Testing the TK Tar Baby

[hartiberlin]

I now see, where you get the high current from
the 9 Volts battery.

See enclosed circuit diagramm.

It is discharging through the Rshunt resistor !

[hartiberlin]

So now here is the solution to avoid the
DC current draining of the 9 Volts battery.

Please TK let us know if you get it to work this way.
Many thanks.

[MileHigh]

Stefan:

The second schematic you posted has problems.  The current flow from the Q2 array source is bypassing the shunt resistor so you are not measuring the current flow through the Q2 array.

The 10K potentiometer concept from TK is unfortunately fundamentally flawed.  The impedance going through the 10K potentiometer is too high and it will choke off the normal current flow.  There is supposed to be an impedance of 50 ohms at all times.

PW suggested a direct resistor connection between the 9-volt battery negative and the Q2 source node.  It would be lower than 50 ohms, perhaps 30 ohms.  This would allow approximately the same amount of current to flow through the circuit as compared to when you have a 50 ohm resistor connected to the function generator negative output.   However, even this has an issue.  This would change the "bounce up" voltage potential when current flows through the circuit as compared to the function generator/50-ohm resistance.

Like I said before, I personally don't see any point in going this route.  The function generator in negative oscillation mode injects power into the circuit, but it does not in any way inject power into the batteries.   The batteries will still discharge with the function generator in the circuit.

MileHigh

[picowatt]

TK,

That "floor" as you cal it is the gate to source turn on voltage.  Once you are up and into the turn on knee of Q2, the voltage at the source will vary only a small amount around that 4 volts unless you try to get the bias current well in the amps range.  If you look at the IRFPG50 data sheet at the turn on characteristics graph, the voltage observed at the source and the current measured will folllow that graph pretty much.  But in the 50ma to 300ma range, you may only see .5 to 1 volt of change at the source.

The series pot would be OK, but a wirewound ,though great for handling the current, will add more inductance.

In Stefan's schematic he has the bias source connected to real ground instead of the CSR, which will not allow the DC bias current to be accounted for and calculated into total power draw.  Also, even if Ibias is set to 25ma, the 9V will eventually drain even at that rate.  A good alkaline might get you a bit more than 500mAh of use, so around 20 hours at 25ma.  A 9volt lithium will provide around 1200mAh, so run time would be longer if used.  The additional load from the 10K pot, at just under 1ma would also add to the battery drain.

I do like the series pot idea you mentioned, as it would allow you to set current while observing the waveforms.  I would try to set the bias so that the oscillations resemble the clipped sines in the first paper FIG 4.

What's your "wild dog's" name?

PW

[TinselKoala]

Stefan:
Sorry, the first diagram is wrong, there is no connection between the pot and the main battery negative, the whole 9v system is "floating", and you have the battery upside down .... and the second diagram is what makes the second type of oscillations as soon as I touch the positive of the 9v to the negative of the main battery, and these oscillations persist regardless of the other connection points until I remove the connection between the 9v + and the main --.
(I just discovered that the color code of the long red-black battery cable connector is backwards. The red wire goes to the negative battery pole. Sorry... it happened because I was using that cable as a "battery" to connect the power supply to the 555 timer, so I wired it backwards in the first place and forgot to change it when it got hooked up TO the battery, instead of AS the battery.  The diagram below shows the correct polarities and hookups for the battery and pot, I've checked it several times and have it running right now behind me.)

The correct hookup for the negative bias pot-battery configuration....:

(ETA: I forgot to put in the Hickok moving-coil milliammeter. It goes in series with the 10R (or 50R etc.)  and the pot wiper.)