How to Measure PWM Duty Cycle with FNIRSI DSO-152 Plus Malaysia

How do you measure the duty cycle of a PWM signal with the FNIRSI DSO-152 Plus Mini Digital Oscilloscope?

This is a useful measurement when troubleshooting:

Arduino PWM outputs

Motor controllers

Fan controllers

LED dimming circuits

555 timer circuits

Microcontroller outputs

Pulse-control circuits

A multimeter may show an average voltage, but an oscilloscope allows you to see the actual switching waveform.

This makes it much easier to understand whether the PWM signal is really changing.

What Is PWM?

PWM means:

Pulse Width Modulation

Instead of continuously changing the output voltage, a circuit rapidly switches between different states.

For a typical digital PWM signal, this may look like:

HIGH → LOW → HIGH → LOW

The proportion of time spent HIGH during each cycle is called the:

Duty cycle

What Does 50% Duty Cycle Mean?

A 50% duty-cycle waveform spends approximately:

Half of each cycle HIGH

and

Half of each cycle LOW

If one complete cycle takes:

10 ms

and the HIGH portion lasts:

5 ms

then:

Duty Cycle = 5 ms / 10 ms × 100%

which gives:

50%

What Does 25% Duty Cycle Mean?

Suppose one complete cycle is:

10 ms

but the HIGH time is only:

2.5 ms

Then:

Duty Cycle = 2.5 / 10 × 100%

which gives:

25%

The pulse will appear narrower than a 50% duty-cycle waveform.

What Does 80% Duty Cycle Mean?

If a waveform remains HIGH for most of the cycle, the duty cycle may be relatively high.

For example:

HIGH time = 8 ms

Total period = 10 ms

Duty cycle:

8 / 10 × 100% = 80%

The oscilloscope makes this difference visually obvious.

Duty Cycle Formula

The basic formula is:

Duty Cycle = HIGH Time / Total Period × 100%

The total period includes:

HIGH time + LOW time

For a repetitive waveform, measure one complete cycle.

Frequency and Duty Cycle Are Different

This is one of the most important concepts to understand.

Frequency tells you how many complete cycles occur every second.

Duty cycle tells you how much of each cycle is spent HIGH.

A PWM controller can change duty cycle without significantly changing frequency.

Example: Motor Speed Control

Suppose a motor controller operates at a fixed PWM frequency.

At low command:

20% duty cycle

At medium command:

50% duty cycle

At high command:

80% duty cycle

The PWM frequency may remain similar while the pulse width changes.

Therefore, checking only frequency may not tell you whether the command is changing.

Why Is an Oscilloscope Useful for PWM?

A multimeter may display an average or interpreted voltage.

For example, a PWM signal switching between approximately:

0 V

and

5 V

may produce a multimeter reading somewhere between those values depending on the signal and meter.

But that does not directly show the pulse pattern.

An oscilloscope allows you to see:

HIGH level

LOW level

Frequency

Pulse width

Duty cycle

Noise

Signal stability

Step 1: Identify the PWM Signal

Before connecting the DSO-152 Plus, identify:

Which wire or test point carries PWM?

Do not assume every wire on a motor, fan or controller is PWM.

For example, a fan system may include:

Power

Ground

PWM command

Tachometer feedback

These signals have different purposes.

Step 2: Determine the Expected Voltage

Find out what voltage level the PWM signal should use.

Common low-voltage electronics may use:

3.3 V logic

or

5 V logic

but other systems can be different.

Never assume a PWM signal is automatically low voltage.

Step 3: Connect the Probe Correctly

Connect the probe to the appropriate PWM signal.

Connect the reference to the correct circuit reference.

Incorrect reference connections can cause:

Wrong readings

Noise

Unstable waveform

and, in some circuits, potentially unsafe conditions.

Understand the circuit before probing.

Step 4: Adjust the Voltage Scale

Set the vertical scale so that both the HIGH and LOW portions are clearly visible.

You want to see the complete switching amplitude.

If the waveform is too small, increase display sensitivity appropriately.

If it goes outside the display, use a more suitable range.

Step 5: Adjust the Time Base

Adjust the horizontal time scale until several PWM cycles are visible.

If too many cycles are compressed together, pulse width becomes difficult to inspect.

If the scale is too fast, you may see only part of a single cycle.

A clear display should make HIGH and LOW timing easy to distinguish.

Step 6: Stabilise the Waveform

Use an appropriate trigger.

For a repetitive PWM waveform, triggering on an edge can help keep the waveform stable on the display.

Choose a suitable trigger level between the LOW and HIGH levels.

Step 7: Measure One Complete Period

Identify two equivalent points on consecutive cycles.

For example:

Rising edge → next rising edge

The time between them is:

One complete period

Call this:

T

Step 8: Measure the HIGH Time

Now measure how long the signal remains HIGH during the same cycle.

