723 + Zener = Fawlty Powers
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- Опубліковано 10 лип 2024
- Electronic Design Flaws: 723 + Zener = Fawlty Powers | 1978 Elektor Power Supply Disproof of Concept
My aim with this video is to provide better understanding of the 723 precision voltage regulator circuit in general, especially to the younger electronics enthusiasts and to show the design flaw of the reviewed power supply (originally published in the July/August 1978 issue of the Elektor magazine) to those who possibly decided to build or already have built this particular solution.
- Electronics tutorial: 0V with 723 | 723 and an auxiliary negative supply voltage
- Elektronik-Tutorial: 0V mit 723 | 723 und negative Hilfsspannung
- Лекција од електроника: 0V со 723 | 723 со помошен негативен напон
Check the link below to this interesting article: “A Collection of Proper Design Practices using the LM723 IC Regulator”, where you can read a good summary of pros and cons for designing with 723:
electronicprojectsforfun.word...
Elektor's response to this video:
(EN) www.elektormagazine.com/news/...
(NL) www.elektormagazine.nl/news/k...
(FR) www.elektormagazine.fr/news/q...
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Best practical testing. Good learning for design aspects to consider.
Thank you!
Thanks. It is so clear and accurate!
Thank you for the feedback!
Great overview and experiments, showing the stability features by stepwise adjustments.
It would be outstanding if you could supplement the experiment using a TL431 as a temperature compensated ‘super-Zener’ with low dynamic impedance - even as 2.5V (no need to go 4.7V) with a lower value resistor driving it, to compare these improvements.
Doing so, it probably be a direct retrofit to formerly made assemblies, or guide new ones.
Thanks for posting it!
Good suggestion. If I manage to do that in the near future, I'll certainly publish a follow up video.
Thank you for your input!
Really good and detailed video. I have made the same regulator lm723 in this. I wanted to get zero to around thirty volts output. I figured there are a couple of more ways to get it to go to zero. A opamp can be added inline as a voltage follower with adjusted gain on the base of the pass transistor. Also a isolated voltage can be added in after R1 in the inverting input of about 3 volts and you adjust the reference side for voltage control. I went with the latter one and made a better reference of 7.5 volts. I got it to work well from 0 to 30.1 volts output and the new reference is a improvement for voltage drift.
Thank you for your input. If you have a chance, please post a short video or link to the schematic of your modified power supply, so we can get a better insight.
Very good! i will be using the 723 CDP in my 0 - ~30v / 0 - 100ma constant power project.
Thank you!
Very interesting presentation indeed. An old but effective countermeasure used for canceling the thermal instability of the zener voltage is to use another one diode connected in series to the zener, but in forward polarity, that is, the anode of the zener should be connected to the anode of the added diode and finally the cathode of that one should go to the 2K2 resistor. This will be then the ground connection point of the 723 (pin 7). This diode addition will add more than about 0,65V to the zener voltage (4,7+0,65V being the typical sum) which is negligible, but it will remove the thermal drift of the negative stabilized voltage. A 1N4148, used usually in such cases, normally cancels out this effect. In any case, a fixed -5V three terminal stabiliser (7905) used in place of the single zener diode solution seems to be the best solution and is common practice...
Thank you for your comment!
Actually, the Zener diodes with a breakdown voltage of up to about 5.6 V have negative temperature coefficient and the Zener diodes with a breakdown voltage above 5.6 V have positive temperature coefficient. So, adding a forward-biased silicon diode (with a temperature coefficient of −2 mV/°C) in series with a Zener diode (to cancel the effects) makes sense only for the Zener diodes with positive temperature coefficients (thus, with voltages above 5.6V, which is not the case here).
And yes, an auxiliary negative voltage supply with 7905 would be a much better solution in this case.
@@GoranDzambazov Yes, you are right, I forgot that detail about them. There is another way in resolving the problem of stabilisation of low voltages, which I also forgot to write. This can be done by the use of common L.E.D diodes as voltage stabilizers, which brings much better result if this method is applied. I have read a relevant article in a German magazine about that, back in 1987...And later on I have seen many times this application in real circuits, naturally because it is a cheap and reliable solution...
Watched video TWICE and understood the Elektor design. Learned a lot. I appreciate the Elektor designer as well as the author of this video. BOTH authors did their BEST. Thanks
Well presented. Kudos to Goran
Thank you!
Bravo! Video perfection 10/10, really enjoyed it, thanks for making!
Thank you for the feedback!
Excellent explanation and analysis Goran! BRAVO!
Thank you Mile! / Благодарам Миле!
Instead using a thermal compensated zenner diode, I, many times used a LED. It is more stable with the temperature and can be used as a indicator also. Sometimes a green and red LED in series.
Excellent report, thanks very much !
Thank you!
