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Non-linear CV/CC control in schematic

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Silver_Pharaoh:
Hi!

I modified this schematic: http://www.zen22142.zen.co.uk/Circuits/Power/1220.htm with some info I read and found in the LM-723 datasheet and a few other LM-723 circuits I found on the web.

It seems to work, but the voltage and current adjustment are not linear, the voltage jumps higher the more the potentiometer is maxed out, same with current... I was hoping for a smooth linear curve for voltage and current adjustment.
In addition, I don't think the original current limiting works anymore, or at least I don't see it on the graphs in LTSpice XVII.

I've attached everything in a zip file.

R1 & R3 was added by me, their valves were found by experimenting with resistance vs the max & min output voltages.
I re-arranged some wires from pin 2, and added in the voltage adjust potentiometer and the current adjust potentiometer, their values are just what I thought seemed to work.
R14 & R15 values were found by measuring the maximum output current across the load, which I limited to ~16A. (I'd like 15A maximum with an overload trip current of 16A.)


So please, pick it apart and see what I did wrong, and with some guidance hopefully I can fix it all properly  :)

EDIT: Attached picture of the original schematic.

coromonadalix:
would be nice to put it in a bmp format or pdf ???

You do know : you have logarithmic pots and linears ones ???

Kleinstein:
Adjusting the voltage by using a pot as a feed back divider gives an 1/x type curve. To get it linear one  could use the pot as a variable resistor only and keep the lower end constant.

For the current adjustment it's unusual to include two BE junctions, just one forward biased BE junction should be enough to compensate for the drop the transistor for the current limit needs.  Due to the contribution from the BE junction the curve is nonlinear. This is slightly intentional to get a larger range. a slightly better position for the pot would across the B-E junction of the Q2 (at the emitter side one could average over the 4 output stages with 4 extra resistors for a little better accuracy)  - this would also eliminate the need for R14,R15.  As the current limit is not very accurate, I would not worry about this so much.

The horizontal connection between R4,R5..R10 is a bad idea - the separate emitter resistors are needed to balance the current. At the low cost end averaging resistors are usually easier than a separate common shunt.

The BMP file formal is bloated and outdated - .PNG is the suitable choice today.

Silver_Pharaoh:

--- Quote from: coromonadalix on March 02, 2019, 03:59:50 am ---would be nice to put it in a bmp format or pdf ???

You do know : you have logarithmic pots and linears ones ???

--- End quote ---
Sorry! Should have though of that before.... I'll add a .PNG verison to the first post  :)

Yes, the potentiometers I am using should be linear if what I'm reading from potentiometers.lib is correct:
The pot variables in LTSpice are: Rtot=10K wiper=.1 the other pot is Rtot=1K wiper=.1 so they should be linear right?


--- Code: ---* A Collection of Potentiometers
* ==============================
* Helmut Sennewald,                          12/23/2003         V1.1
*
*    Models:
*       potentiometer    old style LTSPICE potentiometer
*       pot_lin          k*x
*       pot_pow          x^k
*       pot_plog         exp(k*(1-x))
*       pot_nlog         exp(kx)
*       pot_tab          table(x)
*       pot_piher_plog   piecewise linear, datasheet
*       pot_radiohm_plog piecewise linear, measured
*   
*
*       1 ____    1.0=wiper   
*             | 
*            | |  3
*            | |<---- wiper 0..1
*            | |
*       Rtap | |  Tap
*            | |
*       2 ____|   0.0=wiper
*
*
*     RTOT = total resistance
*     WIPER = ratio of travel of the wiper
*     RTAP = reference resistance at wiper=Tap
*            It is needed only for pot_plog, pot_nlog and pot_pow.
*            RTAP is measured between pin-2 and wiper.
*     TAP = ratio of travel when Rtap is reached

.func LIMIT(x,a,b) {min(max(x, a), b)}

*--------------------- The Linear Potentiometer ---------------------
*     
* 1.0 <----- 0.0
*        |3
*      __V__
*  1--|_____|--2
*       
*  o--R1-o-R2--o
*
.SUBCKT potentiometer 1 2 3 Rtot=1k wiper=.5
* Parameters: Rtot, wiper
.param w=limit(0,wiper,1)
*
R1 1 3 {Rtot*(1-w)}
R2 3 2 {Rtot*(w)}
.ENDS
*

.SUBCKT pot_lin 1 2 3 Rtot=1k wiper=.5
.param w=limit(0.01m,wiper,0.99999)
*
R1 1 3 {Rtot*(1-w)}
R2 3 2 {Rtot*(w)}
.ENDS

*------------- The Ideal Power Function Potentiometer ---------------
*
* It is interesting to know that the power log. curve is a
* good fit to so called "log"-potentiometers, because most of
* them have not true logarithm dependency in reality.
*
* 1.0 <----- 0.0
*        |3
*      __V__
*  1--|_____|--2
*       
*  o--R1-o-R2--o
*
*  RTAP is resistance at travel TAP
*  Example: Rtot=10k, R=1k @ 0.5   (half way)
*           RTAP=1k, TAP=0.5
*  RTAP and TAP define a point of the curve resistance versus ratio.
*
.SUBCKT pot_pow 1 2 3
* Parameters: Rtot, wiper, Rtap, Tap
.param w=limit(0.01m,wiper,0.99999)
*
.param pwrexp=ln(RTAP/RTOT)/ln(TAP)
.param ratio=w**pwrexp
*
R1 1 3 {Rtot*(1-ratio)}
R2 3 2 {Rtot*(ratio)}
.ENDS

--- End code ---


--- Quote from: Kleinstein on March 02, 2019, 08:39:13 am ---Adjusting the voltage by using a pot as a feed back divider gives an 1/x type curve. To get it linear one  could use the pot as a variable resistor only and keep the lower end constant.

For the current adjustment it's unusual to include two BE junctions, just one forward biased BE junction should be enough to compensate for the drop the transistor for the current limit needs.  Due to the contribution from the BE junction the curve is nonlinear. This is slightly intentional to get a larger range. a slightly better position for the pot would across the B-E junction of the Q2 (at the emitter side one could average over the 4 output stages with 4 extra resistors for a little better accuracy)  - this would also eliminate the need for R14,R15.  As the current limit is not very accurate, I would not worry about this so much.

The horizontal connection between R4,R5..R10 is a bad idea - the separate emitter resistors are needed to balance the current. At the low cost end averaging resistors are usually easier than a separate common shunt.

The BMP file formal is bloated and outdated - .PNG is the suitable choice today.

--- End quote ---

Okay, I've modified the current adjust pot and hooked it up correctly from what I believe. Min current is now 3.24A with a 1 Ohm load. I am guessing adding in those 4 extra resistors you are talking about would help, but I don't quite understand where you are saying to put them? I was hoping to get currents as low as possible (250mA would be great.) I am going to modify the values of R3-R10 (Balancing resistors) to try and lower the max current as it is now upwards of 21.6A! (Changing the potentiometer value doesn't seem to affect the max current.)

I also removed the horizontal connections between the load balancing resistors & relabeled all the resistors numerically.

Kleinstein:
I have played a little more the circuit and changed the current limit to a way it allows adjustment to small values too.
The averaging resistors are only included for 2 power stages, but it should be obvious how to include the others - it just gets messy in the diagram.

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