Well i must say that it is odd the manually calculating the average current using the scope values,using the scope to calculate the average current value,and the DMMs average current value all result in the same amount,while using the RMS value results in near twice the valuesuch as Itsu has also found using his current probe RMS value.
Indeed. Using the average current would result in a significant error, wouldn't you agree? But anyway,if i use a battery,and switch the circuit on and off by way of the SG switching a mosfet at the same frequency and duty cycle,we can then trust our DMM to give us an accurate average current valuecorrect?
Yes, using the average DC input current is valid in this case. There is a consideration with this setup however: The device is only using milliwatts of power so there is a chance the SG driving the MOSFET gate could skew the P(in) measurement. In the RA case we were dealing with 30W or so of power, therefore not really a concern. You could insert a hefty series gate resistance to limit the effect, but that could slow down your switching as well. This being the case,and our scopes calculated average current value is the same as that of the DMM,and my manually calculated current value is also the same,then that would mean that my methods of calculating the P/in are correctyes?
Not quite. I suspect you are assuming you will be correcting the 12V battery (for example) by the duty cycle, when in fact that would not be correct. The duty cycle correction already takes place during the current measurement via the DMM's averaging. Since we are already using the DMM to obtain the average current, we don't need nor want to apply the duty cycle correction twice, which is what you would be doing if you corrected the voltage also. Assuming minimal power loss in the wiring and the MOSFET, and assuming that the source power will then equal the power into the transfomer primary, the goal is to determine the power being supplied by the source, agreed? If a CVR is in series with the battery, the loss there can easily be subtracted out. Quick review: The power from a DC source is determined by the average voltage across it times the average current through it. So, you will be measuring voltage across the battery, not the primary of the transformer. As such, you could use 3 DMMs and turn the scope off now that both your source and your load are DC. But how did you average the voltage in the Ainslie circuit,as it would be the supply voltage x duty cycle to get the average voltage over a full cycle. That is how the math works on a scopeis it not?,where V and I are averaged,and then multiplied together ?
As per above, you don't correct for duty cycle when measuring the battery supply voltage, the DMM current measurement already automatically does the correction on the current (which is the parameter that is pulsing) for you. Performing the duty cycle correction twice (once on the battery voltage and once on the battery current) would be really uncool.
« Last Edit: 20180904, 03:27:33 by poynt99 »