Call this:

Ton

Then calculate:

Duty Cycle = Ton / T × 100%

Example: Arduino PWM

Suppose you measure an Arduino PWM output.

One complete period is:

2 ms

The HIGH portion is:

1 ms

Therefore:

Duty Cycle = 1 / 2 × 100%

= 50%

If the software changes the PWM command, you may see the HIGH portion become wider or narrower.

Example: 20% PWM

Suppose:

Period = 5 ms

HIGH time = 1 ms

Then:

Duty Cycle = 1 / 5 × 100%

= 20%

The waveform should show a relatively narrow HIGH pulse.

Example: 80% PWM

Suppose:

Period = 5 ms

HIGH time = 4 ms

Then:

Duty Cycle = 4 / 5 × 100%

= 80%

The signal remains HIGH for most of each cycle.

How PWM Controls LED Brightness

PWM is commonly used to control LEDs.

At lower duty cycle, the LED receives shorter ON periods.

At higher duty cycle, the ON period increases.

Depending on the circuit and switching speed, the human eye may perceive this as a change in brightness rather than visible flashing.

The oscilloscope reveals the switching that your eyes cannot see.

Testing an LED Dimmer

If an LED dimmer is not responding correctly, check:

Is PWM present?

Then adjust the brightness control.

Observe whether:

Duty cycle changes

If the PWM command changes correctly but the LED output does not respond, the problem may be further downstream.

How PWM Controls a DC Motor

PWM is also widely used for DC motor speed control.

A controller may vary the effective power delivered to the motor by changing switching behaviour.

An oscilloscope can help confirm whether the control signal changes when the speed command changes.

However, be careful.

The control PWM signal and the motor power switching node are not necessarily the same measurement point.

Control Signal vs Power Stage

This distinction is important.

A microcontroller may generate a low-voltage PWM signal.

That signal may then drive:

Gate driver

MOSFET

H-bridge

Power transistor

The low-voltage controller output may be suitable for basic testing.

The power stage may involve:

Higher voltage

High current

Fast transients

Floating switching nodes

Do not treat these points as equivalent.

Testing a Fan PWM Signal

Some electronic fans use PWM speed control.

If the fan does not change speed, you may check whether the PWM command changes when the speed setting changes.

For example:

Low speed command → narrower pulse

High speed command → wider pulse

But confirm the actual fan specification.

Do not assume every fan uses the same PWM frequency, voltage or polarity.

PWM and Tachometer Signals Are Different

A fan may also provide a tachometer signal.

The tachometer reports information about:

Fan rotation or speed

The PWM line provides:

Control command

If you probe the wrong wire, you may see pulses but interpret them incorrectly.

Always identify the signal first.

Testing a 555 Timer Duty Cycle

A 555 timer circuit is another good PWM and duty-cycle learning example.

Depending on the circuit configuration, resistor values and capacitor values affect timing.

You can measure:

Total period

HIGH time

LOW time

Then calculate the duty cycle.

This allows students to compare theoretical calculations with actual circuit behaviour.

Why Is My Measured Duty Cycle Different from the Calculation?

Possible reasons include:

Component tolerance

Capacitor tolerance

Circuit design

Measurement estimation

Supply conditions

Oscilloscope limitations

Real circuits do not always produce exactly the theoretical value.

A small difference does not automatically indicate a fault.

Why Does My PWM Waveform Look Rounded?

A PWM waveform ideally has fast transitions.

But the displayed waveform may appear rounded because of:

Oscilloscope bandwidth

Sampling limitations

Probe characteristics

Circuit loading

Signal source characteristics

Measurement connection

This becomes increasingly important at higher frequencies.

Does a Rounded Waveform Mean the PWM Is Bad?

Not necessarily.

The displayed waveform is influenced by both:

Actual circuit

and

Measurement system

Before blaming the controller, consider whether the oscilloscope and probe can reproduce the signal accurately.

Why Is the PWM Waveform Unstable?

Check:

Trigger level

Trigger mode

Probe connection

Reference

Signal noise

Time base

The PWM source itself may also be unstable.

A stable repetitive PWM signal should normally be easier to display with suitable trigger settings.

Why Does Duty Cycle Appear to Change Randomly?

Possible causes include:

Actual changing control command

Poor trigger

Noise

Intermittent connection

Incorrect time-base interpretation

Signal outside measurement capability

Some control systems intentionally change PWM dynamically.

Make sure the duty cycle is expected to remain constant before diagnosing it as unstable.

PWM Is Present but Motor Does Not Run

Do not immediately blame the PWM generator.

Check:

Motor power

Driver supply

Enable signal

Direction control

Gate/driver stage

Motor itself

Mechanical load

Protection condition

The PWM signal is only one part of the complete motor-control system.

PWM Is Present but LED Does Not Light

Possible causes include:

LED fault

Incorrect polarity

Current-limiting component

Driver fault

Missing power

Signal does not reach driver

Output stage problem

Trace the signal from the PWM source toward the load.

