Testing the TK Tar Baby

[polln8r]

"Give a man an egg, he eats for a day. Teach a man to lay an egg..."

[orbut 3000]

By two different measurement methods, all mathematically correct. Right?

Kindest AND best and very well done FOR getting those omelettes.  I hope, soon, TO show you some OF our own.
Orbie

[TinselKoala]

Gaahh.... He still doesn't get it.

Gmeast, here is a very simple challenge for you. Please use your function generator to send a 62.5 kHz positive rectangular pulse of 25 percent duty cycle, at 12 volts peak, directly through a simple 5 Watt automotive light bulb, like a dome light. Apply the exact same power measurement techniques and calculations that you have done in the past three days with your mosfet circuit. Compare with straight DC at 3 volts -- or even 6 volts -- to the bulb. Measure and calculate and report. Use the exact same calculations, from a small shunt for current monitoring, that you have been using, just as if you were measuring your circuit.

[poynt99]

So, getting back to this discussion...

Uh... OK..... but the derivation in the Wiki article is shown to apply to complex waveforms as long as an rms current value can be obtained "somehow". But sure, if you are a bench tech measuring "these types of circuits" presumably meaning noisy PWMs or switching power supplies, you already have an integrating oscilloscope sitting in front of you and you know how to use it.... so you would probably not use your method directly.... even though the scope will be doing it internally.

Even with a good scope and passive probes, making a Pout measurement on an inductive-resistive switched "high-side" load is notoriously difficult, if not impossible to do accurately. Many have been down that road, including myself.

The only way to do it properly, is to use a differential voltage probe across the load resistor, and a current probe in series with it. The other option is to use an isolated front-end scope. These requirements eliminate 99.99% of all users on this forum, and at least 50% of everyone else.

I think the real contribution and novelty is in your insistence and demonstration that one can indeed make good measurements, reliable and accurate, with your method and without an expensive digital scope, as long as some basic knowledge is present.

Yes, but more importantly, this method allows the Pout measurement possible to make at all, fancy equipment or not. The only other way is to use a DC control as Rose and Greg have done, and we know this takes a fair bit of time to do, not to mention the added requirement for a variable power supply, which many don't have. My method works very well, and quickly, and without the need for a scope, nor variable power supply. AND, it is accurate.

[TinselKoala]

So, getting back to this discussion...
Even with a good scope and passive probes, making a Pout measurement on an inductive-resistive switched "high-side" load is notoriously difficult, if not impossible to do accurately. Many have been down that road, including myself.

The only way to do it properly, is to use a differential voltage probe across the load resistor, and a current probe in series with it. The other option is to use an isolated front-end scope. These requirements eliminate 99.99% of all users on this forum, and at least 50% of everyone else.
Yes, but more importantly, this method allows the Pout measurement possible to make at all, fancy equipment or not. The only other way is to use a DC control as Rose and Greg have done, and we know this takes a fair bit of time to do, not to mention the added requirement for a variable power supply, which many don't have. My method works very well, and quickly, and without the need for a scope, nor variable power supply. AND, it is accurate.

Yep, agreed..... I'm spoiled I guess because at the other location I have nice diff probes and that hugely expensive LeCroy DC-100MHz current probe to hand. Your method makes it possible for me to continue (if I want to for some reason) here with lesser equipment.

Ironically..... the Fluke 123 and 199 ScopeMeters that Ainslie used for the Quantum report, or claimed to use (I still don't know if it was really a 123 or 199 that she used) have isolated channel references. You can have 600 V between the "grounds" of the two probes; this allowed one (or Ainslie) to blithely make measurements all over the circuit without worrying.

Which reminds me... how is Gmeast measuring "across the load"? Is he considering Drain-to-Negative Rail to be "across the load"? Or does he have isolated probe references so he can actually measure across the load while also monitoring the main CVR on the neg rail? Or does he have enough channels available to use two probes in a differential manner for the "across the load" measurement?

It's hard for me to imagine anyone trying any kind of work like this without having a regulated, variable DC power supply or several of them. But I am constantly surprised whenever I dip into threads discussing electronics. I consider my own knowledge and bench setup to be rudimentary.... then I see people discussing resonance and coil tuning for example and they don't even have an oscilloscope and are using an old PC power supply (nothing wrong with that, I use one too for special purposes, but still.....)