Accurate Measurements on pulsed system's harder than you think.

[gyulasun]

Hi Laurent,

Just a quick notice: what if you change channel 1 with channel 2 to check how the shift you presently have on the channel 2 waveform affects the mean value of voltage amplitude? perhaps you would have a more accurate reading for the mean current on channel 1 then? 
Can it be a DC shift you have now on channel 2 because of the scope channel 2 input has a DC shift problem?

Gyula

[woopy]

Hi Laurent,

Just a quick notice: what if you change channel 1 with channel 2 to check how the shift you presently have on the channel 2 waveform affects the mean value of voltage amplitude? perhaps you would have a more accurate reading for the mean current on channel 1 then? 
Can it be a DC shift you have now on channel 2 because of the scope channel 2 input has a DC shift problem?

Gyula

Hi Gyula

Thank's for your always  very appreciated  input

Yes there is a very small difference when i invert probe of channel 1 and   probe of channel 2 , which suggest me that i have to invest in brand new probes for my next further experiment.

But this small difference can not explain what i  am working on and this 3.85 time LESS energy calculation of the analogic meters in comparison to the "instant power "
showed  in the Vcc scope shall be deeply investigated

Laurent

[gyulasun]

Hi Laurent,

I am afraid it is not probes problem but input circuit problem of scope channel 2, the problem would stay the same with any new probe I think. 
Perhaps in the scope's Service Manual if there is such somewhere on the web, the DC offset adjustment could be figured out whether it has an inner potmeter to readjust the zero line.

EDIT  perhaps in the Menu there is a possibility for offset calibration for the input channels, try to go through the Manual.
This scope has a Math function too, so you can multiply channel 1 with channel 2 (of course the result you get on display may also be affected by the small offset problem of channel 2)

Gyula

[tinman]

Hi all

Just a video to expose my problem with those "mean input pulsed power calculation"

https://youtu.be/0QWdtTItfHU

It seems to me that analogic instruments just have the ability to feel the right energy, because they don't calculate, they are feeling the right dissipated power all along the pulse and the mechanical device is able to average that power perfectly, perhaps better than the scope.

Some will say that the inertia of the mechanical device will somehow "brake " the reading, but i think that when the frequency of an event is high enough, the max amplitude should be reached at some point. But this is not the case, and i have tried from 50  Hz up to 100 Hz and the result stays the same.

Voila

Laurent

Great video Laurent,and shows very clearly what i mean when saying that measuring pulsed systems is not so easy.

There is much more to come yet,and great to see you are seeing such a big difference between your two measurements- in that you now have reason to look a little closer as to why the measurements are 300%+ out from each other.


Brad

[MileHigh]

I agree that Laurent made a great clip.  The one missing thing is one simple thing:  You can check it by working it out on paper, you don't need to rely on any derived meter readings at all, just the basic measurements will suffice.  The frequency is low so we can ignore reactive effects.

Let's say the current is when the MOSFET is ON is (12.22 volts/101 ohms) = 121 milliamps.

Let's say the voltage is (12.22 volts x 100/101) = 12.10 volts

The instantaneous power is 12.10 volts x 0.121 amps = 1.46 watts

Let's keep it simple and say the 20% duty cycle is ON for one second and OFF for four seconds.  So that 1.46 Joules for every five seconds.

The average power is (1.46 Joules/5 seconds) = 0.292 watts of average power.  We calculated this ourselves and know this to be true within the limits of the data we collected.

If we use the scope voltage of 1.26 volts we calculate the average power as 0.315 watts.

If Laurent scoped the voltage output from the battery and took accurate measurements of the resistances of the two resistors and did as accurate a measurement as possible of the duty cycle with his scope we could fine tune our calculation.  The voltage output from the battery almost certainly takes a dip when the MOSFET switches on and that is an unknown.  We don't know what the voltage drop across the MOSFET is.

The scope method using peak values and factoring in the duty cycle measured 0.337 watts.
The analog meter method measured 0.0875 watts.
The scope averaging method measured 0.0559 watts.

It looks like the scope method using peak values and factoring in the duty cycle is the "correct" value.  I put correct in quotations because your calculated values and your measured values have to be much closer together than we see to get satisfaction.  Why are you seeing these discrepancies?  If you are serious it merits a serious follow-up investigation.  How can you be sure if any of your measurements are going to be good if you don't understand exactly what is taking place in this clip?

There are two things to contemplate here that can affect what's going on:

1.  This is not truly a circuit that pulses a voltage (i.e. the drive voltage for the circuit) that goes from zero volts to 12 volts.  What it is is a MOSFET switch that makes a connection and then breaks a connection and goes high-impedance.  There is no "zero volt" signal.  Open-circuit can be way way different from zero volts, especially as you go towards higher frequencies.

2.  It would be very interesting to see what you would measure with digital multimeters for the voltage and the current measurements.  Do not use an in-line ammeter, just use a digital multimeter to measure the average voltage across the one-ohm resistor.  Digital multimeters can be deadly accurate at making average measurements because they are integration-based measuring devices.

It's amazing how the simple measurements in this simple circuit do indeed present a challenge to the experimenter.  If I was in your shoes, I would investigate this from A to Z and understand exactly what was happening.  Do not forget to use your own head, you don't necessarily need to use the full measurement capabilities of your instruments.  Just do the basic measurements and then do the calculations on paper.  That can be used to double-check what is going on.

MileHigh