### Author Topic: how to understand the functionality of circuit  (Read 4807 times)

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#### audio

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##### how to understand the functionality of circuit
« on: January 14, 2022, 04:53:52 am »
dear all,,

I wanted to understand this circuit, which has cc cv functionality. How to read the circuit and understand the functionality.

#### bob91343

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##### Re: how to understand the functionality of circuit
« Reply #1 on: January 14, 2022, 05:23:36 am »
I would suggest you download the data sheet for the 723 voltage regulator and read the applications section.  It should describe some similar circuits.

Basically the 723 can supply a small amount of current and the power transistors enable much larger currents.  The standard mode for the 723 is CV.  You can get CC by overriding the voltage feedback with a current sensor and some gain.

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#### ledtester

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##### Re: how to understand the functionality of circuit
« Reply #2 on: January 14, 2022, 05:33:42 am »
Besides the datasheet, this is a pretty introduction to the LM723:

https://microcontrollerslab.com/lm723-variable-voltage-regulator/

Then this is a good video going over a simple LM723-based power supply:

LM723 Regulator with Voltages and Maths UEENEEH138A TAFE NSW - Greg Moore
https://youtu.be/tabIyItWlzw

Also check out the write-up that goes along with the video:

https://www.learn.org.au/ueenee-course-pages/ueeneeh138a-fault-complex-power-supplies/psu-week-6/

Alternate link: https://learnorgdata.s3-ap-southeast-2.amazonaws.com/Complex+Power+Sup/Week+7/723+reg+main+sm.pdf

« Last Edit: January 14, 2022, 05:39:35 am by ledtester »

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#### audio

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##### Re: how to understand the functionality of circuit
« Reply #3 on: January 16, 2022, 02:03:50 am »
Thank You sir..

Sir,
In this Circuit Current Sense Resistor 0.33ohm, when 1A current flows through it then there shall be 0.33V . How this would sense 0.7V which is required to turn on current sense transistor. There is a 1K potentiometer which is going to limit the current in  to the base of current sense transistor?

Sir , I am not clear about Current Limiting. Please Explain.

#### basinstreetdesign

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##### Re: how to understand the functionality of circuit
« Reply #4 on: January 16, 2022, 04:04:35 am »
Thank You sir..

Sir,
In this Circuit Current Sense Resistor 0.33ohm, when 1A current flows through it then there shall be 0.33V . How this would sense 0.7V which is required to turn on current sense transistor. There is a 1K potentiometer which is going to limit the current in  to the base of current sense transistor?

Sir , I am not clear about Current Limiting. Please Explain.

This circuit does not current limit at 1A.  It current limits at 2A.
It can SUPPLY 1A safely itself but is designed to limit the short-circuit current to more than that.
The datasheet for the LM723 is here:

https://www.ti.com/lit/ds/symlink/lm723.pdf?ts=1642251339132

On page 4 the short-circuit current limit is quoted as 65 mA when the short-circuit current sense resistor is 10 Ohms.  Thus the burden voltage across that resistor is 0.650V when the SC current clamp kicks in.  That is reasonable since that is the usual base-emitter voltage of a BJT to turn it on.  That transistor is Q16 shown on the chip schematic on page 15.  When that transistor turns on it pulls the base of Q14 and Q15 down removing the base drive from those transistors.

In your circuit the SC clamp would become active at 0.65V / .33Ohms = 2A (approx)
STAND BACK!  I'm going to try SCIENCE!

#### audio

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##### Re: how to understand the functionality of circuit
« Reply #5 on: January 18, 2022, 03:07:48 pm »
Sir,

how 2.5K potentiometer connected  is going to control current?

#### mikerj

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##### Re: how to understand the functionality of circuit
« Reply #6 on: January 18, 2022, 04:19:01 pm »
Thank You sir..

Sir,
In this Circuit Current Sense Resistor 0.33ohm, when 1A current flows through it then there shall be 0.33V . How this would sense 0.7V which is required to turn on current sense transistor. There is a 1K potentiometer which is going to limit the current in  to the base of current sense transistor?

Sir , I am not clear about Current Limiting. Please Explain.

