Power Supply some help needed  - Page 1
Forum section: electronics / beginners

author: DarkZero
author_rank: Regular Contributor
timestamp: March 04, 2014, 09:18:58 pm
content: Hello All!I found one schematic of a power supply that my father designed some 20+ years ago, now im no expert im on a second year of electrotechnical school. The thing is i cant get that -6v that's marked on schematic and this is just my guess either that thing is not oscillating or it cant supply enough current. now if someone familiar/smart can tell me whats going on and also i would like to add some sort of notification like LED when it goes in current limit some hints around that would be great !now i only want to build this as memento, my father died when i was 10 so i cant really ask him about this and my professors seems uninterested to help me although im good student ( 4.57 out of 5.00 if u must know)thanks in advance !

author: Zepnat
author_rank: Regular Contributor
timestamp: March 05, 2014, 11:03:01 pm
content: I cant really help but thought id say that's a pretty cool supply. Have you tried running off AA batteries and measuring the -6v current draw, looks like it should only be a few MA's. Hope you get it going.

author: HalfSpace
author_rank: Regular Contributor
timestamp: March 06, 2014, 12:23:28 am
content: Hi DarkZero,Adding a current limit LED should not a problem. Lets get the power supply working first.The -6V supply is only for IC3 & IC4, are they in sockets? If so remove them and measure if the -6V is now ok. One of the ICs or both may be faulty and loading down the -6V supply.If you have access to an oscilloscope, measure the collector of Q2 to see if the circuit is oscillating. Or maybe a multimeter on its AC range should show a voltage if the circuit is oscillating.If it is not oscillating you will need to test Q1, Q2, D5, D6 & D7 are ok. Also check L1 is not open circuit and C8, C9 are still 10uF.HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: March 06, 2014, 05:27:27 pm
content: ok i removed op-amps from the circuit now im getting 1v above ground so i went to change those two transistors, i turned it on and for a brief moment a heard a hum i tought oh its working something and on the power supply that i used there was 400mA draw so i grabbed instrument to measure voltage and before i did that something smokedand huming stopped and current draw dropped to 15mAin the attachment there is board layout i used, maybe mistake is there but ill first have to sort out what burned out since its not obvious,there is also actual thing built !maybe its a faulty design of that oscillator i guess ill breadboard it and i do have some oscilloscope but i went to simulate the thing and there were some weird shaped signals reaching 600v and i didn't want to probe it with oscilloscope just yet (mainly because i'm novice in doing Measurement's with oscilloscope)

author: HalfSpace
author_rank: Regular Contributor
timestamp: March 06, 2014, 09:48:49 pm
content: Hi DarkZero,If the power supply that you used to power this circuit has adjustable current limit set it to about 50mA. If it does not add a 100 ohm 5 watt resistor in series with the supply this will limit current flow and reduce damage to the circuit if there is a fault.One question where is L1? On the overlay it is next to IC3 but I don’t see it on your PCB.In your simulation did you have a load on its output? With no load it may show some weird signal levels.HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: March 07, 2014, 07:16:15 pm
content: hey HalfSpace!i forgot to drill the holes so i soldered L1 on the bottom side, and just to make sure 102 is 1mH inductor ?actually i didn't have load on that simulation ... shows how much novice i am :/, guess ill give that a try tootomorrow ill be able to use power supply with current limit and ill breadboard that negative voltage converter then ill come back to see if we can sort it outthanks for your efforts !

author: megajocke
author_rank: Regular Contributor
timestamp: March 08, 2014, 05:56:23 pm
content: There seem to be some connections missing from the schematic. Between R4 and R5 for example.

author: DarkZero
author_rank: Regular Contributor
timestamp: March 09, 2014, 11:46:38 am
content: First thing first megajocke, first i need to sort out that -6vI have simulated the thing and i built it on a bread board and i got same results it does oscillate but Q2 (Q1 in simulation) is overheating :/ i measured ~400mA current draw and it was very hot to touch but it does workoh well it beyond my current ability to fix it if someone can suggest something i'm all ears

author: HalfSpace
author_rank: Regular Contributor
timestamp: March 09, 2014, 11:42:56 pm
content: Hi DarkZero,Q1 is overheating because it is biased on too hard. Lower R3 to around 220R this will lower the average current through Q1 to about 50mA with the 600R load. If the circuit is not oscillating, increase R3.HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: March 13, 2014, 04:01:16 pm
content: Hello HalfSpacei lowered R3 as u suggested to ~220R and it starts drawing 30mA then gradually starts drawing more and more so i tought i should put a trimer pot 0-500R and play with it to try and make it stable but i was getting same results and now Q2 is overheating if i set power supply to more then 40mA any suggestions if you still have the will ?

