Testing the TK Tar Baby

[Groundloop]

Oh sorry, you are thinking it's a bipolar supply like for an op amp? My bad, I guess I didn't make it clear. It's just a single 10 volt supply, the positive to the +10V and the negative to the -10 V. I guess I should have labelled it +10 and 0, or something. Sorry about the confusion, it has been a rather long weekend.

A fresh nine-volt battery will work fine but won't be as stable as a regulated power supply.

TK,

OK, I understand. Single power supply, where your -10 is mislabeled and should have read 0 Volt.

Thanks,
GL.

[TinselKoala]

Yep, I went back and put a note on the post to the effect that you had straightened me out. Thanks.... and have fun !

[Groundloop]

Yep, I went back and put a note on the post to the effect that you had straightened me out. Thanks.... and have fun !

TK,

And what are the value of the variable resistor (potmeter)?

[EDIT 1] Oh, I see you put that in the edit also, 100K  :-)

[EDIT 2] Attached is a nice drawing of your 555 circuit, you can copy and paste that into the AN switch drawing.

GL.

[TinselKoala]

That's so pretty.... awww.... thanks GL.



The cap and the pot values can be changed. The pot can be 10k, 100k, or 1 meg, bigger is slower.
The cap can be tiny or up to 1 uF. Again, bigger is slower.

[picowatt]

TK,

I really like the way you hooked up that 555.  Your solution for providing a negative voltage at the Q2 source using the 555 output tied as you did and floating the supply was very creative and elegant!

I would suggest placing a 10R to 27R between the 555 and Q2's source.  The reasoning is as follows.  When operated at around 4-5 volts Vgs, the IRFPG50 has a pretty severe increase in drain current as its temp increases.  Without a source resistor the current will rise as Q2 warms so to mantain Ibias below the 555's 200ma or so limit (it is a bipolar 555 as opposed to CMOS, correct?) you will have to tweak the 555 supply voltage as Q2 temps change.  With a source resistor, as Q2 warms and attempts to draw more current, additional Vdrop will occur across Rsource and reduce the applied Vgs "automatically".  It won't provide perfect current regulation but it will definitely help prevent things from "running away" unnoticed.  The added Rsource may or may not help/hinder the osc.  The value for Rsource will be a tradeoff between low Vdrop (small Rsource) for lower 555 supply operation, and a larger Vdrop (larger Rsource) for better Ireg but also needing a higher 555 supply.  Anyway, try 10-15 ohms and see how it goes.  You'll lose 2 volts of Vgs due to Vdrop using the 10R with 200ma of Ibias and the 555 supply will need to be increased that much.  Adding Rsource could save four expensive mosfets if you aren't there monitoring current all the time.  If you want to know how much the added Rsource increases Ireg versus temp, monitor your loop current and alternately hit your mosfet with a heat gun (hair dryer)/freeze spray (upside down dust-off).  Then add Rsource and repeat the test.  Please carefully monitor current when you warm the mosfets (you don't need to get them "hot") as I wouldn't want you to overcurrent and hurt your magic mosfets!.

Anyway, adding Rsource is just a suggestion.  If you knew all that, well, never mind...

You suggested a 9V battery as a possible 555 supply.  Will it not have to provide the 200ma Ibias?  Typical 9V alkaline would give you what, 2.5 hours or so?  Lithium a bit more?  Of course, Ibias may not have to be 200ma if the circuit will osc below that.  Adjusting Ibias will,however, likely affect the harmonic content (shape) of the osc.

What are you using for a 555 supply now?

Your posted schematic is a pre-computer "copy and paste".  Reminds me of "rip up and retry".

What's up with "deepbunker"?

PW