Regulator driven NPN darlington will not go to 0V, solutions?

[crutschow]

Hi crutschow,

about your post#16.
When FB = 1.5V I expect the output to near zero instead of max.
Am i wrong here?

You are correct, of course.
My brain was in neutral. :???:

 

[schmitt trigger]

My brain was in neutral. :???:

I've a personal policy of mine, not to respond to ANY technical questions unless I've been properly caffeinated. :-D
Dosage varies, but that means at least two cups.

 

[David_]

Hello.

Sorry for this delay.
I feel bad about getting help with something and then just disappear like this, but my head in not always in the game and I think I with this thread proved a point to my self about how I function.
Its funny, every time I in detail plan a activity it never happens. The only way to get things done is for me to not specifically plan to do them...

Anyway I am now sitting looking at the prototype and I have decided to make a new one, after I have hunted down the malfunction of this one. But as I have written I question if its a fault or not just a mistake, I have a hard time making up my mind about if I should define that distinction or if I am just creating confusion in my mind.

I know you should not do in-circuit measurements of component values but I wonder why the following would be:


R7 is a 1% resistor while the other two are actually three resistors partly consisting of random scavenged devices so they can be anything. But why is R6 so very much of?

I have come to feel blind in that my equipment simply don't cut it, but I have no economy to improve on them for some time.
Though within one year I should be able to acquire a better set of instruments.

I find it interesting that my Mastech MS5308 LCR tester which is a ≈200USD meter which I and any others are very pleased with, it has one mode to measure resistance with a AC waveform where you can select 100, 120, 1k-, 10k- or 100k- Hz and then a DC-resistance mode which by the way take many seconds to get a reading out on the screen.

When measuring R6 in the AC mode it shows something like 2,4kΩ below 1kHz and then 12Ω at 10kHz and below 1Ω at 100kHz, I know that this might just as well be the meter misunderstanding what is connected to the probes since as I understands it it does not actually measure any impedance or such but rather some single aspect and then use complex math to get all the parameters you want. But can anyone see a reason for this?

The prototype I am plying with is my first attempt at a PCB design and even though I have no oscillations as far as I can tell I have screwed up the layout inducing instability in the reference voltage which feeds both DAC and ADC and there might actually be some oscillations in the reference buffer. But more to my point is that during this layout I had not one single thought of return currents, Sure I thought about the returning current in some aspects but not in any way as you should and I am quite sure that the performens of the circuit is degraded by my layout where I create long weird return paths.
I'm not happy with it and I have made some other mistakes and I have made some re-defining changes so a new version is in order I think.

Going over the solution for finding the values in the DAC control three resistor divider I see I have not done it right failing to account for voltages here and there and this prototype is simply to sloppy to be a good foreground for anything.

For one thing I have failed to make room for probing important nodes and I need a refresher in my own mind of this circuit function anyway so I'll go ahead and just do It. I have all I need to do it at home so it should not take very long at all.

Regards