author: HalfSpace
author_rank: Regular Contributor
timestamp: March 14, 2014, 11:51:58 am
content: Hi DarkZero,Looks like some thermal runaway. Looking at the simulation diagram try adding a 4.7R resistor with a 10uF electro cap across it between the emitter of Q1 and ground. You may need to trim R3 again. Q1 may require a small heatsink to reduce its temperature and hence the thermal runaway effect. Q2 should not be over heating as it has only a few milliamps flowing through it.HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: March 15, 2014, 02:38:54 pm
content: hey HalfSpacethanks for your time and effort finally some good news i added 4R7 between emiter and the gnd with 10uF cap paralleled to the resistor with 20mA load at 6v and this is what i gotnow i didnt have heatsinks for TO-39 so i made it my self it worked well i guess. with no load voltage at the output was 6v and it was drawing ~17mA, with 20mA load it got lowered to 5.8V and i dont know if this is gona be enough current for the op-amps to work properly, if not is there any way i can increase output current ?

author: HalfSpace
author_rank: Regular Contributor
timestamp: March 16, 2014, 06:35:05 am
content: Hi DarkZero,Increasing R3 or maybe lowering the 4R7 may increase output current. The only way to know if 20mA is enough is to get the -6v circuit on your main PCB working and then add the op-amps.The output voltage will drop a little under load because of the 4R7 resistor, but it should not be a problem for the op-amps as they only need a negative supply so their inputs can work down at zero volts.HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: March 17, 2014, 07:54:51 pm
content: hello !i went full retard on that pcb layout, opamp that controls lm723 on the layout is so badly connected that its a shame, once i rework that ill give it a try.

author: DarkZero
author_rank: Regular Contributor
timestamp: March 19, 2014, 06:35:15 pm
content: Hello!Ok, i fixed errors in wiring i turned it on and i had some weird results that -6v was at +1.2v then i removed opamps while circuit is still on and i reinserted them and then i got -6v, i have to remove them while circuit is on because it happens every time i turn it on :now while it was stable i went to try voltage regulation it seems to workhttps://www.youtube.com/watch?v=WqsFOS7d0w8&feature=youtu.bebut current regulation isnt working here are some messurments at pin 1 and 5https://www.youtube.com/watch?v=pOWj-vNDgzA&feature=youtu.be

author: HalfSpace
author_rank: Regular Contributor
timestamp: March 20, 2014, 02:06:47 am
content: Hi DarkZero,On your schematic IC4 does not match the PCB layout which is a problem to follow. The connections on IC4 of your schematic should be:Differential amp (connected to R12) Pin2 should be Pin5 (+ input), Pin3 should be Pin6 (- input), Pin1 should be Pin7 (Output).Comparator (connected to the LM723) Pin6 should be Pin3 (+ input), Pin5 should be Pin2 (- input), Pin7 should be Pin1 (Output).On your PCB, Pins 11,12 on the LM723 and Pin1 of IC4 are connected to GND. I think you have corrected this is that correct?IC4 may have been damaged if it was powered up with its output connected to GND, best to replace it.The output of the differential amp (first half of IC4) Pin7 should be around zero volts and produce around +10mV for every mA drawn from the power supply. The comparator (Second half of IC4) compares this voltage with the voltage set on P3 (Current Limit), if the current drawn is greater than what is set the comparator output will go low. No it will go -6V! This maybe a design problem as I don’t think the LM723 will like -6V on its + supply pins. Adding a diode between the output of the comparator and the LM723 could be a good idea.If the -6V supply will not start under load you may have to increase R9 in value.HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: March 20, 2014, 10:49:43 am
content: hey HalfSpace!yes i did rework those connections and double checked it they match schematic now.My previous message was bad in explaining things. Whats happening now is that when i power on i get draw of ~80mA and there is 1.2v above the gnd where it should be -6v, then i pull out both op-amps while the circuit is still on now i get ~30mA and then -6v comes back then i just reinsert same op-amps that i pulled out while circuit is still on now i get ~60mA and -6v is present and stable if i turn it off i have to repeat whole process again this video demonstrates it:voltage regulation seems to work but current regulation isnt working pins 1 & 5 are at 21.7v at load of 100mA, i get the part where second op-amp of IC4 is configured as comparator if V- is greater then V+ then it goes close to what ever is on low side of the op-amp supply what i don't know beside the first problem i described, is that differential configuration viable (does schematic really does what you described "+10mV for every mA drawn") i cant confirm math behind it because i dont have knowledge yet