Thanks a lot.
Thank you for sharing this video. I appreciate your hard effort to teach us. I was impressed with the extent of your trials. They were centered around two points... 1) Varying the virtual ground point, by many methods, from the minimum -2V (required to allow the output to reach a true 0V) to an (arbitrary?) -4.7V. Given that the needed offset should set to -2V, I am not sure that I understand the remainder of the negative voltage explorations. Well, admittedly the additional tests do prove the point that the -2V should be firmly affixed. Additional points concerning the Zener reference are found, here. a) The circuit's excellent performance touted in the literature must be considered in the context of the time in which it was written. Meaning, that, in its day, the circuit performed well, in the author's opinion, compared to other circuits of the time. b) You noted that the solution is that the reference must have no less of performance specifications than the LM723. Now that time and technology have advanced, references with better thermal specifications (less thermal drift) are readily available and surely should be substituted in the circuit. 2) For either the original Zener reference, or the newer more thermally stable references of our day... a) Care must be taken to isolate the reference from ambient temperature changes. This is often accomplished with special rooms, or covers, created within the chassis. b) All PSUs, even of today's technologies, in every price range, require a preheating period, in which the cabinet warms to a point and then becomes thermally stable. The PSU is not intended to be used, before that stabilization is reached. Your testing did not include these critical points and this was unfair to the Zener. These considerations are not to find fault with you, or the video. They are simply intended to widen our considerations, in order to learn a bit more. I hope they are helpful, to that end. Cheers.
Thank you for your comment and the suggested considerations.
My analysis in the video is focused on the schematic/circuit that is originally published in the July/August 1978 issue of the Elektor magazine, where an auxiliary negative voltage supply of -4.7V is used. So, that is why I'm doing the tests with this particular value of -4.7V for the auxiliary voltage supply.
Regarding your suggestion that the circuit performance should be considered in the context of the time in which it was written: Perhaps my point would have been even more clearer had I suggested in the video the alternative solutions for an auxiliary negative voltage that were already available at that time, such as 7905 or TL431. These circuits provide better line and load regulation, and better temperature stability than a circuit with a simple Zener-diode. (And also, an auxiliary circuit for a negative supply with another 723 IC was an option.)
Regarding the comments in 2): I agree that in general care must be taken to isolate the reference from the temperature changes. But again, for the circuit in question, if you already have the other options (7905, TL431 or another 723), I do not see a reason to design a more complicated solution with a thermally isolated Zener diode. And even if you do so, you will still have the problem with the poor line regulation due to the dynamic resistance of the Zener diode that I demonstrated in the Test Circuit C / Demo C-1 (starting at 15:09).
Best, Goran
@@GoranDzambazov Those are all super responses. Well thought out and well said. You have helped my understanding of your frame of reference. One of the first circuits that I ever wanted to build was a proper PSU. I found that many/most of open source designs were challenged in developing the negative power rail. Those challenges continue, even with the improvements in IC technology. I still would like to find a simple and successful DIY 30V/3A, Constant Current PSU circuit. If you know of a specific one, please provide a link. Oh, another challenge is always the high cost of the IC used to pass the current. Of course, all of this is in consideration of a Linear Supply. Switch mode designs are becoming very much noise-free and more universally usable. So, my nostalgia is just that, nostalgia. I greatly appreciate your reply.
Thank you again for the kind words.
You might be interested in this article and power supply:
www.analog.com/en/technical-articles/high-performance-portable-dc-bench-power-supply.html
It's a mixed-mode DC power supply with LT3081.
@@GoranDzambazov Thanks!
I build this powersupply when it was published in elektor, I am stil using it. i never felt the need to improve it because most of my hobby electronics are not that critical in the "lab".
Interesting....Thank you for the comment.
Excelente!
Thank you!
Thanks for the video.
Thank you for watching.
Your analysis of the problem of the Elektor circuit is correct. There are uA723 circuits that go from 0 without negative supply, but they have other deficiencies. That is why all modern laboratory power supplies are build using op-amps, not the 723 They give better current limiting too.
Thank you for the feedback!
I wanted also to mention / analyse one other solution for 0V that does not use a negative supply, but the video would have become too long.
I have built a lot better 723 power supply that uses a different topology (also featured in an old magazine) that is a bit more complex by adding extra electronics but most importantly makes the supply floating, meaning there is a separate winding for the control circuit and separate for the output power. This design basically solves all problems mentioned in this video and also allows to use any voltage and any current if your power section is beafy enough.
I'm curious to see that solution. Which magazine and issue is the one you mention or where can I find the schematic of that power supply?
@@GoranDzambazov it's not in English, and I couldn't find it on the internet. However the topology is used in one early school lab power supply that you can find online, it's a good starting point. Look up Tesla BK127 for the schematic (note that it uses the early metal package version and the pinout is different)
Let me know what you think!
