An ideal ammeter has exactly zero internal resistance so it does not alter the current flowing through the circuit under test. In reality, every digital multimeter (DMM) introduces a small series resistance—typically between 0.01 Ω and 10 Ω depending on the selected current range. This resistance originates from the internal shunt resistors, the PCB traces, and the protective fuses. Knowing your exact ammeter resistance is critical when working with low-voltage electronics, where the resulting "burden voltage" can starve your load and yield a falsely low current reading.
Measuring this resistance requires a secondary ohmmeter and a strict adherence to unpowered testing protocols. Below is the exact procedure to characterize your meter's shunts, verify its fuses, and calculate the true burden voltage dropped across your test leads.
Meter Setup and Probe Placement for Ammeter Resistance
To measure the internal resistance of an ammeter, you must use a second, known-accurate multimeter operating in resistance mode. The target ammeter must be completely isolated from any power source and disconnected from all circuits.
Meter Setup Block
- Measuring Meter Dial: Set to Ω (Ohms). If your meter lacks auto-ranging, select the 200 Ω or 2 kΩ manual range.
- Measuring Meter Leads: Insert the black lead into the COM jack and the red lead into the V/Ω jack.
- Target Ammeter: Powered off, dial set to OFF or any voltage setting (never leave it dialed to Amps while probing the jacks with an external ohmmeter, as this can sometimes engage internal switching logic in advanced bench meters).
Probe Placement Technique
The internal shunts are connected between the COM jack and the specific current jacks (mA/µA and 10A). To measure the 10A shunt resistance, insert the probe tips directly into the COM and 10A jacks of the target ammeter. Push firmly to bypass the internal spring contacts and ensure a solid metal-to-metal connection. For the milliamp range, move the red probe to the mA/µA jack while keeping the black probe in the COM jack.
Expected Readings: Good vs. Blown Shunts and Fuses
When you apply the probes, the reading on your measuring meter will indicate the health of the internal current path. A standard handheld DMM (like a Fluke 87V or Brymen BM235) uses a thick manganin wire or a dedicated PCB trace for the 10A range, and a precision wirewound resistor paired with a fuse for the mA range.
| Target Range | Expected Good Resistance | Bad / Blown Reading | Max Burden Voltage (at full scale) |
|---|---|---|---|
| 10A / 20A Jack | 0.01 Ω to 0.05 Ω | OL (Infinite) or > 1.0 Ω | 0.1V to 0.2V @ 10A |
| mA Jack | 1.0 Ω to 5.0 Ω | OL (Infinite) | 0.2V to 0.5V @ 400mA |
| µA Jack | 10 Ω to 100 Ω | OL (Infinite) | 0.1V to 0.5V @ 4000µA |
Worked Example: If you are measuring a 3.3V microcontroller circuit expected to draw 200mA, and you use the mA jack with an internal resistance of 2.5 Ω, the burden voltage is calculated using Ohm's Law: $V = I \times R$.
$V = 0.2A \times 2.5\Omega = 0.5V$.
Your meter drops 0.5V, leaving only 2.8V for the microcontroller. This brownout condition may cause the MCU to reset, meaning the meter will read near 0mA—a misleading reading caused entirely by the ammeter's internal resistance.
Mistakes That Give Misleading Resistance Readings
Measuring sub-ohm resistances is notoriously difficult with standard handheld gear. If your readings seem erratic or higher than the specifications above, check for these common errors:
- Failing to Zero the Test Leads: Standard test leads have 0.1 Ω to 0.3 Ω of wire resistance. If you are measuring a 10A shunt that should read 0.01 Ω, your lead resistance will completely mask the true value. Short the probe tips together, press the REL (Relative) or ZERO button on your measuring meter to subtract the lead resistance, and then probe the ammeter jacks.
- Poor Contact with Jack Springs: The jacks on a DMM rely on internal spring-metal contacts. If the probes are inserted loosely, or if the jacks are oxidized from use in humid environments, contact resistance can add 0.5 Ω or more. Twist the probe tips slightly inside the jack to scrape away oxidation and ensure a solid bite.
- Thermal EMF Errors: When measuring micro-ohm shunts, the heat from your hands on the probe tips can create a thermocouple effect (thermal electromotive force), generating microvolts that the ohmmeter interprets as resistance drift. Let the probes and meter acclimate to room temperature (23°C) and avoid gripping the metal tips tightly.
- Measuring a Live Circuit: Never attempt to measure ammeter resistance while the meter is connected to a powered circuit. The external voltage will backfeed into the measuring meter's ohms circuit, likely blowing its internal PTC thermistor or destroying the ADC.
Safety Categories (CAT Ratings) and Mains Warnings
WARNING: Mains Voltage and Ammeter Fuses
When using an ammeter to measure AC mains current (e.g., testing a 120V/240V branch circuit), the meter must carry a minimum safety rating of CAT III 600V or CAT IV 300V. The internal resistance measurement described above is strictly a bench procedure for an unpowered meter.
If you are measuring mains current, ensure the ammeter's high-current jack is protected by a High Rupturing Capacity (HRC) fuse (typically rated 11A, 1000V, 10kA interrupt capacity). Cheap meters often use standard glass fuses that cannot safely interrupt a mains fault, posing a severe arc flash and shrapnel hazard. Always de-energize the panel, verify dead with a non-contact voltage tester, and use an inline clamp meter for AC mains current whenever possible to avoid breaking the circuit.
For further reading on the physics of shunt design and internal meter protection, refer to the All About Circuits guide on Ammeter Design and the Fluke multimeter basics tutorial.
Frequently Asked Questions
Why does my ammeter have resistance if it's supposed to be zero?
An ideal ammeter has zero resistance, but physical materials inherently resist electron flow. To measure current, a DMM passes the current through a known "shunt" resistor and measures the voltage drop across it using an internal voltmeter. The shunt must have a non-zero resistance to generate a measurable voltage (usually 100mV to 500mV at full scale). Additionally, the protective fuses and PCB traces add unavoidable parasitic resistance to the path.
How does ammeter resistance affect low-voltage circuits?
In low-voltage circuits (like 1.8V or 3.3V logic boards), the ammeter's internal resistance creates a "burden voltage" that subtracts from the supply voltage reaching the load. If the burden voltage drops the supply below the load's minimum operating threshold, the circuit will brownout or behave erratically. In these scenarios, using the 10A range (which has the lowest shunt resistance, ~0.01 Ω) or a dedicated zero-burden active current monitor is necessary to get accurate readings.
Can I measure ammeter resistance while it is connected to a live circuit?
No. Measuring resistance requires the ohmmeter to output a small test current and measure the resulting voltage. If the circuit is live, the external circuit voltage will overpower the ohmmeter's test voltage. At best, this yields a completely invalid reading; at worst, it forces high voltage backward into the ohmmeter's sensitive analog-to-digital converter, permanently destroying the measuring meter.
What is the difference between shunt resistance and burden voltage?
Shunt resistance is a fixed physical property of the meter's internal components, measured in Ohms (Ω). Burden voltage is the dynamic result of that resistance interacting with the circuit's current, measured in Volts (V). You calculate burden voltage by multiplying the circuit current by the shunt resistance ($V = I \times R$). A meter has a constant shunt resistance on a given range, but its burden voltage changes proportionally with the current being drawn by the load.
