Testing the TK Tar Baby

[TinselKoala]

Oh, I want to make sure that it's clear that I'm not monitoring the CVR quantitatively yet; the current values I'm giving lately are just the rough figures from the cheapo DMM inline with the battery.

It's nice to have more than two channels sometimes. Sigh.

@MH:
It's late and I don't think I'm up for much more tonight. I'm going to have to think about this AC issue a bit more. Would one expect a LED to light up in both polarity orientations, if inserted where the CVR is? Or a back-to-back LED pair?

[MileHigh]

TK:

Go "dodo" and get a good rest.

For the LEDs, you want to try putting a back-to-back pair in series with the CSR.  When you get the oscillations, both LEDs should light up.

MileHigh

[picowatt]

TK,

Using the opto's would allow full turn on, and indeed slower turn off, so I can see how they could overlap, but, that deviates from the RA circuit operation.

Are you using the similarly short Q1 on time/very long Q2 biased on time?

Your alternate FET's should work, but for duplicating as close as possible the waveforms and Fosc of the RA circuit, the PG50 would be a closer replication.  You should be able to swap them out with the 830's but the lower capacitances of those devices will flavor the results slightly.

If RA knew what the open circuit votage of the FG was during a test deemed "successful", (i.e., Q2's bias current) an appropriate battey supply equal to the FG open circuit voltage in series with a 50R replacing the FG would have performed similarly (as far as Q2 is concerned).  Of course, the new bias battery would discharge at a rate equal to the bias current and possibly from any assymetrical AC not fully bypassed by the Ciss of Q1/Q2 (though if assymetrical the correct way, possibly the new bias battery would get a slight boost now and again).

I haven't heard if .99 is going to finish his duplication or not, it would be interesting if the two of you could do similar reps and acheive similar measurement results.  But hey, its a lot of time and expense.  More power to both of you.

To infinity and beyond.........

PW

[TinselKoala]

Blah... I made a "tad bit" of a math error earlier. My new load resistor stack of 5 x 50 Ohm 12 Watt resistors in parallel is of course a 10 ohm (measured at 10.3 ohms on the Simpson) resistor capable of handling 60 Watts, not the 250 I somehow came up with earlier. It's immersed in mineral oil though, so hopefully it will be OK.

But I caught the error first.....  or at least you lot are too polite to razz me about it.....   

Anyhow, yes, and good night.

I'm uploading a video describing the optocoupler test so far, but it will be an hour before it's ready, probably.

The waveforms that Ainslie showed are symmetrical, 50 percent duty cycle square waveforms, with the gate drive signal going from 0 down to -5 volts or so. I don't know quite what you mean, PW, about "the similarly short Q1 on time/very long Q2 biased on time". 
With only two channels on my scope I'm really only guessing, based on heating, which mosfets are on when. If I see high currents and the drain signal staying low without sign of pulsing, then I think that at least one mosfet is staying totally on. If I see the oscillations and partial drawdown in "both" phases of the drain signal, then I think that all the mosfets are at least partially turning on, Q1 and Q2 alternating. But I don't have the precision with this kit to be able easily to draw out a full timing diagram for the switching...yet. I have a Tek DPSO at the lab that I'll torture with the Tar Baby later this week and that will tell me a lot... a lot that you lot probably already know.

[TinselKoala]

Musings engendered by me watching my latest video...

So you come into the lab and get set up. You still don't understand the reason for the oscillations but you know if you "tune" long enough they will eventually appear and if you don't touch anything they will stay.
So the load is in the water in the insulated teapot. You log the temperature. You turn on everything and begin tuning. As you tune for the oscillations, you will be going through periods of mosfets totally off with drain trace HIGH, periods of clean switching with drain trace in phase with gate signal from fully ON to fully OFF without oscillations, and periods of mosfet(s) constantly fully on, with drain trace at baseline and no sign of switching even though the gate signal indicates normally.
Then, suddenly, the oscillations appear. You cream your jeans, then you look over at the load temperature. It is amazingly high, like 80 degrees when ambient was 16. You log this, and back off to watch. Hmm.... the oscillations continue, with the drain trace oscillating around the _HIGH_ level as is normal for these oscs. The CVR trace indicates whatever weirdnesses. This is the data you log. Meanwhile, the temperature continues stable, no further rise. So you... retune,change freqs or something. First you lose the oscs, you fiddle around, there are periods of clean switching and  continuous on. Then you reestablish the oscillations and notice that the temp is much higher now, maybe even boiling. Whoopee, the magic oscillations are doing their thing!!

But what you don't realize is that the heating happened during the TUNING, when periods of high current flowed from the battery to the load, and this data wasn't recorded, since you didn't have oscillations and your circuit isn't operating in the magic mode. The load warmed up while you were tuning, and the slight power transfer during the magic oscillations is enough to _overcome the insulation leakage_ and keep the load from cooling off.

Retune, lather rinse repeat. And you've fooled yourself into thinking that the oscillations contribute to the heating of the load, when the major part of the work was actually done during the unmonitored and unanalyzed tuning stage.

It's just a musing, just a guess, a conjecture.