I've found one article about BK 127 in the Amatérské Radio publication (from Czechoslovakia), from 1985. I see that in this topology a fraction of the reference voltage is brought to both inputs of the error amplifier, which is a concept that I'm familiar with. I would like to take a closer look and I'll send you an update (However, it may take a while.).
By the way, I was not familiar with this magazine. Thank you for sending me the info!
Great explanation. Would the 7905 or the TL431 reduce those problems? Since the negative rail depends on the half wave of the AC transformer and the energy storaged on C2 , both turn on and turn off operations would create a similar problem with regulation, right? I can imagine that a soft start circuit could, at least, mitigate one of those problems.
Thank you!
Yes, an auxiliary negative voltage source with 7905 or TL431 should reduce those issues, as these circuits provide better line and load regulation, and better temperature stability than a simple Zener-diode.
I'm not fully sure if I got the problem with the regulation that you are referring to (the part with the turn on and turn off operation of the power supply and the soft start circuit). Please elaborate in more details if possible.
@@GoranDzambazov In the turn on operation, the filter capacitor and the capacitor C2, that is part of the auxiliary negative voltage, start at zero. The positive rail tends to stabilize more fast than the negative rail. In that case, the 2V output will only go to zero when the negative voltage goes to 4,7V. Anything that is already connect to the power supply will take this variation. A 3V3 circuit will get 5V3 for a little fraction of seconds. This could cause some damage for sensitive circuits.
The same happens in the turn off operation. Since the positive filter cap will hold more energy than C2, the negative auxiliary rail will go down faster than the positive voltage and, once again, the power supply will lose it's 0V regulation. Anything that is connected to the power supply while it is turned off will recieve a variation of 2V. But now, for more time.
Thank you for the clarification!
In general, I always try to avoid having the load connected to the output during the turn on and turn off operations of any power supply, to avoid possible stresses to the load caused by the transients/spikes.
Yes, if we go and analyze down to milliseconds or tens of milliseconds upon the turn-on, we can expect different establishing times for the positive and negative supply rails, however which voltage will reach the steady value faster will depend not only on the storage capacitor values, or the phase of the AC input at that time, but also on the load connected to the output (Although the storage capacitors have different values, they also "see" different loads: the current that the auxiliary negative supply sinks from V- is in the range of mA, and the output current can go up to several Amps.).
A good simulation and test measurements with a digital oscilloscope of the output voltage during the power supply turn-on and turn-off should give a better overview of this issue, and this should not be neglected.
@@diegoandradecaruaru I do not consider my last response complete. I would like to revisit the issue of the different establishing times of the positive and negative rails again, but with a simulation and actual test on a protoboard. I'll put my findings in a new comment.
If insteed of 4.7 V Zener diode we use TL431, which, acording with the datasheet is termal compensate?
Yes, an auxiliary negative voltage source with TL431 or 7905 provides better line and load regulation, and better temperature stability than a simple Zener-diode.
let me tell about correction of this mistake. Use 7905 -ve 5v regulator instead of that 4.7 V Zener,the problem is mostly solved. using a 36volt on lm723 (with a series resistor 56 Ohm) can prevent 723 from main 32 V0lts input
Yes, this was already mentioned in the previous comments: an auxiliary negative voltage supply with 7905 is much better solution than a Zener diode.
Thank you for video.I am new in electronic and I want make a current adjustable a power supply with LM723 .I was bought all parts of this circuits too and I will make nowadays.
What is your advice resistance value in negative voltage supply,- 2K2 value- as you had explain at 20:30 minute
Meanwhile please continue to videos about electronics if have not any a reason like health .You have great electronic knowlodge. I was just see your youtube channel (Bad youtube algorithms)
Regards
Thank you for your comment!
I actually do not advise you to make this power supply using the originally published schematic, because of the reasons I explain in the video. Even if you properly set the bias current through the Zener diode to the optimal value by reducing the value of R1, the other factors will still be present, so that alone won't help much.
If you still want to make this power supply, consider replacing the auxiliary negative voltage supply circuit with a circuit with 7905 or TL431. These circuits provide better line and load regulation, and better temperature stability than a simple Zener-diode.
Mas, power supply bell LM723 Tdk bisa keluar teg. Lek protek nyala trs, awalmula teg overload hingga 25volt. Pdhl tr 2N3055 udh tk ganti hasil nihil?
I'm not sure if I understand your comment. Are you trying to repair a LM723 based power supply that has failed?
@@GoranDzambazov
Ok trims 🙏🙏🙏
If you still have the problem and you need advice, please provide more details.