D5, T1 and R5 form a constant current source which sources approximately 1mA through P3, the current limit setting potentiometer. The actual current will depend on the Vf of the LED, and will vary a bit with temperature.

This current creates a voltage drop across P3 which is added to the voltage across the 0.33 current sense resistor.  To get e.g. 1 Amp current limit there would be 0.33v across the sense resistor so P3 would need to be set to add about 0.3-0.4v to this.

#### audio

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##### Re: how to understand the functionality of circuit
« Reply #7 on: January 21, 2022, 01:59:47 am »
Hi to all,

Transformer has two secondary winding, in which one may be higher current which is given to load sharing transistors and other power up Regulator IC with less current.

Terminal 4 and 5 of IC are used to set the output voltage of REgulator.

between terminal 2 and 3 we have an diode which senses current and other end of diode is connected to Variable resistor , Why?

How it controls current?

What is the functionality of Q1 in the circuit?

#### xavier60

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##### Re: how to understand the functionality of circuit
« Reply #8 on: January 21, 2022, 08:09:21 am »
The bottom end of the I set pot could not possibly go to ground, it should connect somewhere early in the output drive path.
The whole design is a mess.
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu,  FY6600 function gen, New! Brymen BM857S-(With Battery)

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#### coromonadalix

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##### Re: how to understand the functionality of circuit
« Reply #9 on: January 21, 2022, 03:40:37 pm »
post # 3   This design is very good   but it has some range limits  in the minimal current setup to the max current

you do have an supplemental power supply with the zenner, to help the lm723 going down to zero in voltage output

for the current section  you can use some trick,  one trick is   add an power rectifier diode in serial of the shunt resistor and recalculate the current limitter

Since the rectifier diode will always have a 0.6 vdc drop  it will help to get a very low current output setting, when set to a minimal value

I have this kind of design  done 15 years ago  still going strong ''0-30 vdc at 10ma to 20 amps'',   i can output current just enough to lit a led without any limitter resistor from zero volts output and up

But i use current foldback limitting ... and all my ''current limiter voltages losses'' are  before the main power transistors
« Last Edit: January 21, 2022, 03:43:29 pm by coromonadalix »

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#### coromonadalix

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##### Re: how to understand the functionality of circuit
« Reply #10 on: January 21, 2022, 03:44:43 pm »
Hi to all,

Transformer has two secondary winding, in which one may be higher current which is given to load sharing transistors and other power up Regulator IC with less current.

Terminal 4 and 5 of IC are used to set the output voltage of REgulator.

between terminal 2 and 3 we have an diode which senses current and other end of diode is connected to Variable resistor , Why?

How it controls current?

What is the functionality of Q1 in the circuit?

This is what we call a crowbar circuit, the scr will short the output in case of an overload

#### coromonadalix

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##### Re: how to understand the functionality of circuit
« Reply #11 on: January 21, 2022, 03:46:44 pm »
Sir,

how 2.5K potentiometer connected  is going to control current?

this trimpot is the voltage reference adjustment

Please read the datasheets   it will help you figure a few things out

#### audio

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##### Re: how to understand the functionality of circuit
« Reply #12 on: January 21, 2022, 04:56:53 pm »
Sir,

Can you explain your schematics in detail. Thank you.

#### coromonadalix

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##### Re: how to understand the functionality of circuit
« Reply #13 on: January 21, 2022, 05:43:49 pm »
Sorry

Can not explain, it was a lot of research at the time ... but the basis was the mentioned post #3 circuit,   not even sure i can find my schematics,  my psu is a mix of a few separated circuits put together

The biggest drawback,  is not having a multitapped secondarie(s) transformer, when you short it to the max  even with a foldback circuit, it will heat ... kinda a lot  loll
------------------------------------------------------------------------------
Here's a beautiful circuit  tested an proven working,  a dual 723 circuit,  explanations and schematics included,  i know there is pcb gerbers available ...

https://www.velleman.eu/products/view/?id=9375

https://www.velleman.eu/downloads/0/manual_k7200.pdf
-------------------------------------------------------------------------------

I dont know your electronic knowledge,  sometimes it can be difficult to explain how some circuit can work, the datasheet helps a lot,  lots of researchs ... explanations of knowledgable people(s)  can help

I am a self taught technician in computers, electronics industrial stuff, broadcast equipements, tv, vcr's, audio sound systems, car mecanics, truck driver ... i did had specialization at school, but i was too advanced  for my classroom    etc ...

