Testing the TK Tar Baby

[TinselKoala]

More data:
Following picowatt's suggestion of trying a DC gate stimulus "tickle" to try to get oscillations without the need for the FG.... I took a 50 ohm resistor and used a jumper to connect it to the board where the FG's leads were connected. Then I "tickled" the positive terminal of the battery at the negative end of my 3-stack.

First I "tickled" the place where the FG's positive lead connects. This turned on the Q1 mosfet hard and clean and it stayed on, essentially a direct short through the mosfet, even after I removed the positive connection (no pulldown) and stayed hard on until I either interrupted the battery connection or "tickled" the negative rail with the 50R on the jumper. I was unable to get oscillations here.

Then... I hooked the jumper to the place where the negative FG lead connects. When I "tickled" the positive battery terminal with the 50R, at first nothing happened... then by accident I heard a buzz from my COMPUTER speakers, checked the scope and the system had entered a wild oscillation state, and my inline DMMs freaked out and stopped indicating, and this oscillation continued, scaring me a bit since I couldn't monitor the current, so I killed it by disconnecting the battery.

It turns out that "tickle" is the key word... if I make a hard connection with the 50R to the batt positive, the Q2 either switches cleanly on and stays on or doesn't switch at all. Just that scratchy brief tickle, though, and you are off screaming like the zombies are after you.

Like I said I was worried about destroying my transistors so I only let it go on for a few seconds. I'll repeat it for a video later on this evening... stay tuned.

[TinselKoala]

@MH:

I don't really see how the LEDs could survive in a series connection. Either the oscillations won't be present if I use an appropriate dropping resistor for the 36 volt positive supply, or the LEDs will pop and open the circuit.

But of course I'll try it... I've got plenty of LEDs and just I love the smell of that magic smoke.....



ETA: Yup... as I thought... with the LED pair in series with a 1800 ohm resistor, right at the +36 volt terminal... only one LED lights and the oscillations are gone and can't be tuned back, so far. The Q2 mosfets are switching cleanly, all 20mA or so of current.

So now what-- reduce the value of the series resistor until the magic smoke _does_ appear?


ETA2: STOP THE PRESSES !! See the latest TKProduction, Tar Baby and the Semi-Deluxe LEDs of Doom... coming to a YT channel near you as soon as it's done uploading.

[MileHigh]

TK:

Didn't you say that your multimeter is showing 200 mA battery draw when the circuit is in oscillation?  To me that sounds like all that you need is two LEDs back-to-back - no series resistor.  200 mA through a vanilla LED won't fry it, correct?  I thought that the typical current through an LED was 100 mA.

If you want to be safe perhaps 2+2 LEDs back-to-back will do it for you.  In theory the LEDs like this are benign, and just stealing 0.7 volts from the battery and otherwise not affecting the circuit.

You can't have any series resistor because that will choke off the whole shebang.

Note that the higher the frequency of the oscillation the lower current consumption of the circuit.  The impedance due to the inductance everywhere increases the higher the frequency.

MileHigh

[Magluvin]

Typical led's are 30ma. 

Mags

[TinselKoala]

TK:

Didn't you say that your multimeter is showing 200 mA battery draw when the circuit is in oscillation?  To me that sounds like all that you need is two LEDs back-to-back - no series resistor.  200 mA through a vanilla LED won't fry it, correct?  I thought that the typical current through an LED was 100 mA.

um...er... I usually calculate for 20 mA for the standard LED. Some will glow very brightly on quite a bit less.

If you want to be safe perhaps 2+2 LEDs back-to-back will do it for you.  In theory the LEDs like this are benign, and just stealing 0.7 volts from the battery and otherwise not affecting the circuit.

Each series LED will drop the voltage. Once they turn on they are just like one-directional resistors, and the voltage drop depends on the current you limit by the dropping series resistor. Sure, if there's enough resistance in the rest of the circuit to limit the current to 200 mA at 36 volts, then ten LEDs in series might light brightly. One... would light bery bery brightly for a very short time.

You can't have any series resistor because that will choke off the whole shebang.

Note that the higher the frequency of the oscillation the lower current consumption of the circuit.  The impedance due to the inductance everywhere increases the higher the frequency.

MileHigh

Ah... yes. OK.... you owe me one green LED. With oscillations indicated, 110 mA showing on the inline meter, I put the green LED in series with no dropping resistor. The current went down to 90 mA indicated and the green LED glowed brilliant orange. It lasted 92 seconds before the light went to dim red ember and the magic white smoke all leaked out.

But the oscillations persisted throughout, until the LED went open totally !!