Let’s explore the power of PicoScope’s Math channels to determine the effective voltage applied to a component when it’s controlled by a negative PWM or pull-down command. This technique is invaluable for automotive diagnostics, providing insights that traditional multimeters can’t.
This article focuses on the Volume Control Valve (VCV) attached to a Bosch High Pressure Diesel Pump, which is controlled via a negative PWM signal. By analyzing the signals from Channels B, C, and D, we can effectively measure the voltage and current flowing through the component.
Channel B below is the Negative PWM control of the VCV
Channel C is the power supply to the VCV
Channel D is the current flow through the VCV (Resistance 3.3 Ohms)
I have added the negative duty cycle math channel to Channel B Duty(-B), approx. 41%
I have also added the math channel C-B, which subtracts the negative PWM from the supply voltage to reveal the voltage differential across the VCV and where there is a differential, there will be current flow (assuming the load (VCV) is serviceable)
If you were to measure this voltage with a multimeter, the frequency of the PWM is changing at approximately 143 times per second (143 Hz) and so, depending on the sample rate of the meter, I am not sure what value will be displayed!
I have tested 2 meters across this VCV, one displays 4.65 V DC (Fluke), the other exactly 5 V DC (Megger)
Just for fun, I then switched the meters to AC to obtain 8.08 V AC non-TRMS (Fluke) and 9.05 V AC TRMS (Megger)
Looking at the measurement values above (all for channel B Negative PWM), our DC Average is 9.739 V and True RMS 12.35 V. These values highlight the ability of the scope (in comparison to the meters) to capture peak-to-peak values and incorporate this data into the numerically displayed measurement value. The sample rate for this capture was 2 MS/s!
Graphing average PWM signal #
Thanks to Martyn once again for his invaluable input. To graph the Average of the PWM, we can use (integral(B))/T, where we obtain approx. 9.597 V (T = Time in our math channel)
A tip here is to use a trigger on the rising edge of the PWM at 0% pre-trigger; this equation would trace a set value for the high part of the PWM cycle, and then slope down to the next rising edge. At this point of the math channel (where you meet the next rising edge), you will capture the average of the cycle. (See image below)
Based on Ohm’s law, I have added the math channel (C-B)/3.3, which demonstrates the theoretical current flow through the VCV based on the effective voltage across this component (Peak 4.408 A)
What I like about this math channel is how it demonstrates how theory and practice differ, as we are not accounting for the impedance of the VCV circuit, PCM control and temperature. The math channel in this scenario divides the voltage by a fixed resistance value (3.3 ohms), hence the instantaneous current rise time and peak value. (Food for thought)
Download the .psdata file here
Graphing RMS PWM #
Following on from graphing PWM above, we demonstrated how the math channel (integral(B))/T returns the average of the PWM (channel B).
How about graphing the RMS of the PWM signal of channel B?
Here we use sqrt((integral(B*B)/T)) to graph the RMS, where we obtain 12.32 V
We hope this helps and goes some way to remove the confusion surrounding the effective voltage across the component.
Download the .psdata file here