author: HalfSpace
author_rank: Regular Contributor
timestamp: March 20, 2014, 12:06:18 pm
content: Hi DarkZero,The -6V supply will have a higher load on its output on power up. C9 needs to be charged up and the op-amps will most likely draw more current until they settle. If the current requirement is too high the -6v will not oscillate and you get +1.2v due to the voltage drop across D5 & D6 being pulled up by the –V supply pins on the op-amps. Increasing the value of R9 may fix the problem.I think we need to get the current limit circuit working first as it may be the cause of the -6V supply loading on power up.Set the voltage output of the power supply to 10v with no load. Pins 5 and 6 of IC4 should be around 9.7v and pin 7 around 0v. Place a 100R 5W resistor on the power supply output as a 100mA load. Pin 7 of IC4 should now read about 1v. If you are not getting these values something is wrong with IC4 it may need replacing or check its wiring.For the “+10mV / 1mA drawn” I would do an LTSpice simulation of the differential amp to understand its gain function.HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: March 21, 2014, 07:47:22 pm
content: Quoteshould i set voltage to around 9.7v with the pot on the pin 6 because that's the non-inverting output of the second comparator or ?

author: HalfSpace
author_rank: Regular Contributor
timestamp: March 21, 2014, 10:49:13 pm
content: Hi DarkZero,Can you fix the errors on your schematic as to match your PCB and post it again? I think there is some confusion still with IC pin outs. We need to be working off the same page (schematic).I think you mean the current limit pot? Set it to maximum.Do you get the voltage readings as per post #17?HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: March 22, 2014, 04:12:10 pm
content: hello !i have set the output to 10v and in the attachment are results with and without load i get same readings even with new op-amps same result

author: HalfSpace
author_rank: Regular Contributor
timestamp: March 22, 2014, 10:36:16 pm
content: Hi DarkZero,I think you have the inputs of the op-amps swapped around. Going by your PCB overlay the wiring should by like the attached picture.HalfSpace

author: SL4P
author_rank: Super Contributor
timestamp: March 23, 2014, 03:24:18 am
content: Just to throw in some recognition for HalfSpace's effort in helping DarkZero.You're a valuable member of groups like this.Cheers

author: DarkZero
author_rank: Regular Contributor
timestamp: March 23, 2014, 08:39:53 am
content: Quotemy thoughts too!Quotealright ill redo everything to be as in your attached picture will be back soon with results.

author: DarkZero
author_rank: Regular Contributor
timestamp: March 24, 2014, 07:42:06 am
content: Hey HalfSpace!I fixed layout to match your attached image and it works !!now i still have problems with that start-up ill try getting it to work with trim pot on the base will post results.thanks a lot i appreciate your help i didn't think this will go this far !

author: DarkZero
author_rank: Regular Contributor
timestamp: March 24, 2014, 10:00:07 am
content: changing base resistor doesnt do anything with the start-up issue it just incises overall current consumption of the whole negative part of the circuit. also i noticed when i set that voltage pot to lowest value my -6v drops to -5.88 and that lm317t is not regulating properly its not 22.5 it has some voltage drop of 2v so if i set 26 volts on the rectifier i get ~23.5v on the output i guess something has happend to lm317 will replace it

author: HalfSpace
author_rank: Regular Contributor
timestamp: March 25, 2014, 11:57:04 am
content: Hi DarkZero,The LM317T has a drop out voltage of about 2V, so drive the circuit with 26-28v. The 5.1k resistor is a little high in value and that’s why you are getting 23.5v. Add a 120k resistor across the 5.1k should drop the LM317 output volts to around 22v.Do you have a digital oscilloscope? If so can you measure the voltage on the -6v supply on power up with the op-amps installed.HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: March 25, 2014, 05:55:42 pm
content: hello !i live alone and my monthly incomes are little over 100 euros and half of that goes to school (monthly bus) and other half is barely enough to eat and pay bills so this is all i really own (see the attachment) .everything else i use is from a guy that was kind enough to lend me his workshop (hes in white goods) until i build this power supply and he has scope that's even worse then mine. and at school professors wont let me use their gear and its outdated also.everything i own didn't cost me more then 20 euros and i fixed this scope by luck by suggestion of someone on this forum !if something can be done with this scope ill put it in a bag and get it to that guy because he has current limited supply hope i don't break it on the way !

