how to understand the functionality of circuit - Page 1
Forum section: electronics / beginners

author: audio
author_rank: Regular Contributor
timestamp: January 14, 2022, 04:53:52 am
content: dear all,,I wanted to understand this circuit, which has cc cv functionality. How to read the circuit and understand the functionality.

author: bob91343
author_rank: Super Contributor
timestamp: January 14, 2022, 05:23:36 am
content: 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.

author: ledtester
author_rank: Super Contributor
timestamp: January 14, 2022, 05:33:42 am
content: 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 Moorehttps://youtu.be/tabIyItWlzwAlso 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

author: audio
author_rank: Regular Contributor
timestamp: January 16, 2022, 02:03:50 am
content: 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.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: January 16, 2022, 04:04:35 am
content: Quote from: audio on January 16, 2022, 02:03:50 amThis circuit does not current limit at 1A. It current limits at 2A.It canSUPPLY1A 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=1642251339132On 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)

author: audio
author_rank: Regular Contributor
timestamp: January 18, 2022, 03:07:48 pm
content: Sir,how 2.5K potentiometer connected is going to control current?

author: mikerj
author_rank: Super Contributor
timestamp: January 18, 2022, 04:19:01 pm
content: Quote from: audio on January 16, 2022, 02:03:50 amD5, 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.

author: audio
author_rank: Regular Contributor
timestamp: January 21, 2022, 01:59:47 am
content: 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?

author: xavier60
author_rank: Super Contributor
timestamp: January 21, 2022, 08:09:21 am
content: 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.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 21, 2022, 03:40:37 pm
content: post # 3 This design is very good but it has some range limits in the minimal current setup to the max currentyou do have an supplemental power supply with the zenner, to help the lm723 going down to zero in voltage outputfor 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 limitterSince 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 valueI 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 upBut i use current foldback limitting ... and all my ''current limiter voltages losses'' are before the main power transistors

author: coromonadalix
author_rank: Super Contributor
timestamp: January 21, 2022, 03:44:43 pm
content: Quote from: audio on January 21, 2022, 01:59:47 amThis is what we call a crowbar circuit, the scr will short the output in case of an overload

author: coromonadalix
author_rank: Super Contributor
timestamp: January 21, 2022, 03:46:44 pm
content: Quote from: audio on January 18, 2022, 03:07:48 pmthis trimpot is the voltage reference adjustmentPlease read the datasheets it will help you figure a few things out

author: audio
author_rank: Regular Contributor
timestamp: January 21, 2022, 04:56:53 pm
content: Sir,Can you explain your schematics in detail. Thank you.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 21, 2022, 05:43:49 pm
content: SorryCan 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 togetherThe 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=9375https://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 helpI 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.

author: xavier60
author_rank: Super Contributor
timestamp: January 21, 2022, 09:47:16 pm
content: Quote from: audio on January 21, 2022, 01:59:47 amWhen "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

author: coromonadalix
author_rank: Super Contributor
timestamp: January 22, 2022, 01:06:31 pm
content: 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 / needthe velleman k7200 employ a different technicYou 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 practicalI 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 lossesand 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 benchThe 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

author: audio
author_rank: Regular Contributor
timestamp: January 23, 2022, 04:48:27 pm
content: Sir,What is the importance of diode 1N4148 between current sense and current limit terminal in Lm723?

author: audio
author_rank: Regular Contributor
timestamp: January 25, 2022, 03:19:18 am
content: 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?

author: xavier60
author_rank: Super Contributor
timestamp: January 25, 2022, 03:49:43 am
content: Quote from: audio on January 25, 2022, 03:19:18 amBecause 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.

author: audio
author_rank: Regular Contributor
timestamp: January 25, 2022, 06:52:58 am
content: 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.

author: xavier60
author_rank: Super Contributor
timestamp: January 25, 2022, 08:26:55 am
content: Quote from: audio on January 25, 2022, 06:52:58 amPin 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.

