Testing the TK Tar Baby

[MileHigh]

Just a few thoughts about the negative oscillation mode to ponder.  I am just speculating about the operation, but this would have to be investigated on the bench to be sure.

You know that the Q2 array (back to the original Ainsley reference schematic) is acting like an oscillator.  All of the wiring is like an inductor, and when the Q2 array switches off in the cycle then all of the inductive energy stored in the wiring starts to discharge.  The "blocking point" for the discharge of the energy stored in the wiring is the switched-off Q2 array.  So it's likely that the Cds capacitance in the Q2 array gets charged up to a high potential.

Once you hit the peak voltage the current flow reverses direction and discharges back though the main loop.

This could be a key to understanding the overall timing.  Once the discharge of the energy stored in the Q2 array capacitance is done then the EMF from the main battery supply takes over and the current starts to flow clockwise again.

The discharge of the Q2 array capacitance could be part of the explanation for the high potential you see on the battery terminals.

All of this would be operating synchronous with the root cause of the Q2 array oscillator, the negative feedback caused by the 50-ohm resistor inside the function generator.

What this ultimately means is this:  Battery energy charges up the capacitance in the Q2 array.  When the voltage hits the peak, the current reverses direction and spits this same energy back at the battery.  It's like throwing a ball against a wall.  You put energy into the ball when you throw it at the wall.  Then the ball changes direction at it's maximum compression point (like the maximum voltage in the Q2 array capacitance) and most of the energy is then coming back at you.  So the battery gets a short pulse of return current.  There is a very good chance that the battery can't really react to a short pulse stream like this and in fact just burns off the return energy.  So the battery acts like a resistor in this case.  I am not a battery expert but that's what I suspect.

You can see that there is a reduced heating efficiency because of this possible effect.  The battery outputs a nugget of energy.  Some of the nugget of energy is thrown back at the battery instead of going to the load to create useful heat.  The nugget of energy that is thrown back of the battery might recharge the battery, but there is a good chance that most of that energy is burned off as heat.

Hence, while the battery is powering the circuit, it's burning off more energy internal to the battery as compared to just driving the load with straight DC current.

MileHigh

[TinselKoala]

I just woke up so I'm still having my first coffee.

I think MH is describing in clear terms what I facetiously described in my "cracks me up" post. In the positive bias mode the bias supply polarity is anti-series : the negative polarity of the bias is hooked to the negative polarity of the main battery.  Part of the circuit sees the main voltage, part sees the bias voltage, and part sees (main-bias) voltage.

On the other hand when in "AC" mode, negative bias mode, the bias supply is in strict, aiding series with the main battery, plus of bias to minus of main. So part of the circuit sees main voltage, part sees bias voltage, and part sees (MAIN + BIAS) voltage.

And to top it all off, the NERD apparently preferred mode of operation is a mixed mode, where the gate HI time is the positive bias mode and the gate LO time has enough negative excursion due to the FG's offset, to make the AC or negative bias oscillation mode. And you cannot do this with a simple 555 timer driven from the main battery only.

But I have to stress that the "AC" mode isn't really AC as far as the bias current goes. When you look at the Bias Current in this mode you will find (or at least I found) that the current is DC and has a small oscillation ripple on top. It would look like AC on a scope if you "AC" coupled the channel, but with DC coupling you can see that this current never reverses sign, therefore it is not AC. The main current is "mostly" AC, that is, it does change sign within the oscillations.... but wait...

IS this the main current providing the heat in the load? Could the current path heating the load be helped by the OTHER path, not the drain-to-source path in Q2, but going through the zener of "off" Q1 when the oscillations are happening on Q2.

So if this hypothesis is right, when in "AC" mode, negative bias, oscillations.... we already know that pulling Q1 (Moses) will not hurt the oscillations themselves....... but what about the current through the load itself? What about the actual load heating during this phase? I know that I haven't really checked these things directly when in AC mode and Q1 is missing ...yet.

What seems clear, still, is that there is no apparent explanation for the zero main current in those several NERD shots that show a definite positive 10 or 12 volts to the Moses (Q1) gate--- other than that the mosfet is open or missing.

[MileHigh]

Going back to the elevated battery voltage, this can tie into the AC impedance of the batteries.  PW suggested making an AC impedance measurement a month ago.

When the capacitance in the Q2 array is at it's maximum voltage, what does the set of batteries look like to the capacitors about to discharge?

The battery set looks like an inductance in series with a resistance - at a given frequency.

The inductance of the battery set is likely much higher than the inductance of the load resistor.  Perhaps more critically, the effective resistance of the battery set at a given high frequency might be quite high.  In other words, for the brief instant (say a microsecond) when you want to pulse current into the battery set, it looks like a 100-ohm resistor (for example.)

So when the Q2 array capacitance is about to discharge, the overall impedance of the battery set could be quite high as compared to the overall impedance of the load resistor.  Therefore the battery set will show a high voltage.  i.e.; there will not be too much of a voltage drop across the load resistor.  The high voltage seen at the Q2 array capacitance is also (mostly) seen at the battery set.

MileHigh

[TinselKoala]

What would happen to the DC mode, the positive bias, no oscillation mode.... if the main battery voltage and the bias supply had the same voltage? (under 15, I guess, to prevent damage to the mosfets).

And same question for the AC mode, the negative bias, yes oscillation mode..?

[MileHigh]

What would happen to the DC mode, the positive bias, no oscillation mode.... if the main battery voltage and the bias supply had the same voltage?

It should be quite similar.  The biasing loop has the 50-ohm resistor so it will always contribute less current.

MileHigh