### Author Topic: Constant current constant voltage linear supply problems/questions  (Read 2780 times)

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#### Sniper1

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##### Constant current constant voltage linear supply problems/questions
« on: September 09, 2023, 11:46:31 am »
So i am trying to include a small constant current constant voltage supply in one of my projects ( active load) and after searching the web i found Dave s design that uses just 1 power element (very important for me since i only have 1 heatsink slot that is not used, the heatsink is recovered from something old and it 2.5x1.5x10cm long i calculated online that it can dissipate 15W with no fan and that would be plenty for me atm) ,
to be more precise i am trying to make something similar to this:   https://eevblog.com/uploads/uSupplyBenchRevA.pdf

But fists i thought it would be a good idea to simulate a bit in LTSpice and apparently what i have works well with ideal op amps but real ones dont work well ( i am also attempting to replace the NPN with a Pmos with no success.
On paper they should work unless i forgot to account for something ......
As for the specs I am aiming for they would e loosely this:
• 0V out ( i plan to use 2 Si diodes and have the output come from a floating GND that is 1-1.5V higher then true GND
• 0-12V output range but i can settle with less if it makes sense ( i think here i am sort of limited by the transistor and op amp input voltages + max dissipated power by the heat sink)
• low noise ( that i why i want ti to be linear)
• As for output current i want like 1A for 5-6V and under
• Stable , aka no oscillation and the CC limit to work well
NOTE: power supply kill be a 12-24V AC-DC adapter so i get isolated DC and sort of regulated input, or some battery)
I think/hope that my requirements are reasonable ( yea at super low voltages it would be harder to get large currents because of the large dissipation but at that point i will simply use a lower input voltage like 5V USB)
Regarding the DACs for control i plan to use I2C 8 bit ones for now ( there is a 12bit drop in replacement that i can se later or for a V2)

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##### Re: Constant current constant voltage linear supply problems/questions
« Reply #1 on: September 09, 2023, 12:10:02 pm »
overdesigned   with an lt3080

you could do it with multi turn pots

and for sur a linear psu will dissipate more than a smps or any switching psu

#### Sniper1

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##### Re: Constant current constant voltage linear supply problems/questions
« Reply #2 on: September 09, 2023, 12:32:52 pm »
well i want to avoid pots and use a DAC since this way i can avoid some mistakes and can also ramp up nice from soft + atm the circuit can accept a pot in the DAC  net labels
##### Re: Constant current constant voltage linear supply problems/questions
« Reply #14 on: September 24, 2023, 11:52:16 pm »
I think its a fine part for your power supply because you are actually taking many hours to make it rather then buying one on ebay for $30 not exactly breaking the bank considering hours spent no matter what part you use and if you built a nice one for yourself with expensive parts, you will find that when you have to make the cheap standard issue cheap crap at work, its interesting since its novel.. infact, you can put a bigger heatsink, and 2x lt3080, and nothing bad should happen IRL, at the cost of about 10$ extra.

each 3080 will respond to over temperature also, so you don't need any mechanical shenanigans with thermal coupling
« Last Edit: September 25, 2023, 12:04:46 am by coppercone2 »

#### Marco

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##### Re: Constant current constant voltage linear supply problems/questions
« Reply #15 on: September 25, 2023, 02:25:39 am »
But it's not much cheaper than LT3081, which is just plain better.

#### coppercone2

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##### Re: Constant current constant voltage linear supply problems/questions
« Reply #19 on: September 26, 2023, 10:44:11 pm »
that will be a powerful project

#### deepfryed

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##### Re: Constant current constant voltage linear supply problems/questions
« Reply #20 on: September 27, 2023, 06:37:02 am »
I haven't read through the data sheet completely, but I don't think LT3083 has programmable current limit.

#### Marco

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##### Re: Constant current constant voltage linear supply problems/questions
« Reply #21 on: September 27, 2023, 07:44:05 am »
Yeah, it's the LT3081. Though if you want a digital programmable one instead of setting it with a pot you need to get a bit tricky. You need to do something like use a 390 Ohm resistor between ilim and the output and then push extra current through that resistor to set the limit.

#### Sniper1

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##### Re: Constant current constant voltage linear supply problems/questions
« Reply #22 on: September 27, 2023, 01:42:49 pm »
True but apparently you can use a pot to change the set pin voltage so you can also use a DAC+ op amp magic BUT then again i am where i was before with the LM317s
BUT if i use a LM317 for current limiting and an LT3080/3083 then i may just make it work

#### Marco

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##### Re: Constant current constant voltage linear supply problems/questions
« Reply #23 on: September 27, 2023, 02:21:51 pm »
If you use the LT3081 you don't need the LM317 and you get a stable and very fast CV/CC control loop which can work with no output capacitance.

#### Sniper1

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##### Re: Constant current constant voltage linear supply problems/questions
« Reply #24 on: September 27, 2023, 02:37:25 pm »
I will also check that one, main problem with it is that it is not available on LCSC as of now , so i would need to move my order to mouser (  and cheek vs the shipped threshold since i want cheap/free shipment)
PS the LT308x series is in a way voltage controlled via a output current in the uA range so i need to be carefull when i convert it to DACs

Smf