723 psu issue - Page 1
Forum section: electronics / projects

author: m3vuv
author_rank: Super Contributor
timestamp: November 14, 2018, 12:53:29 pm
content: Hi all,im putting together a psu using a 723,im using this schematic,the iss use a lower value sense resistorue i have is,i want the current feed back to work from about 100ma upwards,the issue i have is to get 600mv from the current sense resistor,it has to be a high value to produce 600mv across it for 723,this lowers the output to much overall,is it possible to amplify the sense voltage to feed the 723 cl and cl pins,therefor use a lower value sense resistor,ive thought of using a sg3532 but they are expensive!,cheers m3vuv.

author: stj
author_rank: Super Contributor
timestamp: November 14, 2018, 04:42:06 pm
content: why parallel drive?i have stuff i repair that can do atleast 7A that just uses a single 2n6057 darlington.

author: m3vuv
author_rank: Super Contributor
timestamp: November 14, 2018, 04:43:24 pm
content: what has that got to do with what im asking??

author: stj
author_rank: Super Contributor
timestamp: November 14, 2018, 05:09:17 pm
content: it's relevent to the circuit being over-complex.

author: duak
author_rank: Super Contributor
timestamp: November 14, 2018, 06:05:59 pm
content: Back in the 70's I built a 723 based lab supply that still works to this day. The downside is that there's no elegant way of having a wide current limit range without switching. If you have a variable sense resistor, it has to have a large enough value for the low current setting while being able to carry the supply's maximum current.m3vuv's comment made me think that it should be possible to amplify the developed current sense voltage to feed the internal current limiter transistor. It seems like an obvious solution, but I've never seen it. A 1X to 70X variable gain amplifier should be a piece of cake to design. One of the nice things about the 723 current limiter is that it's very fast in that there is only one transistor's delay in reducing the current. An amplifier will add some delay. During high current transients (short circuits, charging capacitors, etc.) I could see the amplifier winding up and applying too much drive current to the current limiter transistor and then possibly damaging it. I would add a few parts to limit the output span and drive of the amplifier.I could also see using the existing current limiter to limit the maximum current, while adding an amplifier and second current limiting transistor in parallel to implement a precision variable current limiter.Hope this makes sense,Cheers,

author: Kleinstein
author_rank: Super Contributor
timestamp: November 14, 2018, 06:56:03 pm
content: It is not very practical to divide down the shunt voltage to adjust the current. The more practical way is to add some voltage to lower the 600 mV threshold. This could be part of transistor BE junction, e.g. the power transistor.To lower the voltage lost, one can also use the emitter resistors that are there for current sharing anyway.Using separate parallel power transistors is a good solution, nothing bad with this. From some point on it gets difficult to use a single large transistor. Much more than about 100 W from a high power TO3 case (e.g. 2N3773) is rather difficult. It is much easier to use several smaller ones at maybe 60 W each.If a low drop out is required, one might consider a combination of NPN and PNP, e.g. in a kind of semi low drop configuration. Already a separate filter cap for the LM723 supply could help.P.s.: at 30 V maximum output and thus some 35 V raw voltage already 2.5 A is a little on the high side for an 2N3055. It would be a good idea to add another 2 or 3 . 0.1 Ohms are also rather low for the emitter resistor.

author: m3vuv
author_rank: Super Contributor
timestamp: November 14, 2018, 09:14:16 pm
content: the first part lost me there klienstien,im not sure what you mean by the be junction etc,im using 6x 2n 3055 pass transistors,im using the drop across the current sharring resistors as it is,there is one on each emitter leg of the pass transistors (6x 0.1 ohm) i still need 5+ amps draw to get it to go into current limiting,i want it to start limiting at 200-300ma up to 30A.

author: iMo
author_rank: Super Contributor
timestamp: November 14, 2018, 09:40:12 pm
content: Quote from: m3vuv on November 14, 2018, 09:14:16 pmyou need a "logarithmic" conversion then,for example an opamp with a diode in the feedback needs to be added into the sense circuitry..