I'm in this stuff since im 14 years old now i'm 51 years old loll   Senior tech at an international company in Canada.

« Last Edit: January 21, 2022, 05:57:27 pm by coromonadalix »

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#### xavier60

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##### Re: how to understand the functionality of circuit
« Reply #14 on: January 21, 2022, 09:47:16 pm »
Hi to all,

Transformer has two secondary winding, in which one may be higher current which is given to load sharing transistors and other power up Regulator IC with less current.

Terminal 4 and 5 of IC are used to set the output voltage of REgulator.

between terminal 2 and 3 we have an diode which senses current and other end of diode is connected to Variable resistor , Why?

How it controls current?

What is the functionality of Q1 in the circuit?
When "foldback" was mentioned, the current limiting started to make a bit more sense.
The data sheet shows the internal current limit transistor Base-Emitter between pins 2 and 3. Pin3(E) connects to the PSU's output. Voltage is applied from the Emitter of Q2 to pin2(B) via R4. The voltage at Q2's Emitter increases with respect to the output with increasing load current. When the Base of the current limit transistor reaches about 0.6V, output drive is limited causing output current limiting.
The way the I ADJ pot is wired with its wiper going to Pin3 via R3 will cause the PSU's output voltage to interact with the current setting.
Increasing output voltage causes higher threshold on current limit onset. This is because increasing current through R3 pulls pin2 more negative WRT Pin3. This results in  Q2's Emitter needing to go to a higher voltage WRT the output to reach the onset of current limiting.
As the PSU's output voltage drops, current also decreases because of R3.
CR3 protects the internal current transistor from excessive reverse bias.
It's still a mess.

https://www.eevblog.com/forum/beginners/how-to-understand-the-functionality-of-circuit/?action=dlattach;attach=1386560
« Last Edit: January 21, 2022, 09:51:54 pm by xavier60 »
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#### coromonadalix

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##### Re: how to understand the functionality of circuit
« Reply #15 on: January 22, 2022, 01:06:31 pm »
basic current limitter is a voltage detection between 2 pins of the lm723,  when you get around 0.6v voltage drop around the shunt resistor  the ''limitting occur''

the lm723 is kinda basic, it depends how you implement a current limitting option / need

the velleman k7200 employ a different technic

You can google find many  mastech topward and other psus schematics, with added secondaries xformer switching, and are based around op amps, they have cc and cv  crossover circuitry,  way more practical

I use a Kikusui PAD xxx   smps (kinda buck boost) / linear regulator, it's a mix of at least 2 technologies to increase density / efficiency and diminish the heat and power losses

and another is an Lambda ATE 75-8 psu, this one is fully linear, and when i short this one at full max, its like an 864 watts heater   loll   weight 56 pounds on a bench

The other (Kikusui) is half of it around 26 pounds and almost the same power specs ??

It really depends of your need(s) and application(s)

BUT if you put sensitive stuff on a self made psu   you could get serious problems, stability issues  etc ...  that's why my ''very old design'' is less used,  the 723 is good  but not perfect

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#### audio

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##### Re: how to understand the functionality of circuit
« Reply #16 on: January 23, 2022, 04:48:27 pm »
Sir,

What is the importance of diode 1N4148 between current sense and current limit terminal in Lm723?
« Last Edit: January 25, 2022, 03:15:30 am by audio »

#### audio

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##### Re: how to understand the functionality of circuit
« Reply #17 on: January 25, 2022, 03:19:18 am »
Sir,

What is the importance of diode 1N4148(CR3) between current sense and current limit terminal in Lm723? What is the function of Non linear Device in the circuit?

