Testing the TK Tar Baby

[TinselKoala]

.99, thanks for doing that.

It is very telling, I think. It's neat how the cap voltage tracks right down the center of the oscillations, showing that the oscs "bounce off" the cap and don't affect its state of charge at all, other than being the "source of the drain" ha ha.  I am especially struck by the "spindle' or turnip shape of the envelope bursts as the cap's charge decreases just before the oscillations die away completely.

So you have a computed negative power product virtually the whole time that the capacitor is discharging. But unlike a battery, we can know the energy in a cap simply by knowing its capacitance and the charge voltage. So a 8uF capacitor charged to 72 volts contains E=CVV/2 Joules in it. So that is (0.000008x72x72)/2 == 0.020736 Joules in there. And it ran the circuit, oscillating, for about 2.5 milliseconds.

So.... Do The Math (tm RA). We find that 0.0208 Joules have been dissipated in 0.0025 seconds, for a REAL average power of 0.0208/0.0025 == about 8.3 Watts _positive_, that is "flowing out" of the  capacitor. Did I do that right?

This power is always flowing OUT of the capacitor as shown by its true voltage value, in spite of the negative mean power value resulting from the computation based on the contaminated measurement locations.


I've not been able to figure out how the weird spindles were made with the NERD circuit.  None of Ainslie's "turnip" shots show the gate drive signal, nor do they show any DC current during the non-oscillation portion of the signal.

What do you think the explanation is for those particular shots from Ainslie? A good example is SCRN0284 but the shapes are weird in shots like SCRN0243 as well.

[poynt99]

So for the blue negative instantaneous power trace, do negative values imply power being returned to the capacitor?  In other words your simulation is so good it even shows how you can be "faked out" and made to believe that power is being returned to the battery/capacitor when in fact the decreasing capacitor voltage is clearly showing that power is being output by the battery/capacitor.

MileHigh

YES.

[poynt99]

What do you think the explanation is for those particular shots from Ainslie? A good example is SCRN0284 but the shapes are weird in shots like SCRN0243 as well.

I recall Rosemary saying that they have used or tried a few different FG's for their tests. I can only assume that one FG they tried didn't cope as well with the oscillations through it. In SCRN0284 we see the oscillation amplitude begin to rise to it's usual level of over 160V, then suddenly it drops to roughly 40Vpp. Both the top and bottom excursions decreased.

My guess is that something to do with the FG caused it.

[MileHigh]

TK:

So.... Do The Math (tm RA). We find that 0.0208 Joules have been dissipated in 0.0025 seconds, for a REAL average power of 0.0208/0.0025 == about 8.3 Watts _positive_, that is "flowing out" of the  capacitor. Did I do that right?

No, because what you are interested in knowing is what the power consumption is for a constant voltage, and not a variable voltage.

If you can imagine using a much larger capacitor so that the voltage is much steadier, then the slope of the cap discharge at say, 72 volts, would be much easier to measure.  So once you have the (nearly constant) slope of the change in voltage at 72 volts, and you know the capacitance, you can derive the current, i = C dv/dt.   So you could then get the power consumption at 72 volts.

I believe that you can also simply drop a power probe on the capacitor itself.  I am not sure if you can low-pass-filter the power probe data (i.e.; some sort of a running average), but I assume that you get the idea.

MileHigh

[poynt99]

With the Watt probe directly on the 8uF capacitor, we get the TRUE instantaneous and average power over the 3ms period.

The average is correctly negative, because sources give up power, while loads dissipate power.