author: xavier60
author_rank: Super Contributor
timestamp: November 15, 2018, 12:29:29 am
content: This should be able to made to work with some experimenting.Q2 and associated circuitry is an adjustable current source which sets a voltage drop across R5 which determines the current limit threshold.One unknown is how much Base current that the 723's current limiter transistor needs.

author: xavier60
author_rank: Super Contributor
timestamp: November 15, 2018, 05:56:40 am
content: The V-I curve will have a soft knee at the lower current settings.

author: not1xor1
author_rank: Frequent Contributor
timestamp: November 15, 2018, 08:35:48 am
content: Quote from: stj on November 14, 2018, 04:42:06 pmIt looks like you do not know the meaning of SOA, so please do not spread potentially dangerous misinformation.A 2N6057 cannot withstand more than 20Vce at 7A and that just in ideal conditions (i.e. infinite heatsink) while the schematic provided by the original poster mentions an input voltage of 40VDC.So a single 2N6057 would work with 7A just for a low voltage PSU.Low voltage, even in a fixed voltage PSU, because you must not forget to take into account Vce in case of shortcircuit.

author: not1xor1
author_rank: Frequent Contributor
timestamp: November 15, 2018, 08:47:06 am
content: Quote from: m3vuv on November 14, 2018, 12:53:29 pmIf you want to build a variable voltage/current PSU (i.e. lab PSU) then it is better to use a 2 opamps design rather than the 723.Any current regulation with the 723 would always be quite coarse and tend to drift unless you add another opamp and at that point IMHO it just does not make sense to use a 723.

author: xavier60
author_rank: Super Contributor
timestamp: November 15, 2018, 09:10:52 am
content: Using a Base Emitter junction for sensing the shunt voltage is such a blunt instrument, only good for about five to one control range.I built a floating type, two op-amp bench supply that is happy to regulate current over a range of 20ma to 20a with a 10 milliohm shunt.I paralleled ten 0.1 ohm resistors.

author: m3vuv
author_rank: Super Contributor
timestamp: November 15, 2018, 01:57:31 pm
content: interesting,would you have the schematic?

author: xavier60
author_rank: Super Contributor
timestamp: November 15, 2018, 09:40:54 pm
content: Yes, it can be found in another thread. I show only the regulator section.I explain the reason for it having a negative reference voltage and some extra circuitry that fixes a problem sometimes mentioned where op-amp current control loops are usually slow to respond to sudden overloads.https://www.eevblog.com/forum/projects/lab-power-supply-the-lost-current/100/

author: xavier60
author_rank: Super Contributor
timestamp: November 15, 2018, 10:25:07 pm
content: I must mention also that it will not be capable of safely supplying 20 amps without pre-regulation.And I don't think that MOSFETs are suitable for paralleling. I haven't really looked into it.

author: xavier60
author_rank: Super Contributor
timestamp: November 18, 2018, 02:08:18 am
content: Quote from: xavier60 on November 15, 2018, 12:29:29 amThis idea had already been thought of in post #27,https://www.eevblog.com/forum/projects/linear-lab-power-supply/25/

author: m3vuv
author_rank: Super Contributor
timestamp: November 19, 2018, 12:02:18 am
content: hi xaviar60 any idea what sort ov size pot v3 should be in the schematic you sent?,cheers Paul m3vuv.

author: m3vuv
author_rank: Super Contributor
timestamp: November 19, 2018, 12:21:00 am
content: i have some of these(op05cp intrument amprifier ic's),could i use one somehow to amplify the voltage generated across the sense resistor to feed the cl/cs pins on the 723 somehow?.

author: xavier60
author_rank: Super Contributor
timestamp: November 19, 2018, 01:22:45 am
content: Quote from: m3vuv on November 19, 2018, 12:02:18 amThe usual 10k linear pot. It will be ok with a lower value so long as R1 is of a value that will pass enough current to keep D2 forward biased at times when the unregulated rail drops in voltage.

