Author Topic: How do make a current limiting knob using LM723 for Linear Reg power supplies?  (Read 25837 times)

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Offline GurumurthyTopic starter

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Dear oldway,

                  True sir, as you said i am a novoice to Practical Electronics. I want to learn power supply design by simple ICs like LM317 and 723 because they cost around 10 rupee, so i can afford to buy these Ics rather than LT308X series ICs. When some smoke comes from my breadboard, It is my money which is wasted. I have learnt certain basic things with LM317 and 723 and TIP142,2N3055,Mj4502 Transistors when things went wrong.This is my study for past 45 days.


                     Now i understand that i have to move to SMPS preregulator to save my Circuits. I am ready to learn things and apply that in practical circuit design. I am more inspired my the way you approach things . Hence please guide me. Give some reference book so that i am strong to handle big Circuit similar to Philips power supply circuits. Not based on university exam. Please help me!!!!!!!!

                         Forgive me if iam wrong...


 

Offline oldway

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If you don't want to waste your money with "magic smoke", you should start with low voltage, low current power supply projects, not a 30V 5A power supply.

For exemple, you could make a very reliable double linear power supply 0 to +/-25V with a LM317 and LM327.

Current limit will not be ajustable but will be safely limited under 1A.

Now, a question for the people who are looking at this topic.... 

I said there was something horribly wrong with the project of the Philips power supplies with pre-regulator as pe-1642....

The problem is with the pre-regulator....

Somebody found what's wrong with it ?
 

Offline MagicSmoker

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I said there was something horribly wrong with the project of the Philips power supplies with pre-regulator as pe-1642....

The problem is with the pre-regulator....

Somebody found what's wrong with it ?

No inductor in series with the output of the half-controlled bridge.
 

Offline oldway

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I said there was something horribly wrong with the project of the Philips power supplies with pre-regulator as pe-1642....

The problem is with the pre-regulator....

Somebody found what's wrong with it ?

No inductor in series with the output of the half-controlled bridge.
No, there is an inductor in the primary to limit the peak current....that's something more complicated....
 

Offline Ian.M

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I've just put together a LTspice sim of the regulator section of the Philips PE 1535-00  0V - 40V, 0A - 0.5A. single output PSU.

Some of the models for the discrete semiconductors are a little rough (heavy use of ako:), but its good enough to see what's going on with the current limit circuit.   I didn't bother with the transformer + bridge or the meter circuit.

Part numbers are as per the Fig. 1000 circuit (page 20 of Philips PSU manual, from Oldway's Reply #7 link). Edit: Link Rot! :( As of 2020 it can be found [here].

Controls and Adjustments

  • R1 - Set output voltage
  • R2 - Set current limit
  • R104 - Trim max current limit (500mA)
  • R108 - Trim zero current limit
  • R123 - Trim zero output
  • R127 - Trim max output (40V)
I've trimmed all the presets to sane values.  R108 is a little fussy, if you trim with the output unloaded it starts dropping the output voltage well before you reach zero current.  The practical lower limit for R2 will be a few tens of mA. 

I've parameterised the R1 and R2 pot positions as vset and iset respectively, for ease of sweeping them in the simulation.   All pots & presets take a parameter wiper which is the position as a number between 0 and 1.

Zipped LTspice simulation attached - Unpack it to a subfolder of your LTspice working folder and enjoy!  :popcorn:  I *think* I've packaged all the custom models it needs - if anything is missing ask.

N.B. The dp() function defined on the schematic wont be seen by the .op data labels unless you copy it to your plot.defs file.  See https://www.eevblog.com/forum/blog/eevblog-516-ltspice-tutorial-dc-operating-point-analysis/msg1383565/#msg1383565 for details (and a better function for limiting displayed data precision).

« Last Edit: December 27, 2020, 04:18:10 pm by Ian.M »
 
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Offline oldway

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Ok, i will try to explain the problem: that's all about phase control and transformers.

To vary the secondary voltage by varying primary voltage with phase control triacs or scr's, that's not recomended.
Why ? because a very little dyssimetry of primary voltage will produce a dc voltage, and this dc voltage will let a hight dc current flowing in the low resistance of the primary.