author: xavier60
author_rank: Super Contributor
timestamp: January 25, 2022, 08:39:11 am
content: 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.

author: audio
author_rank: Regular Contributor
timestamp: January 25, 2022, 09:47:17 am
content: 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.

author: xavier60
author_rank: Super Contributor
timestamp: January 25, 2022, 10:58:09 am
content: 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.

author: audio
author_rank: Regular Contributor
timestamp: January 25, 2022, 04:41:16 pm
content: 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.

author: xavier60
author_rank: Super Contributor
timestamp: January 25, 2022, 11:01:31 pm
content: Quote from: audio on January 25, 2022, 04:41:16 pmAs well as seeing how the PSU's output voltage dips then recovers when there is a sudden load increase, the load transient test will show how stable the feedback loop is.Here is another member testing a PSU with various compensating component values,https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg3893150/#msg3893150It is clear which is the most preferable response.

author: audio
author_rank: Regular Contributor
timestamp: January 28, 2022, 06:52:45 pm
content: Sir,1) what is the need of bc557 T1 transistor?2) why ic1 is present in the circuit? I guess it would make output down to zero, but how?Thank you.

author: xavier60
author_rank: Super Contributor
timestamp: January 29, 2022, 08:54:26 am
content: Quote from: audio on January 28, 2022, 06:52:45 pmT1 is just a driver for the relay coil because the opamp might not have enough output current capability to drive the coil directly.The opamp is configured as voltage detector with some hysteresis, its purpose being to control the transformer's secondary tap switching.Do you understand opamp function and application?

author: audio
author_rank: Regular Contributor
timestamp: January 29, 2022, 04:25:01 pm
content: Sir,when the output is 20V @ 5A, Then Switch should be switched to top end of the coil. But How the System Will sense and Switch?Sorry I cant Understand the operation of the switch and why we have employed opamp in the circuit?

author: xavier60
author_rank: Super Contributor
timestamp: January 29, 2022, 10:00:35 pm
content: Quote from: audio on January 29, 2022, 04:25:01 pmThe opamp is being used as a voltage comparator. Positive feedback through R32 causes it to have 2 threshold voltages, or to have hysteresis. Search for "schmitt trigger"Doing some rough calculations with the 10K VR set half way, the opamp's output will swing low as the PSU's output increases above about 13.5V, turning On the relay. The opamp's output will swing high again when the PSU's output decreases below about 12.5V.

author: audio
author_rank: Regular Contributor
timestamp: January 30, 2022, 01:13:02 am
content: thank You..

author: audio
author_rank: Regular Contributor
timestamp: February 04, 2022, 09:15:43 am
content: Hi to all,I Found this Circuit in internet. What i have Understood from the circuit.1)The Transistor T1 Is parallel transistor or Darlington Transistor to supply current The Load(Connected to High Current source VL). The Circuit has two OPAMP and forms two loop for Voltage and Current Loop to control.2) OA1 is Inverting Amplifier and output depends on ratio of (R2/R1)*Vref3)OA2 is doing positive feedback, could Not Understand how Current Limitation occur in the Circuit??Thank you all

author: xavier60
author_rank: Super Contributor
timestamp: February 04, 2022, 09:33:07 am
content: The voltage at the left of the CS resistor R5 goes negative WRT GND with increasing load current. So the feedback polarity is correct, negative.

author: audio
author_rank: Regular Contributor
timestamp: February 04, 2022, 10:00:22 am
content: Sir,,,How these arrangement Could do Current Limiting Function??Why There is A vref2 source in OA2?Thank You

author: xavier60
author_rank: Super Contributor
timestamp: February 04, 2022, 10:49:36 am
content: With no load, Vref2, R3 and R4 cause an adjustable positive voltage on the non-inverting input of AO2. The output of OA2 will be at the full supply voltage, 33V. D2 will be reverse biased having no control of T1.If load current is increased to the point that causes OA2's non-inverting input to drop slightly below its inverting input. OA2's output will swing low, first causing D2 to forward bias, taking control of T1 from OA1. OA2 might need to continue to pull the Base of T1 lower until the output current settles to a value that causes OA2's inputs to be very close to equal, 0V.The purpose of D1 and D2 is to cause whichever opamp that has the lowest output voltage to take control of T1. They can be called "ORing" diodes, although not strictly correct.BTW, it's actually the current from the Constant Current source that supplies drive current to T1.

