If you are asking what would be the reading of ammeter and voltmeter in a basic DC circuit, the textbook answer is simple: the voltmeter reads the exact potential difference across the component, and the ammeter reads the exact current flowing through it (calculated via Ohm’s Law, I = V/R). But step away from the whiteboard and onto the workbench, and the reality changes.
In the real world, meters are not ideal. A voltmeter has finite input impedance that slightly loads the circuit, and an ammeter introduces a small series resistance (burden voltage) that drops the actual current. Knowing the difference between the 'ideal' reading and the 'actual' reading is what separates a textbook student from a competent troubleshooter. Below, we break down exactly how to set up your meters, what numbers you should expect, and why your readings might be lying to you.
Meter Setup and Safety Categories (CAT Ratings)
Before you touch a probe to a terminal, your multimeter must be configured correctly for the measurement type and the environment. Using a meter rated for electronics on a mains distribution panel is a fast way to cause an arc flash.
The Setup Block
- Dial Position: Set to V⎓ (DC Voltage) or V~ (AC Voltage) for parallel measurements. Set to A⎓ or A~ for series current measurements. Never leave the dial on the Ω (resistance) or continuity setting when probing a live circuit.
- Lead Jacks: Black lead always goes to COM. For voltage, the red lead goes to the V/Ω jack. For current, the red lead must be moved to the A (high current, usually 10A max) or mA/µA (low current, usually fused at 400mA) jack.
- Range: If using a manual-ranging meter, start at the highest range (e.g., 600V or 10A) and step down until you get maximum resolution without hitting the 'OL' (Over Limit) indicator. Auto-ranging meters handle this internally but may take a second to settle.
According to Fluke's safety guidelines on measurement categories, CAT II covers local receptacles and appliances, CAT III covers three-phase distribution and fixed motors, and CAT IV covers the service entrance and utility connections. Always match or exceed the CAT rating of your test point.
Probe Placement and Measurement Procedure
The physical placement of your probes dictates whether you measure voltage or current. Getting this wrong is the most common cause of blown multimeter fuses.
- Voltage Measurement (Parallel): Keep the circuit powered and intact. Place the black probe on the reference node (usually ground or the negative terminal) and the red probe on the test point. The meter bridges the component without interrupting the flow of electrons.
- Current Measurement (Series): You must break the circuit. Power down the circuit. Disconnect one leg of the component. Place the red probe on the side coming from the power source, and the black probe on the disconnected leg of the component. The current must flow through the meter's internal shunt resistor.
- Verify and Record: Power the circuit back on. Wait for the reading to stabilize (typically 1-2 seconds for DC, slightly longer for RMS AC calculations). Record the value and power down before moving probes.
Expected Readings: Good vs. Bad Values
To understand what a good reading looks like numerically, let’s use a standard bench scenario: A 12.0V DC power supply connected to a 100Ω power resistor. According to Ohm's Law, the ideal current is 120mA (0.12A), and the ideal voltage drop across the resistor is 12.0V.
| Measurement | Ideal Textbook Reading | Good Real-World Reading | Bad / Faulty Reading | What the Bad Reading Means |
|---|---|---|---|---|
| Voltmeter (Across Resistor) | 12.00 V | 11.85 V to 12.10 V | 0.00 V | Open circuit, blown fuse in power supply, or probes not making contact. |
| Voltmeter (Across Resistor) | 12.00 V | 11.85 V to 12.10 V | OL (Over Limit) | Meter range is set too low (e.g., set to 2V range while measuring 12V). |
| Ammeter (In Series) | 120.0 mA | 115.0 mA to 119.5 mA | 0.00 mA | Blown internal meter fuse, broken wire, or meter is in parallel (dangerous). |
| Ammeter (In Series) | 120.0 mA | 115.0 mA to 119.5 mA | OL / >10A | Short circuit in the load, or red lead is in the mA jack while current exceeds 400mA. |
Notice that the 'Good Real-World' ammeter reading is slightly lower than 120mA. This is not a faulty meter; it is physics in action, which we will explain next.
Why Your Readings Might Be Misleading
When troubleshooting, you might look at a schematic, calculate the expected values, and assume your meter is broken when the numbers don't match perfectly. Here are the two physical phenomena that alter your readings.
The Ammeter Burden Voltage
An ammeter measures current by passing it through an internal precision shunt resistor and measuring the voltage drop across that resistor. Because it introduces resistance into your circuit, it reduces the total current. This voltage drop is called burden voltage.
For example, a typical bench multimeter might have a burden voltage of 0.3V on the 400mA range. If you are measuring a 3.3V microcontroller circuit drawing 100mA, the meter drops 0.3V, leaving only 3.0V for the circuit. The circuit draws less current because of the meter's presence, meaning the ammeter reads a lower value than what the circuit would draw if the meter weren't there. All About Circuits notes that to minimize this, always use the highest current range possible that still gives you adequate resolution, as higher ranges use lower-resistance shunts.
The Voltmeter Loading Effect
A voltmeter is placed in parallel with a component. Ideally, it has infinite resistance so it draws zero current. In reality, standard digital multimeters have an input impedance of about 10MΩ (10,000,000 ohms). In low-impedance power circuits (like a 12V battery and a 100Ω resistor), 10MΩ in parallel is negligible. The reading is accurate.
However, if you are measuring voltage across high-impedance components—like a 1MΩ resistor in a sensor voltage divider—that 10MΩ meter impedance creates a parallel resistance of roughly 909kΩ. The circuit's actual voltage drops because the meter itself is acting as a load. If your voltmeter reads lower than expected in a high-impedance circuit, the meter is loading the circuit. The fix is to use a meter with a >10GΩ input impedance or use an oscilloscope with a 10x probe.
Frequently Asked Questions
What would be the reading of ammeter and voltmeter if connected in the wrong positions?
If you connect an ammeter in parallel across a voltage source, you are creating a dead short. The ammeter has near-zero resistance. The reading will instantly spike to 'OL' (or maximum scale), and the internal high-speed fuse of the multimeter will blow to protect the shunt. If you connect a voltmeter in series, its massive 10MΩ resistance will choke the circuit. The voltmeter will read the full open-circuit source voltage, the ammeter (if present) will read 0.00A, and the load will not turn on.
What would the voltmeter read if the circuit is open?
If the circuit is open (a broken wire or a switched-off load), no current flows. Because there is no current, there is no voltage drop across the load resistor (V = I × R; if I = 0, V = 0). If you place the voltmeter across the open break itself, it will read the full source voltage (e.g., 12.0V), because the meter's 10MΩ impedance completes the circuit just enough to measure the potential difference of the source.
What would be the reading of ammeter and voltmeter in a short circuit?
In a dead short (where the load resistance drops to nearly 0Ω), the voltmeter placed across the shorted component will read 0.00V (or a few millivolts of wire resistance). The ammeter, placed in series, will read a massive spike in current, limited only by the internal resistance of the power supply and the wires. In a real-world scenario, this current will immediately trip a breaker, blow a fuse, or cause the power supply's over-current protection (OCP) to fold back the voltage to zero.
