Author Topic: Non-linear CV/CC control in schematic  (Read 5661 times)

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Offline Silver_PharaohTopic starter

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Non-linear CV/CC control in schematic
« on: March 02, 2019, 03:47:50 am »
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.
« Last Edit: March 02, 2019, 04:24:44 pm by Silver_Pharaoh »
 

Offline coromonadalix

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Re: Non-linear CV/CC control in schematic
« Reply #1 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 ???
 

Online Kleinstein

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Re: Non-linear CV/CC control in schematic
« Reply #2 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.
 

Offline Silver_PharaohTopic starter

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Re: Non-linear CV/CC control in schematic
« Reply #3 on: March 02, 2019, 04:23:54 pm »
would be nice to put it in a bmp format or pdf ???

You do know : you have logarithmic pots and linears ones ???
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: [Select]
* 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

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.

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.
 

Online Kleinstein

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Re: Non-linear CV/CC control in schematic
« Reply #4 on: March 02, 2019, 04:57:22 pm »
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.
 

Offline coromonadalix

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Re: Non-linear CV/CC control in schematic
« Reply #5 on: March 02, 2019, 05:30:12 pm »
Adding a power diode in serial with the current shunt would help to get lower current settings,  just need to adjust component values

this is an old edited psu schematic found on the web,  a negative 1.2 volts on the voltage pot will help to start a zero volts.
« Last Edit: March 02, 2019, 05:33:59 pm by coromonadalix »
 

Offline Silver_PharaohTopic starter

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Re: Non-linear CV/CC control in schematic
« Reply #6 on: March 02, 2019, 05:43:44 pm »
Okay, I added in the remaining averaging resistors and I also lower the values of the balancing resistors to increase the max current to 12.7A.

That seem to work well  :)

12.5A max is fine as well, 15A was just what I was aiming for.
I think the voltage adjustment is still not as smooth as I wanted, i will try to adjust the pot value for that and see if that help smooth it out.
 

Offline duak

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Re: Non-linear CV/CC control in schematic
« Reply #7 on: March 02, 2019, 07:58:43 pm »
If you want a linear output voltage adjustment, change it to something like the attached image where only the upper portion of the voltage divider is variable.

Cheers,

 

Offline Silver_PharaohTopic starter

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Re: Non-linear CV/CC control in schematic
« Reply #8 on: March 03, 2019, 12:54:01 am »
If you want a linear output voltage adjustment, change it to something like the attached image where only the upper portion of the voltage divider is variable.

Cheers,

I cannot for the life of me figure this out! I've tried connecting pin 4 to the top leg of the pot & the wiper, no luck.
Tried moving the pot over to pins 5&6 like I see in the LM723 datasheet, no luck - I have no control over voltage at all.

I'm missing something here....


I've also been reading about getting voltage control down to 0V with the LM723, it seems I need a negative voltage fed into the error amplifier of the LM723..... Not sure how that will play out.
 

Offline xavier60

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Re: Non-linear CV/CC control in schematic
« Reply #9 on: March 03, 2019, 02:02:38 am »
If you want a linear output voltage adjustment, change it to something like the attached image where only the upper portion of the voltage divider is variable.

Cheers,

I cannot for the life of me figure this out! I've tried connecting pin 4 to the top leg of the pot & the wiper, no luck.
Tried moving the pot over to pins 5&6 like I see in the LM723 datasheet, no luck - I have no control over voltage at all.

I'm missing something here....


I've also been reading about getting voltage control down to 0V with the LM723, it seems I need a negative voltage fed into the error amplifier of the LM723..... Not sure how that will play out.
It would be better to have a fixed divider from the output terminals to the error amp's inverting input. The Pot then is used to adjust the reference to the non-inverting input.
Applying a small positive offset to the inverting input will allow the output to be adjusted to a lower voltage.
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Offline coromonadalix

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Re: Non-linear CV/CC control in schematic
« Reply #10 on: March 03, 2019, 04:57:33 am »
another control variant

Velleman kit  k7200
 

Offline Silver_PharaohTopic starter

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Re: Non-linear CV/CC control in schematic
« Reply #11 on: March 03, 2019, 04:55:01 pm »
If you want a linear output voltage adjustment, change it to something like the attached image where only the upper portion of the voltage divider is variable.

Cheers,

I cannot for the life of me figure this out! I've tried connecting pin 4 to the top leg of the pot & the wiper, no luck.
Tried moving the pot over to pins 5&6 like I see in the LM723 datasheet, no luck - I have no control over voltage at all.

I'm missing something here....


I've also been reading about getting voltage control down to 0V with the LM723, it seems I need a negative voltage fed into the error amplifier of the LM723..... Not sure how that will play out.
It would be better to have a fixed divider from the output terminals to the error amp's inverting input. The Pot then is used to adjust the reference to the non-inverting input.
Applying a small positive offset to the inverting input will allow the output to be adjusted to a lower voltage.

the error amp's inverting input is pin 2, which is connected to the current adjustment pot, so I'm not sure how to connect up the voltage adjust pot if the current adjust pot is connected to pin 2?
 