author: audio
author_rank: Regular Contributor
timestamp: February 04, 2022, 12:14:22 pm
content: Sir,,"it's actually the current from the Constant Current source that supplies drive current to T1"1)Why current source is in between 33v source and Output of OPamp ?2)Does that mean Current is going to come from source and opamp(milli amps) , Summed and supplied to Transistor?3)If i Construct the circuit, how to select(a)opamp(b) Resistance Vales and Wattage.4) Whether This RPS is Practically Usable in Day today use, Like charging the battery(high Current 10 A)thank you

author: xavier60
author_rank: Super Contributor
timestamp: February 04, 2022, 12:29:01 pm
content: Quote from: audio on February 04, 2022, 12:14:22 pmKeep it simple by thinking of the opamps controlling only their output voltages and so the Base voltage of T1. The Emitter of T1 follows its Base voltage. The load determines the Emitter current. T1 determines the required Base current which is drawn from the CC source. All surplus CC current is sunk to GND by either one of the opamps.I dont think I want to calculate practical values.

author: audio
author_rank: Regular Contributor
timestamp: February 04, 2022, 12:43:41 pm
content: Sir,How to construct Constant Current Source? marked in image.Thank you sir.

author: xavier60
author_rank: Super Contributor
timestamp: February 04, 2022, 03:11:55 pm
content: There are a few simple ways,https://www.nexperia.com/applications/sub-systems/constant-current-source.htmlSearch for "Constant current source"

author: richard.cs
author_rank: Super Contributor
timestamp: February 04, 2022, 04:50:29 pm
content: This is a perfect example of a situation where the schematic becomes hugely more readable with an IC symbol that reflects the internal functionality, example attached. It may help to re-draw your schematic in this way.

author: audio
author_rank: Regular Contributor
timestamp: February 05, 2022, 02:17:49 am
content: Sir,1)Why There is a current Source in the circuit, OA1 itself can Provide few milliamps of current or we could use Darlington Pair ?2)What is the need of current source between unregulated supply and output of OA1?Still not clear sir.

author: xavier60
author_rank: Super Contributor
timestamp: February 05, 2022, 02:44:15 am
content: Quote from: audio on February 05, 2022, 02:17:49 amThe CC source doesn't connect to the output of OA1, it connects to the junction of the 2 diodes anodes.The CC source isn't strictly needed. It could be replaced by a pull-up resistor or one of the diodes could be replaced by a resistor.Replacing the CC source with a pull-up resistors forces a compromise. If the value is made low enough to supply enough current at high output voltage settings, it's dissipation might be too high at low output voltage settings.Another advantage of the CC source is that it will inject almost no ripple current into the control path.

author: audio
author_rank: Regular Contributor
timestamp: February 05, 2022, 05:38:00 am
content: sir,,how current limit LED circuit is operating?T1 is a pnp transistor, I have applied KVL in Base emitter loop and Current is nearly 2mA. How it will going to operate as current limit indicator?

author: xavier60
author_rank: Super Contributor
timestamp: February 05, 2022, 06:51:53 am
content: Quote from: audio on February 05, 2022, 05:38:00 amWith a red LED used, that CC source should supply about 1mA. It's not critical anyway. This current mainly flows through P3 to cause a voltage drop that's proportional to its set resistance. This adjustable voltage becomes added to any voltage drop across R8 and is applied to the current limit BJT inside of the LM723. The whole idea is so that the PSU has a current limit range that can adjust down zero.

author: xavier60
author_rank: Super Contributor
timestamp: February 05, 2022, 07:04:41 am
content: I should have also mentioned that the LED function is as a crude voltage reference for the CC source circuit.Two or more Silicon diodes in series are often used for the same purpose.

