Understanding oscilloscope signals from brush cutter trigger

Thread Starter


Joined Oct 18, 2015
Hello everybody,

I dismantled the top part of a 36V Ryobi brush cutter. This machine has the following finger trigger, that controls the speed of its motor:


A voltmeter shows some 38V at the input and, when pressed (no matter how much), the same at the output. When not pressed, the output reading is 0V.

With the oscilloscope I used a probe with the 10x attenuation.

A. while the input coupling was set to DC, the volts/div to 2V, I got the following readings:

A.1 trigger not pressed:


A.2 trigger pressed in any position:


The line was sharp; only the picture is blurry. The line is positioned vertically at just below 2 units, which, seems to match the 38V on the voltmeter.

Decreasing the volts/div and lowering the waveform with the help of the Y-POS knob, led to a zoomed image, which started to reveal a blurred line, which is consistent to the AC reading. Unfortunately, I could not lower the waveform to infinity, since the Y-POS knob is limited. In other words, I could not zoom in to a point where I could see the details causing the blur.

B. while the input coupling was set to AC, the volts/div to 50mV, I got the following readings:

B.1 when the trigger was not pressed at all:


B.2 when the trigger was pressed a little bit:


C: when the trigger is pressed a little bit more:


D: when the trigger is pressed a even more:


When pressed all the way, the scope is showing the flat line in the beginning.

I am assuming this is PWM.

How does this actually work? Does the engine receive power during the time segments corresponding to the flat line?

The battery pack has the following markings:

36.0V, 144 Wh
Li-Ion 4.0Ah

The appliance label is:


Thank you.
Last edited:


Joined Mar 14, 2008
Why did you take it apart?
Is it not working?

With PWM the motor inertia and inductance averages the voltage value of the pulses to vary the speed.
The frequency of the pulses is unrelated to the motor speed, only the duty-cycle is of interest.
Thus a 50% duty-cycle of the PWM signal gives an average motor voltage of 1/2 the battery voltage.

Thread Starter


Joined Oct 18, 2015
Thank you very much for looking into it.

This appliance consists of a stick made up of two parts: a top one, with the battery and trigger and the bottom one with the motor and knife.

The motor gave up the ghost, so I sent in the entire bottom part to be serviced. Eventually I got a replacement comprising of both top part and bottom part. So now I have an extra top part. Being a bit of a tinkerer, I realized I could use the battery socket in the top part together with the battery for something else (not sure what for, actually).


Your explanation is at a level exceeding my current knowledge of electronics, but I am getting there. If you want, you can get into the weeds and possibly others would enjoy and benefit from our exchange. If not, maybe you could recommend some online resource starting with the basics of PWM for power electronics as applied here (assuming I got this much right). I barely know how to make simple measurements with an antique oscilloscope, although I know quite a bit of math.

I understand the basics of PWM with clean square signals, but in this case we don’t have a clean square signal...

I am not sure I understand what inductance and duty cycle are.

Found the following:

Inductance is the property of an electric conductor or circuit that causes an electromotive force to be generated by a change in the current flowing.

A duty cycle is the fraction of one period when a system or signal is active. We typically express a duty cycle as a ratio or percentage. A period is the time it takes for a signal to conclude a full ON-OFF cycle.

I am assuming:
  • a period is a block in the graph (between two imaginary vertical lines) that keeps repeating, in this case being composed of a falling curved line, a horizontal line and a sharply ascending line
  • the curved line would be a sort of a break, in which the motor doesn’t receive any power

Did I get this right?
Last edited:


Joined Nov 24, 2008
The duty cycle is the ratio of time spent in the different states 100*(active/(active+inactive)). This seems to be active low.

One question though. Is your scope set up for DC coupling or AC coupling? Hard to see with images posted.
Last edited:


Joined Nov 6, 2012
The Motor is actually "running on it's own stored-up magnetic-Energy",
for a very short period of time,
which is of course,
during the very short time-period(s) when there is no Current flowing from the Battery.

The stored-up Magnetic-Energy in the Motor is possible because
of the principles of Inductance and Mechanical-Inertia.

With PWM waveforms,
the only time that the Waveform has perfectly squared-off edges
is when the theory of it's operation is illustrated for the purpose of
understanding how it works.

PWM Power-Control can be very effective,
even with wild deviations from a perfectly Square-Edged-Waveform.

There will always be some deviation from "theoretical" "square-edged-Waveforms" that are
caused by "real-World" inefficiencies, and
Inductive and Capacitive interactions within the overall Circuit, which can not be completely avoided.

Sometimes, what might be considered "poor-design-practices" are really
a manufacturing "cost-cutting" consideration.

In other words,
the Engineers who designed the Speed-Regulating-System come up with
what they consider to be a fair and reasonable compromise in expense vs reliability.
Then the next guy down the line starts removing supposedly "unnecessary" components
until the device either doesn't work any more, or blows-up.
Then next time around he will remove all the previous parts except the last one which caused a failure.
Only then is the device is considered suitable for mass-production.

Jon Chandler

Joined Jun 12, 2008
The shape of the waveform is because the scope is AC coupled. A PWM signal should toggle between maximum and minimum voltage, but the corners of the square wave will usually show some distortion.

Keep in mind the positive voltage does not always equate to "on", and the switching mechanism may invert the signal. Motors are often connected to V+, and the other lead connected to ground to turn the motor "on".

In this case, the line from the switch may not be switching the motor directly – it may be activating some switching element like a mosfet.

The duty cycle is the on/off ratio of the square wave. 100% means the motor runs at full speed; 0% means the motor is stopped. If the duty cycle is 50%, the motor is on half the time and off half the time, so it's essentially operating at half the supply voltage and will be turning about 50% of full speed. Remember that 100% might be at V+ or ground, depending on how the motor (or LED, incandescent lamp, whatever) is wired.

For future reference, internal combustion "rotating things" are usually called engines; electrical "rotating things" are usually called motors.

Thread Starter


Joined Oct 18, 2015
Thank you, everybody, for looking further into this.

The original pictures were taken with AC input coupling. I just updated them and added two new ones with the DC coupling. I also removed some questions from my second post.

Concerning the word inverted, the +/- button was not pressed.

I never opened the motor assembly.

It seems to me that:
  1. the motor gets controlled at about 38V by varying the lengths of the curved and of the straight lines
  2. these lines form a rectangular waveform of sorts, with the top part distorted and the bottom part clean (not distorted)
  3. the straight lines at the bottom represent the active state, when the motor receives power
  4. the curved lines at the top represent the inactive state, when the motor does not receives power
  5. the signal is inverted because the active part is at the bottom and the inactive at the top and not the other way around
  6. this variation changes the duty cycle from 0 (or a small number very close to 0), corresponding to the trigger/switch not being pressed at all (or being pressed just a little bit) to 1, corresponding to the trigger being pressed all the way
  7. without inspecting the motor assembly, we cannot know for sure if this control is direct or indirect via some other components close to the motor (like a MOSFET transistor)

Did I get this right?
Last edited:


Joined Nov 6, 2012
A PWM Circuit Switches On and Off at a specific Frequency,
which normally never changes,
only the Off-Time vs the On-Time is adjustable.