PWM Exists at Microcontroller but Not at Driver

This is an excellent PCB troubleshooting clue.

Suppose:

MCU output = PWM present

but

Driver input = Flat line

Investigate the path between them.

Possible components include:

Resistor

Buffer

Optocoupler

Logic gate

Connector

PCB track

Signal tracing helps identify where the waveform disappears.

PWM Reaches Driver but No Power Output

Now the fault investigation moves to another stage.

Check:

Driver supply

Enable

Protection

Power transistor

Load

But be careful when moving from low-voltage control electronics into power circuitry.

A DSO-152 Plus should not be assumed suitable for arbitrary power-switching measurements.

Can DSO-152 Plus Measure Every PWM Signal?

No.

The DSO-152 Plus is an entry-level mini oscilloscope with limited bandwidth and sampling performance compared with higher-performance oscilloscopes.

It is best suited to appropriate lower-frequency electronics and basic waveform troubleshooting.

Fast PWM, high-speed digital signals and precision switching analysis may require a more capable instrument.

Does Trigger Setting Increase the Maximum Frequency?

No.

Triggering helps:

Stabilise

or

Capture

a waveform.

It does not increase:

Bandwidth

Sampling rate

Input rating

or

Measurement accuracy

These are separate instrument characteristics.

PWM Frequency vs Switching Edge Speed

Even if the PWM repetition frequency appears relatively low, the rising and falling edges may contain much faster components.

Therefore, a low PWM frequency does not guarantee that an entry-level oscilloscope will reproduce every detail of the switching edge accurately.

For basic duty-cycle checking, this may be acceptable.

For detailed switching analysis, use more appropriate equipment.

DSO-152 Plus vs Multimeter for PWM

A multimeter is useful for:

Supply voltage

Continuity

Resistance

Basic DC checks

The DSO-152 Plus is useful for seeing:

PWM waveform

Frequency

HIGH time

LOW time

Duty cycle

Intermittent pulses

For PWM troubleshooting, these instruments complement each other.

DSO-152 Plus vs Logic Analyzer for PWM

If you mainly need to see:

Actual voltage waveform

an oscilloscope is useful.

If you need to analyse many digital channels simultaneously, a logic analyser may be more suitable.

For a single basic PWM signal, the oscilloscope provides a direct view of the electrical waveform.

Quick PWM Duty Cycle Troubleshooting Checklist

Before trusting your measurement, check:

1. Is this definitely the PWM signal?

2. What voltage should it use?

3. Is the circuit reference correct?

4. Is the probe connected properly?

5. Can you clearly see HIGH and LOW?

6. Is the waveform stable?

7. What is the complete period?

8. What is the HIGH time?

9. Does duty cycle change when the command changes?

10. Is the signal within the oscilloscope's practical capability?

This provides a much more reliable approach than looking only at average voltage.

Safety When Measuring PWM

PWM signals are used in both low-voltage electronics and high-energy power systems.

Do not assume:

“It is PWM, therefore it is safe.”

PWM may exist in:

Arduino circuits

but also in:

Inverters

Motor drives

Power supplies

H-bridges

High-voltage switching circuits

Identify the voltage, reference and circuit topology before connecting an oscilloscope.

Do not casually probe mains-referenced or floating power switching nodes with an entry-level oscilloscope.

FNIRSI DSO-152 Plus PWM Duty Cycle Measurement Malaysia

The FNIRSI DSO-152 Plus can be a practical entry-level oscilloscope for learning and troubleshooting suitable lower-frequency PWM signals.

It can help answer:

Is PWM present?

What is the approximate frequency?

Is the pulse getting wider or narrower?

Does duty cycle change with the control command?

Does the PWM signal reach the next circuit stage?

For students, DIY electronics users and technicians, these measurements are useful for understanding:

Arduino

555 timers

LED controllers

Fan controls

Motor-control circuits

Microcontroller outputs

MTM Precision supplies FNIRSI oscilloscopes and electronic test instruments in Malaysia.

If you are troubleshooting a PWM circuit, send us:

A photo of the PCB or controller

Supply voltage

Expected PWM frequency if known

Application – motor, fan, LED, Arduino or other

Photo or video of the waveform

through WhatsApp.

We can help determine whether the FNIRSI DSO-152 Plus is suitable or whether a higher-performance oscilloscope is required.

MTM Precision Sdn Bhd

Showroom & Service Centre:
No. 29-1 & 2, Jalan Bandar 18,
Pusat Bandar Puchong,
47160 Puchong, Selangor, Malaysia.

Tel: 03-8080 7172
WhatsApp: +6016-660 7346
Email: mtmpre@yahoo.com
Website: www.mtmpre.com.my

Supplying FNIRSI and electronic test instruments throughout Selangor, Kuala Lumpur, Johor, Penang, Melaka, Negeri Sembilan, Perak, Pahang, Kelantan, Terengganu, Kedah, Perlis, Sabah and Sarawak.