2. Voltage, Current, Resistance, and Power in Industrial Automation

Introduction

Every industrial automation system is built on four fundamental electrical concepts:

• Voltage
• Current
• Resistance
• Power

These four concepts explain why a sensor works, why a relay energizes, why a motor turns, and why electrical components fail.

Many technicians learn PLC programming, HMI configuration, and VFD parameters, but troubleshooting becomes much easier when they understand what is physically happening inside the electrical circuit.

A PLC instruction may say:

“Turn ON output.”

But electricity must still travel through a real circuit:

Power Supply
      ↓
Protection
      ↓
PLC Output
      ↓
Field Wiring
      ↓
Load
      ↓
Return Path

Understanding electrical quantities allows a technician to move from guessing to measuring.

The Four Electrical Quantities

1. Voltage — The Electrical Pressure

Voltage is the electrical potential difference that pushes electrons through a circuit.

The unit of measurement is:

Volt (V)

A useful analogy is water pressure.

Imagine a pipe system:

High Pressure → Water Movement

In electricity:

Higher Voltage → Ability to Push Current

Voltage does not mean that current is flowing.

A circuit can have voltage present but no current flow if the path is open.

Example:

A 24 VDC power supply may measure:

+24 VDC

at the output terminals.

However, if a wire is broken, the connected device will not operate because the circuit is incomplete.

Industrial Voltage Examples

Automation technicians commonly work with multiple voltage levels:

A modern industrial machine can contain several voltage systems at the same time.

Example:

480 VAC
   ↓
VFD
   ↓
Motor


120 VAC
   ↓
Control Transformer
   ↓
24 VDC
   ↓
PLC + Sensors

2. Current — The Movement of Electricity

Current is the actual movement of electrical charge through a conductor.

The unit of measurement is:

Ampere (A)

Using the water analogy:

Voltage is pressure.

Current is the amount of water flowing through the pipe.

In electricity:

Voltage pushes.

Current flows.

Example: Sensor Circuit

A typical proximity sensor:

24 VDC Supply

Brown  → +24 VDC

Blue   → 0 VDC

Black  → Signal Output

The sensor may only consume:

100 mA

while a motor may require:

20 A

The voltage may be the same:

24 VDC

but the current requirement is very different.

Why Current Matters

Excessive current causes:

• Overheating
• Damaged wiring
• Blown fuses
• Tripped breakers
• Failed components

Example:

A solenoid valve designed for:

24 VDC
0.5 A

is connected incorrectly and draws:

3 A

The result may be:

• Output module failure
• Fuse opening
• Damaged wiring

3. Resistance — Opposition to Current Flow

Resistance is the opposition that limits current flow.

The unit of measurement is:

Ohm (Ω)

Every electrical component has resistance.

Examples:

Resistance determines how much current flows when voltage is applied.

Example: Relay Coil

A relay coil may have:

Voltage:
24 VDC

Resistance:
240 Ω

Using Ohm’s Law:

I = V / R

I = 24 / 240

I = 0.1 A

The relay consumes:

100 mA

This helps determine whether a PLC output can drive the load directly.

Ohm’s Law

The relationship between voltage, current, and resistance is:

V = I × R

Where:

V = Voltage

I = Current

R = Resistance

This formula is one of the most important tools for troubleshooting.

Practical Automation Examples Using Ohm’s Law

Example 1 — Solenoid Valve

A solenoid has:

24 VDC

Resistance = 48 Ω

Calculate current:

I = V / R

I = 24 / 48

I = 0.5 A

The solenoid requires:

500 mA

If the PLC output is rated for only:

250 mA

the output can fail.

Example 2 — Finding a Failed Coil

A contactor coil should measure:

100 Ω

During troubleshooting you measure:

OL (Open Loop)

This indicates:

• Broken coil winding
• Failed contactor coil

The electrical path is open.

4. Power — The Work Performed by Electricity

Power is the rate at which electrical energy is converted into useful work.