author: HalfSpace
author_rank: Regular Contributor
timestamp: March 25, 2014, 10:52:45 pm
content: Hi DarkZero,Yes I have been a student in the same circumstances trying to make money last to the end of each month. We will work with what you have available to you.I have noticed that capacitors C3 and C4 negative leads are connected to the ADJ pin of the LM317. When I do a simulation in LTSpice of your power supply circuit the -6v supply will not start up with the current configuration of C3 and C4. This maybe this is why the -6v won’t start up under load. They should be connected to GND. (See attached picture).HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: March 25, 2014, 11:33:18 pm
content: hello !hmm didn't notice that ill try that tomorrow after school !

author: DarkZero
author_rank: Regular Contributor
timestamp: March 26, 2014, 11:06:37 am
content: hello!i still have issue with start-up i properly grounded those caps and removed all part's of that 7815 regulator but good news is when i turn the voltage pot to its minimum that -6v doesnt drop to 5.88 anymore its very stable now

author: HalfSpace
author_rank: Regular Contributor
timestamp: March 27, 2014, 01:25:58 am
content: Hi DarkZero,Good to hear we are getting a little closer to a working power supply.Simulating the -6v supply in LTSpice I have found that it requires a fast turn on of its supply rail (22v). If the rise time of the 22v rail is longer that 20mS the -6v supply will not start. When using your friends power supply to drive this circuit it may have a long rise time if it’s going into constant current mode when connected to your circuit due to the time it takes to charge up the two 2200uF caps.Can you try running your power supply with the transformer that you plan using with it. With no current limiting in charging the supply caps the 22v rail may rise fast enough.HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: March 27, 2014, 02:26:39 pm
content: Hello !i powered it directly from transformer i plan to use and its the same as with the other supply goes to 1.2v.Quotei did simulation also and start with 0 volts and with just 2uF cap on the supply and it wouldnt start.i guess this will be hard to solve

author: HalfSpace
author_rank: Regular Contributor
timestamp: March 28, 2014, 08:25:19 am
content: Hi DarkZero,After a lot of simulations in LTSpice the only thing that seems to work is what you have been doing and that is wait for the -6v supply to fully power up and then connect it to the op-amps.In the attached screen shot I have added a delay circuit C5, R7, Q5 and D5. The circuit requires the -6v supply to power up and will then connect it to the op-amps after about 20mS. You may need to increase the zener (D3) to compensate for the voltage drop across Q5. Any NPN transistor with a gain of 200+ and collector current 200mA+ will do for Q5.HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: March 28, 2014, 09:33:58 am
content: Hello HalfSpace!when i get 7.5v zener and transistor ill try it out !thanks again for helping me out!

author: DarkZero
author_rank: Regular Contributor
timestamp: March 29, 2014, 02:33:18 pm
content: hello !it works ! i noticed one thing though, when its in current limit transistor that is with heat sink on that -6v is going to 98oC and when its not it goes to like 40oC max but voltage is stable -6.2v with 7.5v zener else no problems.

author: HalfSpace
author_rank: Regular Contributor
timestamp: March 29, 2014, 10:57:55 pm
content: Hi DarkZero,Excellent, good to see it’s working.I think a bigger heatsink is required as anything over 70C could be a problem and once installed into an enclosure it will get even hotter!On you first post you inquired about a constant current LED for your PS. See the attached circuit. I have added the LED circuit to the output of the CC comparator. Any PNP transistor with a gain of 200+ will do. If the LED is still alight when not in CC mode add another 1N4148 in series with the LED.HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: March 30, 2014, 10:03:35 am
content: hello HalfSpace!i forgot about LED hehe good thing you were one step ahead ! all TO-39 heat sinks i found at my local suppliers are small i guess ill have to make it custom or add a fan or something. once i finish it ill post it here maybe someone will be interested who knowsand i just want to say thanks for everything !

