Difference Between Ammeter and Voltmeter

An ammeter measures electric current and is connected in series so the same current flows through the meter and the circuit. A voltmeter measures voltage and is connected in parallel across a component. An ammeter has very low internal resistance, while a voltmeter has very high internal resistance.

Both instruments are used to measure electrical circuits, troubleshoot power supplies and monitor batteries. A digital volt-amp meter combines both functions in one panel unit, allowing you to monitor voltage and current at the same time.

Ammeter vs Voltmeter: Quick Comparison

Remember: An ammeter goes in series with the load. A voltmeter goes in parallel across the load or power supply.

What Is an Ammeter?

An ammeter is an electrical measuring instrument used to measure the current flowing through a circuit. Current is measured in amperes (A), milliamperes (mA) or microamperes (µA), depending on the circuit.

To measure current, the ammeter is placed in series with the load. This makes the circuit current pass through the meter.

Why Does an Ammeter Have Low Resistance?

An ammeter should introduce as little resistance as possible into the circuit. If its internal resistance were high, it would reduce the current being measured and change the circuit's behaviour.

In practice, ammeters have a small internal resistance or use a shunt resistor to measure current. The instrument measures the small voltage drop across the shunt and uses it to calculate current.

Safety warning: Never connect a conventional ammeter directly across a battery or power supply. This can create a near-short circuit and damage the meter, wiring or power source.

What Is a Voltmeter?

A voltmeter measures the electrical potential difference between two points. Voltage is measured in volts (V), millivolts (mV) or kilovolts (kV), depending on the application.

To measure voltage, connect the voltmeter in parallel across the battery, power supply or component being tested. The meter measures the difference in potential between its two terminals.

Why Does a Voltmeter Have High Resistance?

A voltmeter should draw very little current from the circuit. Its high internal resistance helps it measure voltage without significantly changing the circuit's behaviour.

For example, to measure a 12V battery, place the voltmeter probes across the battery terminals: positive to positive and negative to negative.

Ammeter vs Voltmeter: Wiring Guide

The main difference between these instruments is how they connect to a circuit.

1. Connecting an Ammeter in Series

To measure current, the ammeter must be inserted into the circuit so the load current passes through it.

1. Switch off the power.

2. Open the circuit at the point where you want to measure current.

3. Insert the ammeter in series with the load.

4. Select the correct current range and input terminal.

5. Check the polarity and wiring before restoring power.

For a digital multimeter, move the red lead to the correct current input if required by the meter's design. Start with an appropriate higher current range when the expected current is uncertain.

Important: An ammeter's low resistance means that connecting it directly across a supply can create a short circuit.

2. Connecting a Voltmeter in Parallel

To measure voltage, connect the voltmeter across the two points whose potential difference you want to measure.

1. Set the meter to the correct voltage mode, such as DC voltage for a battery.

2. Connect the probes across the load or supply terminals.

3. Match the polarity for a positive DC reading.

4. Read the displayed voltage.

You do not need to break the circuit to measure voltage with a typical digital multimeter.

Safety rule: Never leave a multimeter in current mode and then connect it across a battery or power supply. Current mode provides a low-resistance path and can cause a short circuit.

How to Wire a Dual Digital Voltmeter-Ammeter

A dual digital volt-amp meter displays voltage and current on one unit. It is useful for monitoring a DC power supply, battery-powered project, charger or electronic load.

For a typical DC panel meter:

• The voltage-sensing leads connect in parallel across the supply or load.

• The current-sensing path is connected in series with the load.

• Some models use a built-in shunt, while others require an external shunt.

The exact wire colours, terminal arrangement and power connections vary between models. Always follow the wiring diagram supplied with your specific meter.

Thick Wires vs Thin Wires

Many dual digital meters use separate wires for voltage sensing and current measurement.

• Thick current wires: Commonly carry load current through the meter's current path or external shunt. They must be rated for the maximum current.

• Thin sensing wires: Commonly measure voltage and may also supply power to the display, depending on the design.

Do not determine wire function from thickness or colour alone. Check the product's wiring diagram, especially if it has separate supply and measurement connections.

Choosing a 10A or 100A Meter

The current rating must match the maximum current the meter is designed to measure.

A 100A meter commonly uses an external shunt. The shunt and wiring must match the meter's specifications. Never exceed the rated current of the meter, shunt, terminals or cables.

Recommended Product: Digital Voltmeter Ammeter

For applications requiring higher current, choose a suitable 100A version and confirm whether it uses an external shunt.

Using an Ammeter and Voltmeter for Battery and Solar Monitoring

Voltage and current readings provide different information about a battery or solar-powered system.

• Voltage monitoring: Helps you check battery or supply voltage under different operating conditions.

• Current monitoring: Shows how much current a load draws or how much current flows through a charging circuit.

• Power calculation: If voltage and current are measured at the same operating point, DC power can be estimated using \(P = V \times I\).

For example, if a DC load operates at 12V and draws 2A, its electrical input power is approximately 24W.

For solar and battery systems, current direction also matters. A suitable bidirectional monitor may be needed to distinguish charging current from discharging current.

Conclusion

The main difference between an ammeter and a voltmeter is what each measures and how it connects to a circuit. An ammeter measures current in series, while a voltmeter measures voltage in parallel. Both must be used within their specified measurement ranges.

For projects where you need both readings at once, a dual digital volt-amp meter offers a compact monitoring solution. The Digital Voltmeter Ammeter DC 0–100V, 10A is an option for compatible DC circuits. Verify the current rating, shunt arrangement and wiring instructions before connecting it.