Electronics > Projects, Designs, and Technical Stuff
0.13x vs 1.0x gain op amp regulated power supply - fail
HendriXML:
I could just fit 2x parallel resistors of 15K. Without desoldering the previous ones.
The new results amaze me! The response has improved a lot! Wasn't expecting that :-//.
-- So I verified the test setup.... no magic to be seen here..
My circuit 2 not saturated! 0.13 V 0.12 V 0.10 V 0.08 V
HendriXML:
Hmm made a mistake with the signal generator in these test runs: the load Mosfet did not become saturated..
The table with comparable conditions is:
Resistor Siglent powersupply My circuit My circuit 2 5R1 2.08 V 0.60 V 0.78 V 6R8 1.90 V 0.50 V 0.65 V 10R 1.62 V 0.40 V 0.48 V 15R 1.36 V 0.30 V 0.29 V
It's worse as one would expect, but not much! With less amps its on par.
HendriXML:
So I altered the circuit to have the lowest gain possible (by looking at the curve). Now I wanted to check whether it could go without an endcap (C7): it now seems to be stable at resistive and inductive loads.
I attached some scope images to show how it responds to sudden loads. It’s not good (the scales are very different!) but it stabilizes nicely.
These results should be discarded, reason down below.
HendriXML:
I was doing some measurements and things didn't add up. Because of the better resolution I did the measurements in AC mode. But sometimes that hides stuff you would like to know. One thing that would have shown otherwise was that the capless measurements were taken when the circuit did not regulate properly. One of mod resistors had a loose contact. :palm: The other measurements after the mod, seem to be fine. I did some retesting and got even (a bit) better results.
But this thread was not started to make the world’s finest supply, it is mostly about a circuit which can give some insight in how to model the amplification of a IRLZ44N. Because it was at first not stable without a cap, the needed measurement could not be done. I'm optimistic that there's now a situation in which some interesting conclusions can be draw. There're probably books which provide much better information :=\. But this is more fun.
Back to the –regulated- capless response:
I posted 2 curves, the difference are the reference caps (C1, C4, C5, C6). The first one is with C4, C5, C6 on board, thus 300nF. The second is with only C4, thus 100nF.
It is clearly to see the caps need to charge up before a new balance is found. This takes about 200us per 100nF. Because that is a relatively long time, I’ll assume the ref voltage as being constant, when analysing the first few us. This will be done in following posts, but it might take a while.
Image 005 shows 2 curves
* Green: the drain of the pull down (load) Mosfet
* Yellow: VCC out
The difference between those is the voltage over a 5R1 resistor and should give us the current drawn at each moment. Will make some graphs to show this more clearly!
HendriXML:
--- Quote from: HendriXML on February 13, 2019, 11:47:03 pm ---
--- Quote from: HendriXML on February 13, 2019, 06:13:21 pm ---The question I want to answer is what range it can regulate without significant drops. Somehow I think the integration of the ratio curve in the other post has a relation to it. My thoughts are if it’s less than 1 (or 0.5 because of the resistive divider), it’s okay. If its more, the reference capacitor needs to charge a bit further, so the output voltage will drop.
--- End quote ---
From the measurements done above this does not seem the case.
That raises also the question about how important the raising of the ref voltage is.
Or wether it is possible to drive the Mosfet with even lower "gain".
To bad I've soldered the resistors down so that isn't just a quick check. The legs are also the traces...perfboard experimenting issue.
--- End quote ---
My initial "hypothesis" might still be correct! The endcap had such a huge impact, that no conclusions could be drawn. If the integration/integral stuff seems to be true, then I should somehow incorporate that in my "mental model" of "amplification". A empirical answer needs probably also some more (advanced) testing. Maybe one of you already knows a theoretical one?
The question is - given the curves in the posting below- how much gain is needed to keep a steady Vsd (and Vout from not dropping) in a given range of Vsg (which corresponds to a initial amperage - new amperage). Without the effect of the "delay" of the amplification/sensing, so keeping time out of the picture.
https://www.eevblog.com/forum/testgear/this-graph-brings-tears-to-my-eye/msg2189625/#msg2189625
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