Offline xavier60

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Re: Non-linear CV/CC control in schematic
« Reply #12 on: March 03, 2019, 05:10:49 pm »
I meant the voltage error amp's input pins, 4 and 5.
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Offline duak

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Re: Non-linear CV/CC control in schematic
« Reply #13 on: March 03, 2019, 05:13:48 pm »
Silver, what is your present voltage control circuit?  The linear Vout adjust I presented works very well.

If you want the output voltage to go down to zero, you will have to provide a negative supply voltage to pin 7 of the 723 or figure out a way to operate the 723's core with voltages offset from the actual output voltage.  The latter can be done with some reference diodes such as the LM385.
 

Offline Silver_PharaohTopic starter

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Re: Non-linear CV/CC control in schematic
« Reply #14 on: March 03, 2019, 05:27:39 pm »
I meant the voltage error amp's input pins, 4 and 5.
Ahh, I must have misread. Thansk for clarifying, I will give it another go  :)

Silver, what is your present voltage control circuit?  The linear Vout adjust I presented works very well.

If you want the output voltage to go down to zero, you will have to provide a negative supply voltage to pin 7 of the 723 or figure out a way to operate the 723's core with voltages offset from the actual output voltage.  The latter can be done with some reference diodes such as the LM385.

Currently, the top leg of the pot is connected directly to pin 3 which is Vout, the wiper comes from pin 4, the inverting input (which is coupled to pin 13 with a capacitor) and the other leg of the pot goes to ground where pin 7 connects.

I've attached a picture of the current schematic
 

Offline xavier60

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Re: Non-linear CV/CC control in schematic
« Reply #15 on: March 03, 2019, 05:43:54 pm »
I'm not certain if the idea I mentioned will get t all the way down to 0V output because of the way the voltage error amp is designed in the LM723.
There needs to be enough voltage applied to pin 4 to keep Q13 turned on inside the IC but the Comp pin 13 can't pull much lower than pin 4.

EDIT: I had the pin numbers all wrong from the data sheet.
« Last Edit: March 03, 2019, 06:03:38 pm by xavier60 »
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Offline Silver_PharaohTopic starter

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Re: Non-linear CV/CC control in schematic
« Reply #16 on: March 03, 2019, 05:47:29 pm »
I'm not certain if the idea I mentioned will get t all the way down to 0V output because of the way the voltage error amp is designed in the LM723.
There needs to be enough voltage applied to pin 2 to keep Q13 turned on inside the IC but the Comp pin 9 can't pull much lower than pin 2.

Doesn't need to be perfect, I don't think I'll need anything lower than 1V. I was only asking if I need to test something low voltage like an LED or something :)


I think I'm getting somewhere though, I modified the schematic again and moved the volt adjust pot over to where I think you were telling me to move it.
It seems pretty linear, each 0.1 increase or decrease in the pot turn raises or lowers Vout by ~2V. That's not too bad I think.

I've attached the latest revision below.
 

Offline xavier60

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Re: Non-linear CV/CC control in schematic
« Reply #17 on: March 03, 2019, 05:54:17 pm »
Notice that I had to correct the pin numbers in post #15. I keep forgetting that there a 2 packages with different pin outs.
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Offline xavier60

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Re: Non-linear CV/CC control in schematic
« Reply #18 on: March 03, 2019, 06:15:19 pm »
It will need a resistor from Vref to the Inv pin to keep some minimum voltage on it but I'm having trouble figuring out how much it needs.
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Offline Silver_PharaohTopic starter

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Re: Non-linear CV/CC control in schematic
« Reply #19 on: March 03, 2019, 06:30:48 pm »
Notice that I had to correct the pin numbers in post #15. I keep forgetting that there a 2 packages with different pin outs.

Haha I see! I better pay attention when I'm ordering the LM723.
Wonder why they do that? wouldn't it make more sense to keep the pinout the same?


I think the circuit is pretty much done. I'm happy with it anyway. I've attached it to the post.

Can anyone see any issues with it? I added in the resistor wattages as well. They should be good I think.
 

Offline xavier60

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Re: Non-linear CV/CC control in schematic
« Reply #20 on: March 03, 2019, 06:41:23 pm »
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.
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Offline duak

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Re: Non-linear CV/CC control in schematic
« Reply #21 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?
 

Offline xavier60

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Re: Non-linear CV/CC control in schematic
« Reply #22 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
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Offline Silver_PharaohTopic starter

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Re: Non-linear CV/CC control in schematic
« Reply #23 on: March 03, 2019, 10:33:28 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?

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.

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

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.
 

Offline xavier60

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Re: Non-linear CV/CC control in schematic
« Reply #24 on: March 03, 2019, 10:58:42 pm »
I drew this up quickly. Ill check for possible mistakes later.
The values aren't supper critical.
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