Power Measurement Basics

[poynt99]

Since I have no previous training or background in EE I can understand the logic in the measurement practice you've presented. So I think we can move forward from here.

Good. I think you have a solid enough foundation to decide if a process or the logic of one makes any good sense or not. These videos are aimed more or less at a beginner level of electronics, so they will not go into the degree of detail that others might feel is necessary.

I've been around these forums for quite some time as you know, so I feel I have a good sense for what points need to be stressed in order that important concepts are understood. And for me that is the ultimate goal of these videos, to understand the concepts and be able to apply them in your testing.

However, I'm noting your logic is affecting those with EE background and wondering how it will all transform?

Thanks for being very open

Luc

There is often more than one way to accomplish a task, as you well know. And often you will find different parties unable to agree upon the "best" or "most standardized" way of performing some task, such as power measurement. Such is the case here with Mark and I.

Again, I have the advantage of being involved in these forums for years, and as such I feel I can get to the root of the problem quickly and efficiently without overloading anyone's learning process with minor, relatively insignificant details.

The bottom line is this: Do we measure a voltage source with the probe's gnd lead on the source's +'ve or -'ve terminal? I have never seen anyone on these forums measure a battery, DC lab supply, or AC source for that matter with the probe's gnd lead on the positive or hot terminal, have you? And would you ever do that? No, I doubt it. I know I would only in some rare case.

So that decision in itself establishes your approach and procedures that follow. And unless someone can convincingly explain why this approach is flawed, I shall continue along these lines with future videos. That is; Pos to Pos, and Neg to Neg wrt to sources.

[gotoluc]

GoToLuc Poynt99's engineering is first rate.  He absolutely knows his subject matter.  My entire concerns revolve around how to effectively get his message across.

Thanks MarkE for making that point clear

Luc

[TinselKoala]

The issue of the proper use of the scope's AC coupled input function has come up before and apparently it is still confusing.

Maybe it would help to know how the scope accomplishes the AC coupled input. Every scope I've seen does this simply by switching in a capacitor in series with the probe input lead, immediately where it enters the scope's pre-amp/attenuator circuit. Old analog scopes like the Tek RM503 use a manual slide switch and a capacitor. Newer DSOs use a software-controlled relay.... and a capacitor. You might be able to hear the relay clicking, when you select AC coupled in the software on a modern DSO; I can hear it easily in the older Link DSO (when the relay doesn't stick, that is.)

The capacitor will block DC and very slow AC oscillations (like below 5 Hz or whatever, determined by the coupling capacitor value) and will only allow the AC component of any signal to pass to the scope's electronics. This has the effect of bringing the _average_ value of the input signal, to the scope channel's "zero voltage reference" line. This could be either a raising up or a lowering down of the peak voltage values, depending on the value and sign of the blocked DC component. This is why the math is screwed up when AC coupled is selected.

AC coupling capacitors and selection switches in the Tek RM503 precision low-frequency analog oscilloscope:

[gotoluc]

The issue of the proper use of the scope's AC coupled input function has come up before and apparently it is still confusing.

Maybe it would help to know how the scope accomplishes the AC coupled input. Every scope I've seen does this simply by switching in a capacitor in series with the probe input lead, immediately where it enters the scope's pre-amp/attenuator circuit. Old analog scopes like the Tek RM503 use a manual slide switch and a capacitor. Newer DSOs use a software-controlled relay.... and a capacitor. You might be able to hear the relay clicking, when you select AC coupled in the software on a modern DSO; I can hear it easily in the older Link DSO (when the relay doesn't stick, that is.)

The capacitor will block DC and very slow AC oscillations (like below 5 Hz or whatever, determined by the coupling capacitor value) and will only allow the AC component of any signal to pass to the scope's electronics. This has the effect of bringing the _average_ value of the input signal, to the scope channel's "zero voltage reference" line. This could be either a raising up or a lowering down of the peak voltage values, depending on the value and sign of the blocked DC component. This is why the math is screwed up when AC coupled is selected.

AC coupling capacitors and selection switches in the Tek RM503 precision low-frequency analog oscilloscope:

WOW TK!... you are one dedicated man to do all that work on the picture just for this post.

Thank you

Luc

[poynt99]

Thanks for opening up the wonderful antique for us to take a peek TK.