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Non-linear CV/CC control in schematic

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xavier60:
My most recent experience was with the SG3525. Its error amp works with the input common mode range down to 1V.
I would aim for about 1.5V minimum on the Inv pin on the LM723.
I don't have the IC to experiment with.
With the Ninv pin floating I would find what minimum voltage is needed on the Inv pin to keep the Comp pin pulled low.

duak:
To work properly the 723 requires a minimum of 2 volts between the both the inverting and non-inverting inputs and pin 7.  It may work lower but performance may suffer.  Your circuit violates this by allowing the non-inverting input to go to 0 volts.  Try exchanging R1 and the Vadjust pot and recalculating the values.  In the limit, the non-inverting input will vary between Vref (~7.15 V) and whatever your voltage divider delivers.  The 723 then tries to deliver an output voltage equal to the ratio of R21/(R20+R21) times the non-inverting input voltage.  With the present values or R20 and R21 it will be 11X the voltage on the non-inverting input.

Does this make sense?

xavier60:
I would start with a 4:1 divider, 15K/4.7K. 10K from Vref to Inv should set the 2V minimum.
Use something higher like 10K for the voltage adj Pot and use it as a  variable divider not as a VR

Silver_Pharaoh:

--- Quote from: duak on March 03, 2019, 06:58:50 pm ---To work properly the 723 requires a minimum of 2 volts between the both the inverting and non-inverting inputs and pin 7.  It may work lower but performance may suffer.  Your circuit violates this by allowing the non-inverting input to go to 0 volts.  Try exchanging R1 and the Vadjust pot and recalculating the values.  In the limit, the non-inverting input will vary between Vref (~7.15 V) and whatever your voltage divider delivers.  The 723 then tries to deliver an output voltage equal to the ratio of R21/(R20+R21) times the non-inverting input voltage.  With the present values or R20 and R21 it will be 11X the voltage on the non-inverting input.

Does this make sense?

--- End quote ---

Not fully no...

Inv & NInv pins should both have a difference of 2V between them and pin 7 correct? So those pins should be connected to Pin 7 with a voltage divider to ground? (Pin 7 is usually connected to ground no?)

I tried swapping R1 and the pot leaving everything else alone and no luck. When you say recalc the values, I assume you mean R20 & R21?
The last bit about Vout being X times the NInv voltage makes sense.


--- Quote from: xavier60 on March 03, 2019, 07:22:36 pm ---I would start with a 4:1 divider, 15K/4.7K. 10K from Vref to Inv should set the 2V minimum.
Use something higher like 10K for the voltage adj Pot and use it as a  variable divider not as a VR

--- End quote ---

I tried this, but I wasn't able to get anything to work....


So bottom line here is Inv pin and NInv pins should be connected together? What about Pin 7 (-V?)


A physical picture example would probably help me. I don't learn so well from just text unfortunately. I need to see something hands on to grasp it.

xavier60:
I drew this up quickly. Ill check for possible mistakes later.
The values aren't supper critical.

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