Measurement
DC Measuring Instruments
Overview
One of the challenges of electricity is that we cannot see it. We rely on instruments to detect its presence and measure it. The three most common measuring instruments for DC circuits are the Ammeter, Voltmeter, and Ohmmeter.
1. Ammeter
Definition
An ammeter is an instrument used to measure electric current in a circuit. It is specifically designed to measure the flow of electrical charge (typically electrons) passing through a specific point in a conductor per unit of time.
Connection
The ammeter must be connected in series with the rest of the circuit. Any current flowing through the circuit must pass through the meter.
Key Characteristics
- Measures current flow (amperes)
- Must be placed in the current path
- Low internal resistance to minimize circuit interference
- Can damage if connected in parallel
Unit of Measurement
Ampere (A)
2. Voltmeter
Definition
A voltmeter is an instrument used to measure the voltage (electric potential difference) between two points in an electrical circuit. It quantifies the electrical potential energy per unit charge at a specific location within the circuit.
Connection
To measure voltage, the voltmeter must be connected across the component (i.e., in parallel with it).
Key Characteristics
- Measures potential difference (volts)
- Connected in parallel with the component
- High internal resistance to avoid drawing significant current
- Does not significantly affect the circuit being measured
Unit of Measurement
Volt (V)
3. Ohmmeter
Definition
An ohmmeter is a specialized instrument used to measure the electrical resistance of a component or circuit. Unlike voltmeters and ammeters, an ohmmeter is designed specifically to quantify resistance.
Connection
- Must be connected to the component when the circuit is de-energized (powered off)
- Connected across the component (in parallel)
- Cannot be used on live circuits
Key Characteristics
- Measures resistance (ohms)
- Contains internal battery to pass test current
- Must be used only on isolated components
- Typically has multiple resistance ranges
Unit of Measurement
Ohm (Ω)
4. Multimeter
Definition
A multimeter, also known as a multitester or VOM (volt-ohm-milliammeter), is a versatile electronic instrument that combines the functions of a voltmeter, ammeter, and ohmmeter into a single portable device.
Functions
-
Voltage Measurement
- AC voltage
- DC voltage
- Multiple voltage ranges
-
Current Measurement
- DC current (typically up to 10A)
- AC current (on some models)
- Multiple current ranges
-
Resistance Measurement
- Ohm ranges from few ohms to megaohms
- Continuity testing
-
Additional Features (on digital models)
- Diode testing
- Capacitance measurement
- Frequency measurement
- Temperature measurement
- Transistor testing
Types of Multimeters
Analog Multimeter
- Uses moving needle display
- Reads values from graduated scales
- No battery required for voltage/current measurement
- Lower cost
Digital Multimeter (DMM)
- LCD/LED display showing numerical values
- Higher accuracy and precision
- Auto-ranging capabilities
- Additional functions
- Easier to read
Key Components (Digital Multimeter)
- LCD Display: Shows measured values numerically
- Range Selection Dial: Selects measurement type and range
- Probe Lead Sockets:
- Common socket (black/negative lead)
- Voltage/Resistance/Diode socket (red lead)
- Current socket (10A high current and mA low current)
- Function Buttons: Mode selection, hold, backlight, etc.
Safety Considerations
- Always start with the highest range when measuring unknown values
- Never measure resistance on live circuits
- Ensure proper probe connection for current measurement
- Observe polarity for DC measurements
- Use appropriate fuse-protected ranges
- Check probe condition regularly
Applications in Electric Vehicles
- Battery voltage monitoring
- Current draw testing
- Continuity checks in wiring harnesses
- Resistance measurements in motor windings
- Sensor output verification
- Charging system diagnostics
Comparison Table
| Instrument | Measures | Connection | Unit | Special Notes |
|---|---|---|---|---|
| Ammeter | Current | Series | Ampere (A) | Low internal resistance |
| Voltmeter | Voltage | Parallel | Volt (V) | High internal resistance |
| Ohmmeter | Resistance | Parallel (de-energized) | Ohm (Ω) | Contains internal battery |
| Multimeter | Voltage, Current, Resistance | Varies by function | V, A, Ω | Combines all functions |
Best Practices for Using DC Measuring Instruments
General Guidelines
- Know Your Instrument: Understand specifications, ranges, and limitations
- Circuit Condition: De-energize circuits when measuring resistance
- Proper Connection: Use correct connection method (series/parallel)
- Range Selection: Start with highest range for unknown values
- Probe Condition: Inspect probes for damage before use
For Ammeters
- Never connect in parallel with a component
- Ensure circuit can handle the meter's resistance
- Use appropriate current range
For Voltmeters
- Never connect in series with the circuit
- Check AC/DC setting matches circuit type
- Observe polarity for DC measurements
For Ohmmeters
- Always disconnect component from circuit
- Discharge capacitors before testing
- Zero the meter if required (analog types)
For Multimeters
- Select correct function before connecting
- Use proper input jacks for measurement type
- Auto-ranging simplifies measurements but verify accuracy
- Store with probes disconnected
Troubleshooting Common Issues
Ammeter Shows No Reading
- Check series connection
- Verify circuit is energized
- Check fuse in meter
- Ensure range is appropriate
Voltmeter Shows Zero or Incorrect Reading
- Verify parallel connection across component
- Check probe contact
- Ensure AC/DC setting is correct
- Verify circuit has voltage present
Ohmmeter Shows Infinite Resistance
- Check component is disconnected from circuit
- Verify probe contact
- Component may be open circuit
- Battery in meter may be depleted
Multimeter Errors
- "OL" (overload) - value exceeds range
- Unstable readings - poor probe contact or noisy environment
- No display - check battery
- Incorrect mode - verify function selection
Applications in Electric Vehicles
Battery System Testing
- Voltage measurement of individual cells
- Current draw during charging/discharging
- Resistance of connections and busbars
- Continuity of battery management system wiring
Motor and Controller Testing
- Motor phase resistance
- Controller output voltage
- Current consumption under load
- Insulation resistance testing
Charging System Diagnostics
- Input AC voltage verification
- DC output voltage monitoring
- Charging current measurement
- Ground continuity verification
General Electrical System
- Fuse and relay testing
- Sensor output verification
- Wiring harness continuity
- Component resistance checks