author: homebrew
author_rank: Frequent Contributor
timestamp: March 30, 2014, 10:59:15 am
content: Just a (maybe stupid) question, regarding the current limiting part of the schematic.Basically I see a opamp in subtraction configuration, followed by a comparator, comparing the voltage drop over the shunt with a constant value, set by the potentiometer. When the tripping point is reached, the output of the comparator will switch to low (something around -6V). This output voltage is then feed to the LM723's pins 11 and 12 (Vc and V+).That doesn't look right (negative Voltage on the supply rail of the lm723) and it also could badly oscillate ...Any thoughts on that?

author: DarkZero
author_rank: Regular Contributor
timestamp: March 30, 2014, 05:04:18 pm
content: hello homebrew!Quoteill check that maybe that's why one of my transistors are overheating in current limit mode

author: DarkZero
author_rank: Regular Contributor
timestamp: April 01, 2014, 01:12:46 pm
content: Hello !I guess lm723 really doesnt like -v on those pins so i added a diode one the output pin and something aint right with diode or without (i also try grounding that negative supply in witch case it doesnt work at all) when i set current limit to lowest and i short the output its working but if turn up current limit just a little bit i lose regulation on -6v it goes to +2v and it will draw as much as i can supply it recovers only when i remove short im afraid to put it on transformer :/ any suggestions ?

author: homebrew
author_rank: Frequent Contributor
timestamp: April 01, 2014, 02:46:29 pm
content: Well, even with the diode, the lm723 is completely out of spec with those conditions. According to the data sheet the input voltage has to be at least 9.5V. When your current limit engages, you essentially drop it to 0 Volts (with your diode protection) and short out the opamp comparator.Your negative voltage generator (charge pump) can't stand that load (opamp might deliver something around 20 mA) and thats why the negative supply stops working.But anyway, why are you using the lm723 that way at all? In your configuration it acts more as a voltage reference. That is fine (although there certainly are better references available, as the lm723 isn't designed to be one in the first place) but I would suggest to feed back the current comparator output to the opamp, rather than to the voltage source. Hope the attachment makes clear what I mean ...The snippet of the schematic works as follows: As long as the output of the current comparator is higher than the set voltage, the diode is non-conductive and thus the set voltage appears at the opamp output. If in an overcurrent situation, the output of the current comparator will be smaller than the set voltage. In that case the diode becomes conductive and the set voltage is lowered until the current matches the preset value.But you really have to test your loop stability to prevent oscillations. Loop stability isn't an easy topic at all...Good luck ...

author: homebrew
author_rank: Frequent Contributor
timestamp: April 01, 2014, 02:56:35 pm
content: Just to add a reference, those guys are also using the same technique as I described in my previous post:http://diyfan.blogspot.ch/2013/03/adjustable-lab-power-supply-take-two.htmlLooks like a decent supply to me ...

author: DarkZero
author_rank: Regular Contributor
timestamp: April 01, 2014, 04:56:13 pm
content: Hello!Quotebeats me its not me who designed the circuit im not engineer yet.looking at your attached image where from should i power lm327 from ? or its the same with just adding the diode.

author: spirulerlocus
author_rank: Contributor
timestamp: April 01, 2014, 08:31:12 pm
content: I don't want to derail this thread, but what type of oscillator is used to generate the negative rail?

author: homebrew
author_rank: Frequent Contributor
timestamp: April 01, 2014, 10:22:25 pm
content: Quote from: DarkZero on April 01, 2014, 04:56:13 pmYeah, of course you can stick with the lm723. The attached image was just an idea of keeping everything in your schematic as it is, just using the current feedback to override the set voltage directly. You can power the lm723 from any voltage source greater than +9.5V.I didn't want to offend you by any means. I know that you didn't do the design and maybe 20 years ago there were no affordable alternatives available. However, I just want to help you to get your project working. And one of the first things to make sure is that all components work within their specifications. Otherwise random things can and will occur ...Good luck!

author: HalfSpace
author_rank: Regular Contributor
timestamp: April 01, 2014, 10:48:23 pm
content: Hi DarkZero,I think we have a missing capacitor on the CC comparator see attached picture. It was bugging me why there was a 3.3k resistor on the inverting input I think this is why. This should improve loop stability, but you may need to use a larger value capacitor than what is shown to get it stable.I had suggested that the LM723 could have – 6v applied to its +V back on post #15 but now I think in normal operation it won’t.I would get the power supply stable and then look at the LM723 it may not be the problem.HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: April 02, 2014, 08:47:55 pm
content: hello!Quotei added the capacitor i have gone from 470pF - 1uF nothing changed when i short the output i added capacitor between non-inverting and the output of the cc comparatorQuotetried with diode too same thing i lose -6v regulation on shorted output. and don't worry nothing on the internet can offend me.maybe one of the components gone faulty ill check that first. i also noticed when it goes in current limit it doesnt go -v on the output it goes to around 0.4v also i noticed when i turn cc pot just a little bit up (under short output) i hear noise from power transistor like a hiss and thats when regulation stops and until i remove short nothing works.i dont know what to measure to get better help :/

