Why Is My Oscilloscope Voltage Reading Wrong? FNIRSI DSO-152 Plus Troubleshooting Malaysia
Why Is My Oscilloscope Voltage Reading Wrong? FNIRSI DSO-152 Plus Troubleshooting Malaysia
You connect the FNIRSI DSO-152 Plus to an electronic circuit and successfully see a waveform.
But something does not look right.
You expected approximately 5 V, but the voltage shown on the oscilloscope appears different.
Does this mean the DSO-152 Plus is inaccurate or faulty?
Not necessarily.
Incorrect or unexpected oscilloscope voltage readings can result from several factors, including probe attenuation settings, measurement method, waveform type, coupling selection, signal reference and instrument limitations.
Before deciding that the oscilloscope has a problem, check the complete measurement setup.
1. Check the Probe Attenuation First
One of the most common causes of an incorrect oscilloscope voltage reading is a mismatch between the probe attenuation and the oscilloscope setting.
Oscilloscope probes may use attenuation ratios such as:
1X
or
10X
depending on the probe and application.
If the physical probe is set to one attenuation ratio while the oscilloscope assumes another, the displayed voltage can be incorrect.
For example, the waveform shape may look normal while the amplitude appears much higher or lower than expected.
Before troubleshooting anything else, confirm:
What attenuation is the probe using?
and:
Is the oscilloscope configured for the same setting?
2. Why Does 1X vs 10X Matter?
A 10X probe attenuates the signal before it reaches the oscilloscope input.
The oscilloscope must account for this attenuation when displaying the voltage.
If the settings do not match, the displayed value may be off by a significant factor.
This can create a situation where a beginner thinks:
“The circuit voltage is wrong.”
when the actual problem is simply:
“The probe setting is wrong.”
Always check the probe configuration before making conclusions about the circuit.
3. Check What Voltage You Are Comparing
Another common mistake is comparing two different types of voltage measurement.
For example, you may be comparing:
Peak voltage
with
Peak-to-peak voltage
or:
RMS voltage
with
Peak voltage
These values are not necessarily the same.
A waveform can therefore have several valid voltage values depending on how it is measured.
Before saying the oscilloscope and multimeter disagree, determine exactly what each instrument is displaying.
4. What Is Peak-to-Peak Voltage?
Peak-to-peak voltage describes the difference between the highest and lowest points of a waveform.
For a waveform that swings between:
0 V and 5 V
the peak-to-peak value is approximately:
5 Vpp
But consider a waveform that swings between:
-5 V and +5 V
Its peak-to-peak voltage is approximately:
10 Vpp
even though the positive peak is only +5 V.
Understanding this distinction is essential when interpreting oscilloscope measurements.
5. Why Does the Multimeter Show a Different Voltage?
A digital multimeter and an oscilloscope do not necessarily measure and display signals in the same way.
A multimeter may display:
DC voltage
AC RMS voltage
or another processed numerical value depending on the meter and selected function.
An oscilloscope displays the waveform versus time.
This means two instruments can show different numbers without either instrument necessarily being defective.
The first question should be:
Are both instruments measuring the same electrical quantity?
6. Example: PWM Signal
PWM is a good example.
Imagine a digital signal switching between approximately:
0 V and 5 V
A multimeter may display an intermediate numerical value depending on the duty cycle and measurement method.
The oscilloscope allows you to see that the actual signal is switching between LOW and HIGH states.
For example, the oscilloscope may show a pulse waveform reaching approximately 5 V while the multimeter displays a lower average-type reading.
Both readings may provide useful information.
They are simply describing the signal differently.
7. Check AC or DC Coupling
Coupling selection can also change what appears on the screen.
With DC coupling, the oscilloscope displays both the DC component and changing portion of the signal.
With AC coupling, the DC component is blocked.
This can be useful when you want to inspect a smaller AC variation sitting on top of a larger DC level.
But if you forget that AC coupling is selected, the waveform position and interpretation may be different from what you expect.
Before comparing voltage measurements, check the coupling setting.
8. Check the Zero Reference
Make sure you understand where 0 V is located on the oscilloscope display.
If the waveform is vertically shifted, a beginner may incorrectly interpret its amplitude or voltage level.
For example, a waveform displayed above the centre of the screen does not automatically mean the centre line represents 0 V.
Check the channel's reference position.
This is particularly important when evaluating DC offset.
9. Check the Volts per Division Setting
The vertical scale determines how much voltage each division on the display represents.
If the setting is:
1 V/div
then one vertical division represents 1 V.