Gorane, ispravi me ako grešim ali na paru tranzistora u darlington spoju imamo pad napona od cca 1.4v(2x0.7v, po jedan na svakom od spojeva baza emiter), tako da je napon izlaza manji za toliko u odnosu na Vout koji dobijamo na pinu broj 10 regulatora lm723...još ako se uzme obzir i mali pad napona na šant otporniku od 0.33oma(čini mi se da je označen sa R6) to dalje implicira da je min napon izlaza blizak nuli, umesto 2v...govorim za kolo koje je objavljeno u Elektoru 78 godine a čiju je šema prikazana na 1:15.....ja imam regulisano laboratorijsko napajanje sa LM723 kolom i čak tri tranzistora u darlington spoju pa je pad napona na njima i veći, čak 2.1v, pus je dodata jedna snažna dioda na red sa darlington triom tako da je pad napona u odnosu na Vout tj pin10 regulatora ukupno 2.8v.....videću da slikam šemu mog ispravljača pa da nekako okačim da se vidi...
Tačno je da se na svakom od spojeva B-E javlja pad napona od oko 0,7V, a tu još imamo i pad napona na rednom otporniku za ograničenje struje (koji dostiže vrednost od oko 0,6-0,7V kad je izlazna struja maksimalna). Ali ako želite reći da bismo time mogli postići (ili vec postigli) da izlazni napon počinje od nule umesto od 2V čak i bez pomoćnog napajanja za negativni napon, to u ovom slučaju / za ovu šemu povezivanja nije istina. Zašto? Pa zato što se signal koji se nosi pojačivaču greške uvek uzima iz tačke koju smatramo da je izlaz, a to je u ovom slučaju izlazni terminal napajanja (a ne izlaz 10 integrisanog kola 723). Pogledajte da u ovom slučaju pojačivač greške uzima signal od otporničkog delitelja P2 i R8, a gornji kraj P2 je povezan na izlazu. Ako stavimo gornji kraj P2 na izlazu 10 integrisanog kola, onda bi izlaz 10 bio regulisan, a svi padovi napona dalje bi umanjili izlazni napon i napon na izlaznom terminalu u tom slučaju bio bi blizu 0V i bez pomoćnog napona. Ali u tom slučaju izlaz nije regulisan (već samo napon na izvodu 10) i izlazni napon (napon na izlaznom terminalu) bi ovisio puno više od toga kolika je izlazna struja.
Nadam se da sam vam ovim komentarom objasnio što je razlika i kako se postize regulacija. Možete još jednom pogledati i deo videa od 2:41.
@@GoranDzambazov da, sve jasno...sumnjao sam da može biti tako kao što kažete ali sam želeo vašu potvrdu...hvala na objašnjenju, video je zaista detaljan, jasan i kvalitetno urađen, takođe.
@@ishsnikaishsnika2066, hvala!
@@GoranDzambazov jedino mi ostaje nejasno kako moje napajanje ima regulaciju od nule na izlazu..
Postoji i jedno drugo rešenje da izlazni napon počinje od nule (bez pomoćnog izvora negativnog napona), gde se na invertujućem ulazu pojačivača greške dovodi deo izlaznog napona ali dopunski i deo referentnog napona (sa izvoda 6) i tako se kolo (na neki način) "vara" na nižim naponima. Ali i to rešenje ima svoje nedostatke.
Proverite šemu vaseg napajanja dali je to možda slučaj kod vas, a možete i da mi nekako pošaljete šemu (recimo preko Google Drive-a) i onda obrisati link iz komentara, pa ću vam potvrditi kad budem imao malo više vremena.
Elektor was its time far ahead. Its now common practise to dream up specification and publish them. The ebay and amazon chinese electronics are often far off, what is specified. Everybody who does understand something about Zenerdiodes now and 30 years ago knows those are not as stable as the 723 reference. You want to prove the publisher after 30 years wrong and you did. But would it not be far more useful and interesting to prove yourself right and come with a solution.
replace the zener with a TL431 and the output will remain stable 0.01 V between 0 and full load could be acceptable for most viewers.
Thank you for your feedback.
I agree, the video would have been more useful had I recommended a better solution for an auxiliary voltage (I became aware of that soon after I started preparing it and I even considered that option). That version would have involved at least a short analysis of the other solutions and then repeating the experiments and measurements. However, the video was already too long, so I finally decided to go with this version.
Hi Goran, a prequel to your nice video with the 723 essentials: ua-cam.com/video/tabIyItWlzw/v-deo.html
Thank you Greg for referring to my video in your article and at the end of your video!
If you are interested to hear some of the minor remarks that I found in your video, please send me your e-mail address as a reply to this message. Note that my response will probably be delayed.
Another circuit of elektor: drive.google.com/file/d/14T0tWEYLFJYM3g6Hg-US5IPsIVyoHWH6/view?usp=sharing