Hi guys, everyone still here? I've been side tracked with LTO battery testing.

I'm trying to reverse engineer a 2.7v shunt board that is a little more sophisticated than the normal 2 component design.
I can't draw it properly, as some parts are unknown. All numbers of active devices removed.
This is tentative layout. May be a mistake crept in. Still checking.

It seems the original design uses the 5a npn driver to disipate half the heat. 2p-4r7 resistors have been changed to lamp that draws 800ma @ 2.75v.
I'm trying to get saturation, with lamp doing all the work.
200 ohm (51k) base resistor saturates output npn but not the driver, which then runs hot.
I'm stuck with this basic circuit, as boards are cheaper than I can build.
Bottom two devices don't match pinout for 431reg, pnp, or darlington. The .8v drop from p1-p2 with test meter, should be a clue.
Anyone willing take a look at this?

 

I'm trying to get saturation, with lamp doing all the work.

can't be done unless the lamp resistance is exactly right for the amount of current available; Mr. Ohm won't allow it Or are you looking for bang-bang control?

Keep in mind that incandescent lamps have a substantial positive coefficient of resistance - cold resistance is typically at least 10 times normal operating resistance.

 

Thank you for reply.
Not sure I follow. I know it won't be saturated during turn on. I only want to bring everything within limits.
Changing the base resistor drops the "0n" voltage drop from .8v to .1v.
In the original circuit, the 2 ^ 1w load is very near max when circuit is fully on.
Ideally circuit will trigger with schmitt trigger action. Staying on until discharging from 2.7 to 2.6v level.

 

I was taking saturation to mean that you wanted the transistor operating the lamp to turn on hard or fully off, not operate in the linear zone, presumably so it won't need a heatsink.

Turing full on and off at separate thresholds is "hysteric control", which is often (maybe not, anymore) called bang-bang control. Think about a solenoid valve controller water level in a tank - "bang" it opens when the water level is at the upper limit then "bang" it closes when the water reaches the lower limit.

Did the original circuit operate in hysteritic mode?

I'm not clear about which part is running hot.

What does 2p-4r7 mean? Two 4r7 resistors in parallel?

 

Yes on parallel R.
Original seemed to have hysteresis, in that it latched on at one voltage and off at another. However it still had a small period with slow rise.
That part, I believe I can figure out by adding some feedback.
For now I just want to increase the current a little without swapping transistors. Over 2 or 3 changes, it's easier to start over.
This is what works for now.
Green values piggy backed on smd parts.

 

2sd1805 is marked. Other parts are not.

 

Looking like the unknown device lower right is a put.
As when p2 exceeds p1 by .5v it drops below p1 potential.
Now if I can find an sot-23 put with proper pinout. Or a similar acting 3 leg device,