There is a comment in the forum: "How to use Picoscope software to display the "effective" voltage on the component when the component is controlled by negative PWM?"
I have been testing the flow control valve (VCV) on the Bosch high-pressure diesel pump, which is controlled by a negative PWM signal. In Figure 1, channel B is the negative PWM control signal of VCV, channel C is the power supply voltage of VCV, and channel D is the current flowing through VCV (resistance is 3.3Ω).
I applied the negative duty cycle math channel to the B channel: duty (-B), which is about 41%. I also added the math channel CB to subtract the negative PWM voltage from the power supply voltage to calculate the voltage difference across VCV. If there is a voltage difference , VCV in it will be a current flows (assuming VCV available).
Figure 1 Measurement function
The PWM signal changes approximately 143 times per second (143 Hz), so if I want to measure this voltage with a multimeter, I am not sure what value will be displayed!
I have used two multimeters to measure VCV, one shows 4.65V DC (Fluke multimeter) and the other shows 5V DC (Megger multimeter). Out of curiosity, I switched the multimeter to the AC gear and continued to measure. One showed 8.08V AC non-TRMS (Fluke multimeter) and 9.05V AC TRMS (Megger multimeter).
Check the measurement value in Figure 1 (all measurements are the negative PWM signal of channel B), the average DC is 9.739 V, the true root mean square is 12.35V, this is the measurement function of the software. Compared with a multimeter, an oscilloscope captures the signal between each wave crest, integrates and quantifies the data into a measurement value. The sampling rate when the signal is captured in Figure 1 is 2MS/s!
Plot the average PWM waveform:
To plot the average value of the PWM waveform, we can use the math channel (integral(B))/T, and calculate the average value of the PWM to be about 9.597 V (T is the time in the math channel).
Here is a reminder that you can use the trigger on the rising edge of the PWM, and the pre-trigger time is set to 0%. In this setting, the software will trigger from the higher set voltage value in a certain period of the PWM signal, and then the PWM signal will slope downward, and then to the next rising edge. At this point, when the math channel encounters the next rising edge, the software will capture the average value within the period (see Figure 2).
Figure 2 PWM average waveform
According to Ohm's law, I added math channel (CB)/3.3 to Figure 2. This formula measures the theoretical current value (peak 4.408 A) under the effective voltage across the VCV.
The advantage of this math channel is that it clearly shows the difference between the theoretical value and the actual value. This is because we did not take into account the impedance, PCM control and temperature of the VCV circuit. In this case, the math channel divides the voltage by a fixed resistance value (3.3Ω), so the instantaneous current becomes larger and peaks appear.
After acquiring the PWM signal (channel B), we have previously demonstrated how to use the math channel (integral(B))/T to draw the average PWM waveform. But how to draw the curve of the effective value of the B- channel PWM signal ?
As shown in Figure 3, we used the mathematical channel sqrt((integral(B*B)/T)) to draw a 12.32 V RMS curve.
Figure 3 PWM effective value waveform