The transformer get saturated and a very high ac current flows in the primary....this will blows up the fuses and can even damage the transformer.

But you will answer:the scr's are in the secondary, then there can not have any problems....

But is this really a secondary phase control ?
 

Offline MagicSmoker

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Why ? because a very little dyssimetry of primary voltage will produce a dc voltage, and this dc voltage will let a hight dc current flowing in the low resistance of the primary.

Ah, yeah. Also the inevitable asymmetry in the SCR conduction times or their forward voltages can set up a DC bias in the transformer, too. I was so hung up on the current integration inductor not being in the right place I hadn't thought of any other problems.
 

Offline oldway

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But indeed the inductor is not at the right place ....It can't be in the primary because any asymetry of the secundary current will apply a dyssimetrical voltage on the primary of the transformer....And asymetrical current occur during transients, when load vary rapidly....

This is why this power supply is basically unstable and the pre-regulator has a tendency to oscillate like a hell, saturate the transformer, consume a primary current of 30 to 50A and blow up the fuses and circuit breakers.
With the years, it gets worse because the capacitor C27 loses its value and then there is no way to prevent it from oscillating

Wat would be the solution of this problem ?
There are two solutions:
- to place the inductance not in serie with primary of the transformer, but in serie with the secundary of the transformer.
- no need of inductance if you use a transformer with high short circuit impedance like MOT.

Quote
R108 is a little fussy, if you trim with the output unloaded it starts dropping the output voltage well before you reach zero current.  The practical lower limit for R2 will be a few tens of mA.
Current limitation of this power supply is very basic and not intended to work as a precision current source.
It only use a single diferential amplifier and gain is very low.
But it is very stable for this reason and can be very fast ....
The greatest problem of this power supply (0-20V and 0-7V) are the shunt resistors: they are made with thin wires and prone to failure.
« Last Edit: May 19, 2017, 06:13:40 pm by oldway »
 

Offline GurumurthyTopic starter

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Dear oldway,
                I requested yesterday for  nice reference book so that i can learn things and design. so please give me some advice to design circuit.
 


Offline BravoV

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Lot of thanks to mr.bravo , mr old ways and all those good heart who replied to my post.   

Alot of those good hearts people helps which are so sincere got wasted  :--,  just because you want an easy circuit and keep ignoring those good replies.

What happened to your old threads ? Why you abandoned it and create a new one every time there is no replies or replies that you didn't like ?

Offline GurumurthyTopic starter

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Dear Bravo,

               These replies Made me to learn electronics. For Sure i have not wasted the energy of all those who have given there knowledge of designing circuits.

                 But my knowledge is not sufficient to digest these stuff. So i want to take some time to learn and design the circuit. No new Post will come here after.

                   I want good literature survey on Regulated Power supply design and Good Books to study Practical electronics. I already have Electronics Principles By Albert paul Malvino. Guide me in this regard...

 You can all suggest me good Books to learn "The ART OF ELECTRONIC DESIGN". Thank you all.... Regret inconvenience..
 

Offline oldway

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Thousands of circuits: encyclopedia-of-electronic-circuits 7 volumes.....
For exemple, ....
http://docslide.us/documents/graf-encyclopedia-of-electronic-circuits-vol-6pdf.html
 
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Offline GurumurthyTopic starter

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Dear sir, 

          Give me a book which could give nice description of principle of operations of discrete power supplies and integrated circuits linear supplies.
 

Offline oldway

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What about you to do something for yourself and to look on Internet after such a book?  :popcorn:
 

Offline GurumurthyTopic starter

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Dear sir,
            Internet may give data at any instant but a book has  a starting and ending. So mostly I should have hard copy of any book. Next time I should refer this to others.
 

Offline oldway

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Some basic informations on linear power supplies........

http://sound.whsites.net/power-supplies.htm
 

Offline GurumurthyTopic starter

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« Last Edit: May 21, 2017, 03:04:04 pm by Gurumurthy »
 

Offline MARSHALBSB

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Thanks man. I'm loving the lecture!
 


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