuit?
In theoretical circuit analysis, the ideal resistance of an ammeter is exactly 0 Ω. This ensures that inserting the meter into the circuit does not alter the total resistance or change the current flow you are trying to measure. In practice, achieving 0 Ω is impossible, which is why precision engineers use current shunts and oscilloscopes or specialized zero-flux current transducers for highly sensitive measurements.
Can I measure the resistance of an ammeter while it is connected to a live circuit?
No. You must never measure resistance on a live circuit. The measuring multimeter injects a small test current to calculate resistance. If external voltage is present, it will conflict with the test current, yielding meaningless readings and potentially blowing the internal fuse or destroying the ADC (analog-to-digital converter) of your measuring multimeter. Always de-energize and isolate the circuit first.
Why does my multimeter show OL when measuring the 10A ammeter jack?
An "OL" (Open Loop) reading across the COM and 10A jacks almost always indicates a blown internal fuse. The 10A jack is protected by a high-rupturing-capacity (HBC) ceramic fuse designed to safely interrupt massive fault currents without exploding. If you accidentally placed the meter in parallel with a voltage source while the leads were in the current jacks, this fuse sacrificed itself to protect you and the meter. You will need to open the meter case and replace it with an exact-match OEM fuse.
Does a higher ammeter resistance affect low-voltage microcontroller projects?
Yes, significantly. When debugging a 3.3V or 1.8V microcontroller (like an ESP32 or STM32) powered by a bench supply, inserting an ammeter with a 5 Ω internal shunt in the mA range will drop the supply voltage seen by the chip as current draw increases. This causes voltage sag, erratic behavior, and brownout resets. For low-voltage embedded work, use a dedicated current monitor IC (like the INA219), a specialized low-burden current probe, or calculate the burden voltage and compensate by raising your bench supply voltage accordingly.