#### xavier60

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##### Re: how to understand the functionality of circuit
« Reply #18 on: January 25, 2022, 03:49:43 am »
Sir,

What is the importance of diode 1N4148(CR3) between current sense and current limit terminal in Lm723? What is the function of Non linear Device in the circuit?
Because of the way the current limiting foldback works, the ICs internal current sense transistor normally has reverse bias voltage applied to its Base-Emitter junction. The diode is there to clamp the voltage to a safe approximately 0.7V.
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#### audio

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##### Re: how to understand the functionality of circuit
« Reply #19 on: January 25, 2022, 06:52:58 am »
sir,

1) when load current is larger than o.5milli ampere, the T3 PNP Darlington will turn on and All series pass transistor will share equal amount current because of the emitter resistor of .22 ohms with respect to corresponding load.(attached schematics)

2) pin 4 and 5 of IC2 set the output voltage as desired.

3) pin 2 of IC2 and pin 10 of IC3 are connected with resistive divider with an LED why???

4) how pin 4 and pin 5 of IC3 is going to work as Current control IC?

5) how six .33ohm parallel resistor is going to sense current?

6) what is the reason that six .33ohm parallel resistor are below ground potential?

Thank you sir.

#### xavier60

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##### Re: how to understand the functionality of circuit
« Reply #20 on: January 25, 2022, 08:26:55 am »
sir,

1) when load current is larger than o.5milli ampere, the T3 PNP Darlington will turn on and All series pass transistor will share equal amount current because of the emitter resistor of .22 ohms with respect to corresponding load.(attached schematics)

2) pin 4 and 5 of IC2 set the output voltage as desired.

3) pin 2 of IC2 and pin 10 of IC3 are connected with resistive divider with an LED why???

4) how pin 4 and pin 5 of IC3 is going to work as Current control IC?

5) how six .33ohm parallel resistor is going to sense current?

6) what is the reason that six .33ohm parallel resistor are below ground potential?

Thank you sir.
Pin 10 of IC3 is the Emitter follower output option for controlling external circuitry which is able to swing positive, sourcing current.
In this case, when the current setting threshold is sensed by the internal opamp in IC3, its pin 10 swings high, lighting the CC LED and applying voltage to the Base of the current limit transistor in IC2 via its pin 2 and the divider.

The six .33ohm parallel resistors form a 0.055Ω Current Sense resistor. The internal opamp of IC3 compares the voltage drop across the CS resistor applied to pin 4 to a voltage on pin 5 set by RV3.
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#### xavier60

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##### Re: how to understand the functionality of circuit
« Reply #21 on: January 25, 2022, 08:39:11 am »
The bottom end of the CS resistor becomes more negative WRT GND with increasing load current. The reason is just that's the way it has to be.
« Last Edit: February 01, 2022, 12:11:35 am by xavier60 »
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#### audio

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##### Re: how to understand the functionality of circuit
« Reply #22 on: January 25, 2022, 09:47:17 am »
Sir,

1)The function of R21 and 22 is current limiting in inv and non inv terminals of voltage control IC2. R25 and R21 act as voltage divider and the voltage is feed to the inverting input and why the is Rv5,Rv4,R14 network which is feed to non inverting terminal?

2)Similar topology for current Limiting, Why?

3)why capacitor is present at terminal of both Comparator like c3,c5,c6?

Thank you.

#### xavier60

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##### Re: how to understand the functionality of circuit
« Reply #23 on: January 25, 2022, 10:58:09 am »
I think there are mistakes in your questions.
The voltage regulation section works by the same principles as described here. https://en.wikipedia.org/wiki/Error_amplifier_(electronics)
Rv5,Rv4,R14  supply a variable portion of the 7V reference to the non-Inverting input.
Divided output  voltage is applied to the Inverting input via R5,R23,R21.
Most of the capacitors are bypass capacitors for filtering out hum and other noise.
Capacitors connected to pin 13 are for frequency compensation to hopefully make the PSU stable and not oscillate.
« Last Edit: January 25, 2022, 11:05:38 am by xavier60 »
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#### audio

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##### Re: how to understand the functionality of circuit
« Reply #24 on: January 25, 2022, 04:41:16 pm »
sir,

Why Transient analysis is done in Power supplies, even though it is a DC System?(attached Document)
Normally why capacitors are connected in parallel with the Load?

Thank you sir.

Smf