author: audio
author_rank: Regular Contributor
timestamp: February 05, 2022, 08:54:30 am
content: Sir,,The 0.33ohm at 2 ampere load Current will develop .7V .Hence it will be 0.7V needed to turn current limiting transistor.1)how this p3 variable resistor is going to be used control current of 1A?2)how 1mA emmitter current of T1 is going to be used as Indicator?3)Still Current limiting Function Not Clear Sir.

author: xavier60
author_rank: Super Contributor
timestamp: February 05, 2022, 10:45:49 am
content: Quote from: audio on February 05, 2022, 08:54:30 amIll try to answer some of that now.Assuming the CC source is really supplying 1mA.At 1A, the voltage across R8 will be 0.33V.If P3 is set to 270Ω, it will have 0.27V across it which will up to 0.6V applied to the CL BJT in the LM723 and cause PSU output current limiting.What are you planning to do eventually? Build something?I don't fully understand question 2. The 1mA constant current always normally stays the same.

author: audio
author_rank: Regular Contributor
timestamp: February 05, 2022, 04:13:33 pm
content: Sir,If Variable resistance P3 is at 1K and 1mA Current Flows will Devolop 1V and already o.33V across Current sense resistor because Load current is 1A. Then voltage developed across CL and CS terminal is 1.33V.so Excess base voltage results more Ib,then more Ic which reduce base current of series pass transistor of LM723 IC. REduces Load Current and succesfully Reduces load voltage. This will be the action.But as per your Explanation when P3 is 270ohm, current limiting will begin?1)why p3 should be 1K,why not 270 ohm?2)Actually circuit does current limiting up to 1Ampere, How Sir?3) i cannot correlate question 1 and 2?thank you sir

author: xavier60
author_rank: Super Contributor
timestamp: February 05, 2022, 10:22:51 pm
content: Quote from: audio on February 05, 2022, 04:13:33 pmCurrent limiting occurs when the voltage between CL and CS reaches about 0.6V. With P3 set to 0Ω, current limit will be about 1.8A.With P3 set to 600Ω there will be 0.6V applied between CL and CS causing the current limit to be near 0A.All this assumes that the CC source is really 1mA. Maybe it's not.Do you have a link to the whole project?

author: xavier60
author_rank: Super Contributor
timestamp: February 06, 2022, 12:50:18 am
content: I found this,https://learnorgdata.s3-ap-southeast-2.amazonaws.com/Complex+Power+Sup/Week+7/723+reg+main+sm.pdfIt says that the output is up to 1A. Maybe I have made a mistake. The current limiting still works as I explained.Has someone built this PSU design?When P3 is 270Ω, the voltage drop across it at 1mA will 0.27V. So the current limiting will begin at about 1A.

author: audio
author_rank: Regular Contributor
timestamp: February 06, 2022, 08:27:48 am
content: Sir,Is there any way to include current limit indicator circuit. PFA.

author: xavier60
author_rank: Super Contributor
timestamp: February 06, 2022, 11:05:45 am
content: One way is to use another BJT to sense the voltage between pins 2 and 3 with a resistor in series with the Base. The best resistor value might need to be found by testing. A 1K to start with.The indication can never be trusted to be precise.I guess it wouldn't hurt to drive an LED with a few mA using the REF pin as the supply.

author: xavier60
author_rank: Super Contributor
timestamp: February 06, 2022, 11:18:03 am
content: There is a chance that the indicator BJT might never turn on enough. To fix it, a high value resistor might need to be connected from the REF pin to the Base of the indicator BJT.

author: audio
author_rank: Regular Contributor
timestamp: March 03, 2022, 04:38:03 pm
content: Hi to all,How this circuit is having control on Current?R11 should be 240ohm or 120 ohm but in the circuit it is 4.7K. does this affect the q current of regulator.what is the function of T1,T2,T3,D8,D9 in current regulation???