author: xavier60
author_rank: Super Contributor
timestamp: November 19, 2018, 01:40:05 am
content: Quote from: m3vuv on November 19, 2018, 12:21:00 amProblem is that even if a single rail type op-amp is used, it will need a + supply rail referenced to the PSU's + output terminal. When the PSU is working near its full output voltage, the + rail for the op-amp will approach or even need to exceed the unregulated rail voltage.There are a few options to make this work. It is starting to evolve into a floating type regulator design.Extra: It is difficult to ignore the fact that the LM723 design looks so uncomplicated.

author: xavier60
author_rank: Super Contributor
timestamp: November 19, 2018, 01:47:03 am
content: Does anyone know what the dropout voltage is for a typical LM723 design like in the first post?

author: xavier60
author_rank: Super Contributor
timestamp: November 19, 2018, 03:32:37 am
content: An op-amp could be powered from the full unregulated rail voltage if one can be found that is rated for what the maximum unregulated voltage is and its input common mode range has to include its - supply voltage to within about 2V of its + supply voltage.

author: perieanuo
author_rank: Frequent Contributor
timestamp: November 19, 2018, 02:59:11 pm
content: Quote from: stj on November 14, 2018, 04:42:06 pmWith 150w transistor you stabilise 5V from over 30Vdc and output 7 amps?Envoyé de mon iPad en utilisant Tapatalk

author: David Hess
author_rank: Super Contributor
timestamp: November 21, 2018, 01:27:22 pm
content: Xavier60 has the right idea for a simple variable current limit; the current through the resistors in series with the base subtracts from the base-emitter voltage lowering the current limit. The same trick works for 317 based current limit circuits.Xavier60 also identifies the problem with Vbe current limiting; the curve is soft and this only gets worse as the added offset is increased limiting control range.If a higher performance current limit is desired, then instead of an amplifier, I would consider using the error amplifier from a second 723 to duplicate how the MC1466L works.

author: m3vuv
author_rank: Super Contributor
timestamp: December 13, 2018, 12:51:23 am
content: hi any schematics of an example i can try?,cheers m3uv.

author: xavier60
author_rank: Super Contributor
timestamp: December 13, 2018, 01:58:19 am
content: Quote from: m3vuv on December 13, 2018, 12:51:23 amI have been submitting the results of a design that is being tested on another thread.The big test will be when I commit the design to a PCB. I won't say much more until it's done.The original ,https://dangerfromdeer.com/tag/power-supply/, doesn't work properly.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 14, 2018, 08:13:38 am
content: normally if you add an power diode in serial or the sensing resistors, you'll get more voltage to control your current limitter, but you will lose more voltage at the outputTo negate this, you have to put the current control circuit before the power outputs collector. That way the lm723 is managing / regulating the voltages fluctuations better, and you have less voltages drop at the output.See the attached photos for an example, i think the l146 can be subbed by the lm723I have an very old lm723 design similar to this, but have more parts in the current limitter section, i cant find my schematics anymore, it use the foldback regulation technique.It can drive leds down to 10-20 ma ... up to many amps ...You have the same applicable principle to the 3 legged variables regulators too ..