The unit is:

Watt (W)

The formula is:

P = V × I

Where:

P = Power

V = Voltage

I = Current

Industrial Power Examples

Sensor

24 VDC

0.1 A

Power:

P = 24 × 0.1

P = 2.4 Watts

A small amount of power.

Motor

Example:

480 VAC

20 A

Power:

P = 480 × 20

P = 9600 Watts

Approximately:

9.6 kW

A motor requires much more energy than a sensor.

Why Automation Technicians Need These Concepts

Troubleshooting a Dead Sensor

Operator:

“The sensor is not working.”

Possible checks:

Step 1

Measure voltage:

Brown wire → Blue wire

Should be approximately:

24 VDC

If no voltage:

Problem may be:

• Power supply
• Fuse
• Wiring
• Terminal block

Step 2

Measure signal output:

Sensor activated:

Black wire ≈ 24 VDC

If no signal:

Possible causes:

• Sensor failure
• Incorrect wiring
• Wrong sensor type
• Target distance problem

Troubleshooting a Motor Starter

Problem:

Motor does not start.

The technician should ask:

Is voltage available?

Measure:

L1-L2
L2-L3
L1-L3

Is control voltage available?

Check:

Contactor coil voltage

Is current flowing?

Measure:

Motor current

Is resistance correct?

Check:

• Motor winding
• Cable insulation
• Coil resistance

Multimeter Measurements

A technician uses different meter functions depending on what needs to be verified.

Voltage Measurement

Used to verify:

• Power supply output
• Control voltage
• Sensor voltage
• Motor supply

Example:

24.1 VDC

Resistance Measurement

Used to verify:

• Coils
• Continuity
• Wiring integrity

Important:

Never measure resistance on an energized circuit.

Current Measurement

Used to verify:

• Motor load
• Heater current
• Solenoid consumption

Common Mistakes

Mistake 1

“I have voltage, so the device should work.”

Not always.

You may have voltage but no current path.

Mistake 2

Replacing a sensor without measuring.

The problem may be:

• Broken wire
• Missing common
• PLC input failure

Mistake 3

Ignoring resistance.

Resistance measurements can reveal:

• Open coils
• Shorted windings
• Damaged cables

Mistake 4

Confusing voltage with power.

A small sensor and a large motor can have different power requirements even if the voltage is similar.

Real Plant Example

A technician receives this call:

“The valve output is ON, but the valve does not move.”

A structured electrical approach:

Verify voltage

At PLC output:

24 VDC?

Verify wiring

At terminal block:

24 VDC?

Verify load

At solenoid:

24 VDC?

Verify resistance

Solenoid coil:

Expected resistance?

Verify current

Is the coil drawing current?

The problem can now be isolated without replacing unnecessary components.

Electrical Troubleshooting Mindset

A good automation technician thinks:

Voltage creates the push.

Current creates the action.

Resistance controls the flow.

Power creates the work.

Every electrical failure can be investigated by asking:

1. Is the correct voltage present?
2. Can current flow?
3. Is the resistance correct?
4. Is the device receiving enough power?

Key Takeaways

Voltage, current, resistance, and power are the foundation of industrial troubleshooting.

An automation technician who understands these concepts can:

✅ Diagnose sensors
✅ Troubleshoot PLC outputs
✅ Understand motor circuits
✅ Verify VFD problems
✅ Identify wiring failures
✅ Avoid unnecessary part replacement

Before troubleshooting advanced systems like PLCs, VFDs, and robots, master the electricity that makes them operate.

Practice Questions

1

A sensor requires:

24 VDC
100 mA

How much power does it consume?

2

A relay coil measures:

120 Ω

and is powered by:

24 VDC

What current does it draw?

3

A PLC output is ON, but a solenoid does not activate.

List three electrical measurements you would perform.

4

Explain the difference between:

• Voltage
• Current
• Resistance
• Power

using your own words.