If the setting changes to:
2 V/div
the same waveform will occupy less vertical space.
Understanding V/div is one of the fundamental skills when using any oscilloscope.
Do not judge voltage only by how “big” the waveform looks on the screen.
Always consider the scale.
10. Is the Signal Stable?
An unstable signal can also cause changing voltage measurements.
If the amplitude continuously changes, you may see different values from one moment to another.
Possible causes include:
Unstable power supply
Changing load
Intermittent connection
Circuit fault
Electrical noise
Changing control signal
In this case, the oscilloscope may actually be showing useful information about the circuit.
11. Check the Probe Connection
Poor probe contact can affect measurement quality.
Make sure the probe tip has a stable connection to the intended test point.
Also check the reference connection.
An intermittent connection can cause:
Changing amplitude
Noisy waveform
Signal disappearing
Unexpected voltage readings
If touching or moving the probe changes the measurement significantly, inspect the physical connection before blaming the instrument.
12. Is the Signal Within the Oscilloscope Capability?
The DSO-152 Plus is an entry-level mini oscilloscope.
Like every oscilloscope, it has measurement limits.
If the signal frequency, rise time or other characteristics exceed the instrument's capability, the displayed waveform may not accurately represent the original signal.
This can also affect amplitude interpretation.
A waveform appearing on the screen does not automatically mean every characteristic of that waveform is being measured accurately.
For demanding measurements, use an oscilloscope with suitable performance for the signal.
13. Don't Compare a Mini Oscilloscope with a Multimeter Without Understanding the Signal
Suppose you measure the same test point with:
Digital Multimeter = 2.4 V
and
Oscilloscope = pulses from approximately 0 V to 5 V
Which instrument is correct?
Potentially both.
The multimeter may be showing a numerical representation of the changing signal.
The oscilloscope is showing the actual voltage variation over time.
This is exactly why oscilloscopes are useful in electronics troubleshooting.
They reveal information that one numerical value cannot always explain.
14. Use a Known Voltage or Signal for Checking
If you suspect the oscilloscope measurement is incorrect, start with a known suitable source.
The objective is to separate:
Instrument/setup problem
from
Unknown circuit behaviour
Use a safe and appropriate known source within the instrument's specifications.
Then check whether the displayed measurement behaves as expected.
If it does, return to the original circuit and investigate the signal itself.
15. When Should You Suspect an Instrument Problem?
After checking:
Probe attenuation
Probe connection
Voltage scale
Zero reference
AC/DC coupling
Signal type
Measurement method
Instrument limits
and testing against an appropriate known source, you may have stronger reason to investigate the instrument itself if readings remain clearly abnormal.
Avoid concluding that the oscilloscope is faulty based on only one unknown circuit measurement.
Quick Voltage Troubleshooting Checklist
If your FNIRSI DSO-152 Plus voltage reading looks wrong, check:
1. Is the probe set correctly?
2. Does the oscilloscope probe setting match?
3. Are you reading peak, peak-to-peak, RMS or DC voltage?
4. Is AC or DC coupling selected correctly?
5. Where is the 0 V reference?
6. What is the V/div setting?
7. Is the probe connection stable?
8. Is the circuit reference correct?
9. Is the signal within the oscilloscope's measurement capability?
10. Have you compared it with a known suitable signal or voltage?
These checks can solve many apparent voltage-reading problems without replacing the instrument.
FNIRSI DSO-152 Plus Voltage Troubleshooting Malaysia
The FNIRSI DSO-152 Plus Mini Digital Oscilloscope can help technicians and electronics learners understand signals that cannot be fully explained by a normal voltage reading.
But using an oscilloscope correctly requires understanding the measurement setup.
If the displayed voltage seems wrong, do not immediately conclude:
“The oscilloscope is inaccurate.”
First check the probe, attenuation, scale, coupling, waveform type and measurement method.
MTM Precision supplies FNIRSI electronic test and measurement instruments in Malaysia.
If you are unsure about a measurement, you can send us:
A photo of the oscilloscope screen
A photo of the probe connection
The circuit or equipment being tested
The voltage you expected
Your multimeter reading if available
through WhatsApp.
This can help us understand whether the difference is caused by the instrument setup, waveform characteristics or application.
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 test and measurement instruments throughout Selangor, Kuala Lumpur, Johor, Penang, Melaka, Negeri Sembilan, Perak, Pahang, Kelantan, Terengganu, Kedah, Perlis, Sabah and Sarawak.
06 Oct 2026
