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Bench CC/CV PSU Based on Daves uSupply (Not Anymore)
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jaycee:
With no error amps, it wont regulate at all
iMo:
Your TIP120 and CCS. By changing the R17 from 0(zero)..5kohm you set the output voltage from 0 to about 20V.

KC0PPH:

--- Quote from: imo on March 17, 2019, 05:38:45 pm ---The stuff works such the 4ma current source produces a constant current which flows from the CCS's collector "somewhere".

Without the opamps wired the whole CCS current flows into the TIP120 base and the output voltage shall be at its Max.

You have to "redirect/steal/sink" the CCS current by pulling it off the TIP's base in order to decrease the voltage at the output.

--- End quote ---

I believe I understood this, and to allow that current to go "somewhere" I added a Pot that gave it a path to GND. My knowledge level of transistors is pretty limited so my understanding may not be correct and the pot may not be doing the correct thing. One side is connected to V+, the other to GND and the wiper to the Transistor.


--- Quote from: Kleinstein on March 17, 2019, 05:42:23 pm ---A darlington transistor has about 1.5 V drop, as there are 2 BE junctions. So I would expect a little more than 3.3 V, but not much. With load current there is additional drop at the shunt. The circuit shown here is never made to work with a low drop - more like good enough to get 12 V out from a 18 V source (e.g. laptop supply). The other points are keeping the circuit simple and cheap and use a high side shunt, so that it could be used in combination with an LM317 / LM7805 or similar regulator from the same raw source.

Just using a pot to set the base voltage would be only a very crude voltage adjustment. So this would be a much simpler circuit and the test on this circuit would not tell much about the final circuit.

--- End quote ---

I had no idea the darlington had 2 junctions in it. I always thought it was just 1 junction with a current amp. I am still lost as to why I am only getting 3V3. I will connect up to 12V here soon and see what happens.


--- Quote from: jaycee on March 17, 2019, 05:53:20 pm ---With no error amps, it wont regulate at all

--- End quote ---

I dont understand how this circuit works, so I am trying to decompose it and test each "system" independently to learn about it. My current understanding is that I should get a constant voltage/current out of the transistor with what I have currently breadboarded up. (The 4mA current source and the TIP120 with 100 ohm to ground as a load). I understand that with varying loads and such I am going to get inconsistent results,


--- Quote from: imo on March 17, 2019, 05:57:43 pm ---Your TIP120 and CCS. By changing the R17 from 0(zero)..5kohm you set the output voltage from 0 to about 20V.



--- End quote ---

What you have drawn up is almost exactly what I have breadboarded. I am hesitant to go on until I can figure out why my LED is turning off.
KC0PPH:
Quick Update: I have everything excluding the "current limit" items wired up. I am now powering it from 12V. I get a range of 10.3V to .6V. I am thinking i need a negative supply on the op-amp now to get all the way down to zero as I was able to get to zero before I added in the op-amp. I did not put in the 10K and 470p cap on the op-amp yet (although I think that has nothing to do with the feedback of the OP, just some sort of filtering I would presume).

I might try and rig up my signal generator to a transistor and make a variable load to start playing with this.
Kleinstein:
The small caps at the OPs are usually needed to prevent the regulator from oscillating. So they are not just for a smoother output, but really essential parts.  A main purpose of the simulations is to find the right values for the caps. This circuit is type is not that critical, but the cap value should still be about right.

How far one can go down depends on the OPs. The constant current load needs a negative supply to work - so some 0.6 V sound like a reasonable lower limit for the constant current load.
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