author: HalfSpace
author_rank: Regular Contributor
timestamp: April 02, 2014, 10:51:57 pm
content: I think it would be better to use a resistive load to test the current limit circuit stability not a short circuit. A 10R 5 watt resistor in a glass of water to increase its power rating or something similar. Have a look at the output of the power supply under load with your oscilloscope to see when it is unstable (oscillating). Try the capacitor again on the CC comparator at different settings of current limit and see if a particular capacitor value is improving the stability.The hiss from the power transistor is because the power supply is oscillating.The wiring from your PCB to the pots and power transistors should be as short as possible as long wires may cause stability issues.HalfSpace

author: homebrew
author_rank: Frequent Contributor
timestamp: April 03, 2014, 06:08:47 am
content: If you would like me to fiddle around with the simulation, could someone post the LTSpice model?Would save me a lot of time if I don't have to redraw it ...

author: HalfSpace
author_rank: Regular Contributor
timestamp: April 03, 2014, 10:08:57 pm
content: LTSpice model

author: DarkZero
author_rank: Regular Contributor
timestamp: April 05, 2014, 06:13:40 pm
content: hello !i didnt give up yet ! tomorrow ill be able to do some scope measurement's ill post pictures or videos hope someone can help i made 10ohm load from heater wire and later ill build Dave's dummy load bare with me !

author: DarkZero
author_rank: Regular Contributor
timestamp: April 07, 2014, 06:57:11 pm
content: hello !ok. i have done some scoping here is the setup 10R load with 10v across it first i turn down current pot slowly and then slowly upmost of the time it starts getting bad when i turn current pot up

author: HalfSpace
author_rank: Regular Contributor
timestamp: April 08, 2014, 08:25:10 am
content: Hi DarkZero,When you did the videos what value capacitor did you have on the CC comparator and what was the timebase setting on the comparator output video? I can see the position of the timebase knob but cannot read the value.Where are you connecting the negative side of your load too? The best place would be on the negative side of the main filter capacitor C1 or C2. Anywhere else on the PCB could cause stability issues due to large currents flowing through the PCBs ground trace.HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: April 08, 2014, 09:27:12 am
content: hello HalfSpace!timebase in all videos were 0.2ms, all my gnd connections are behind filter caps on their negative side, cap on the CC comparator 470pFone thing is unclear to me why is in 3rd video that i made output of the CC comparator following pot turning should there be no change at all until the tripping point then it goes to -6 or those oscillations are making those things happen.

author: HalfSpace
author_rank: Regular Contributor
timestamp: April 11, 2014, 02:37:28 am
content: Hi DarkZero,When the comparator is in constant current mode it functions like an op-amp with feedback. The feedback loop will be all of the op-amps, the LM723 and output transistors.For example if you set the voltage output to 10v, set the current pot to 1Amp and attach a 5R load to the output the following will happen.The output voltage will drop from 10v to 5v to allow only 1amp to flow through the load (5R) because the current limit has been set to 1amp.For the output to drop to half the voltage set, the LM723’s output must lower from 20v to 10v which in turn lowers the voltage set by the volts pot from 10v to 5v. This will only happen if the cc comparator output which supplies the LM723 is lowered to approximately half its output voltage. It will be a little higher to allow for any voltage drop across the LM723.Now add another 5R load. The output voltage on the cc comparator, the LM723 and across the loads will drop by half again to maintain the 1amp set by the current pot.If the cc comparator output was 0v or -6v the supply output voltage would drop to zero and would not maintain the current of 1amp through the load.So the cc comparator output under normal operation should not go below zero volts even with a short on the output of the power supply. Because the circuit is oscillating it becomes difficult to predict what the cc comparator will output.Try adding some decoupling caps (100nf or higher) from the 22v and -6v supply rails as close as possible to the op-amp pins and ground.Adding a larger cap across the cc comparator, does that make a difference to stability?HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: April 11, 2014, 07:12:57 pm
content: Hello HalfSpace!It Works !First i want to say that you were on spot with the capacitor on CC comparator any value higher then 200pF will work. Why it didn't work ? well when i was redoing wiring i cut the connection from middle of those 2 resistors at the output of LM723 and the pin 4 because there is no that connection in the schematic but was on the board then i was checking the datasheet and every example there had that connection so i connected it again it was still oscillating i added 100nF as you suggested and it stopped oscillating with very clean dc outputs works with short too thou i get max 1.5 amps but thats expected guess ill have to change that 15v zener or something.im happy this turned out all right i just cant thank you enough !

author: HalfSpace
author_rank: Regular Contributor
timestamp: April 11, 2014, 11:28:14 pm
content: Hi DarkZero,Excellent to see you have got it working.If you change both 33k resistors on the differential amp to 22k that will rise the maximum current limit to 2amps.HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: April 15, 2014, 06:34:44 pm
content: Hello!ok i got everything to work but i cant make CC LED to work and i noticed that if the output is shorted and CC isnt at its minimum power supply wont start properly i fried power transistor and one opamp but i think its due to power supply that im powering the thing i didnt try it yet with transformer if it cant be fixed its not a big deal ill just never short it before i start it up

author: HalfSpace
author_rank: Regular Contributor
timestamp: April 15, 2014, 11:21:51 pm
content: Hi DarkZero,Turning your power supply on with a load or DUT (device under test) already connected may damage the DUT as the power supply may produce a higher voltage than what is set until it starts regulating. A load switch that disconnects the power supplies output terminals is a good idea.Are you using a 2N3906 in the CC led circuit? Any PNP transistor with a gain of 100+ will do. To test the CC led circuit disconnect the op-amp side of the 47K resistor and connect that side of the resistor to 0V. This should turn on the led.HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: April 16, 2014, 01:55:13 pm
content: Hello HalfSpace !i used BC556B transistor and it works i was just lazy to check bias of a LED hahahah ....ok now one last thing is there any chance to protect transformer to limit its current to lets say 2.3A so if i by mistake short the output before turn on to atleast save the transformer and to be more reliable then fuse

author: HalfSpace
author_rank: Regular Contributor
timestamp: April 17, 2014, 12:31:31 pm
content: Hi DarkZero,Transformers are quite robust if over loaded for a short period of time. Having a fuse on the primary side of the transformer is more important to limit mains current when a short is on the secondary.What is the secondary voltage and current rating of the transformer?HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: April 17, 2014, 04:36:15 pm
content: Hello HalfSpace!I made transformer my self i made it for 3A at 24VAC secondary wire is little thicker then calculated to give it a little bit of margin and i had space but i welded it so if it gets smoked gotta get a new one and thats pretty much most expensive part in this build considering i really bought only a few things to build this which costed like nothing.

author: HalfSpace
author_rank: Regular Contributor
timestamp: April 17, 2014, 11:36:32 pm
content: Hi DarkZero,A time delay fuse on the primary side of the transformer will give the most protection. Do you know what the transformers output voltage and current was before you re-wound it? Also what is your mains supply voltage?HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: April 18, 2014, 11:13:30 am
content: Hello HalfSpace!I have a lot EI laminations all transformers i make are squared or close to square i use B=1.1T and J=2.5A/mm2and thats for 230VAC 50Hz also it has to survive 10sec of short circuit on the secondary i made a LOT of them for my father when i was kid

author: HalfSpace
author_rank: Regular Contributor
timestamp: April 18, 2014, 11:26:20 pm
content: Hi DarkZero,If the transformer’s power rating is 60VA (24V x 2.5Amps) and primary voltage is 230Vac, the primary current for the full load is around 60/230 = 0.26Amps plus 10% for transformer’s losses =0.29Amps and multiply by 2 =0.58Amps. A time delay (slow blow) fuse of around 0.5Amps on the primary side should provide enough protection for the transformer.HalfSpace

author: DarkZero
author_rank: Regular Contributor
timestamp: June 13, 2014, 07:26:51 pm
content: hey all !sorry to bring this very old post up but i finished it and here are some pictures of the results!powering my newly built 10MHz frequency counter out of the old chips my telecom company was trowing away !thanks for everything HalfSpace you are the best !!

author: HalfSpace
author_rank: Regular Contributor
timestamp: June 14, 2014, 11:19:09 pm
content: Hi DarkZero,Fantastic to see the project finished and big thumbs up for your perseverance to get it all working.Recycling of an old CB radio covers is a great idea.HalfSpace