author: coromonadalix
author_rank: Super Contributor
timestamp: December 14, 2018, 08:36:44 am
content: oh and check this attached photo for negating the minimal 1.2 volt minimum at the output, the lm723 internal design will never get to 0 volts at the output.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 14, 2018, 11:13:02 am
content: The circuit with the negative supply to get an output voltage down to zero has 2 serious drawbacks:1) the reference voltage is now to a large part the external zener diode to stabilize the negative supply - this would likely be rather poor quality compared to the usually rather good 723 internal reference. In the shown circuit there would also be quite some ripple from the negative supply.2) The negative voltage adds to the supply of the 723 - so the maximum voltage would be lower by some 5 V. The voltage to the 723 is already too high without it, if 30 V transformer is used.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 14, 2018, 11:48:07 pm
content: The - 1.2 volts is only applied to the voltage potentiometer, the 723 will maintain its stability, this montage has been proven effective, if not the other schematic is good too, i dont know many projects who need less than 1.2 volts ?? and for the - 1.2 volts you can use Ldo regulators too, better stability.If you tweak the pin 5 and 6 on the 723 you may get lower than 1.2volts, i could go down to almost 1 volts.Normally there is no other way to counter act the minimal output voltage unless you do something like this. If not you can build an power supply with op ampslike some mastech psus who goes down to a real zero volts, the circuitry is not very complicated to replicate.You can have or build this circuit Velleman k7200 psu, its a very well build circuit, 1x 723 as the voltage regulator and 1x 723 for the current regulator.https://www.velleman.eu/downloads/0/manual_k7200.pdfHaving a relais on a tapped transformer will help to minimize the disspipated temperature if you set low voltages.your 2) assumption is wrong in part, the -1.2 volt will be substracted in the potentiometer voltage (and 723 feedback circuit) if set at the minimum, you will get to zero volts, and it will be -1.2v less than any voltage set for sure.The important thing is never to get past 40vdc for the 723 supply pin, 25vac is going near that 40vdc (Full bridge rectifier) if you want to get past the 35-37volts output, you have to use an floating lm723 design. There is some 723 circuits on google who goes at 70vdc.You have the rare 3 legged pin tl783 who does 1.2 to 125 vdc at the output, i managed to get a few of them, and build some psu, but the dissipated heat is enormous.Thats why for now i use already build Ebay snatched psu's like Sorensen, Kepco Ate 75-8, an my newest Kikusui Pad70-5, they use ac phase scr controlled bridge rectifiers, they will dissipate only the required heat (power) no more.Still my old 723 based psu is rock solid, but if i compare them for the same voltage and current settings, the 723 based one dissipate a lot more heat, i have active variable speed cooling on it snatched from an atx psu, and all my 2n3773 are matched in the HFe (gain) to have equilibrium in their dissipation on the heatsink.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 15, 2018, 10:09:12 am
content: The last circuit diagram shows a -4.7 V supply to the LM723. This would reduce the maximum output voltage by about these 4.7 V. There is a much more elegant way to make the 723 regulate down to 0 V (actually some 100 mV open circuit, depending on the minimum load): just add a a small fraction (about 1.5 V at the divider) of the reference voltage to the feedback divider. The voltage is than adjusted at the non inverting input. As a positive side effect this avoids a loop gain that depends on the set voltage. This happens if the voltage is adjusted with a variable resistor in the feedback.The 40 V maximum voltage of the 723 normally (no extra measures to limit the maximum voltage) means the transformer should not be higher than about 25 V. Under low grid conditions this gives a raw voltage of close to 30 V at the positive end. With some 10% ripple and about 3 V dropout for the 723 the maximum output voltage would thus be at around 24 V under load. To get a little more one could use something like separate filtering for less ripple at the 723 without using excessive filter caps for the main load current. Another point would than be using voltage limiting for the 723 so that a 27-30 V transformer could be used.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 15, 2018, 01:30:01 pm
content: here's a classic project : with op amps and goes down to zerohttp://electronics-diy.com/50v-bench-power-supply.phphere's an 70v 2amp psu with an 723https://www.eleccircuit.com/0-70-volt-2-amp-dc-variable-power-supply/The current limitter is before the collector output transistor.My psu is somewhat similar to this one: with a few tweaks, variable current adjustment and an added power diode in serie with the sense resistor to go down at 10-20 ma, enough to power a led without blowing it.https://www.eleccircuit.com/wp-content/uploads/2014/06/The-circuit-diagram-of-0-50V-3A-Variable-DC-power-supply.jpg

author: Dzambaz
author_rank: Newbie
timestamp: June 13, 2019, 06:10:39 pm
content: The power supplies with 723 that use an auxiliary negative power supply with a Zener-diode to achieve 0V (as in the proposed schematic) have a fundamental design flaw, because any deviation of this auxiliary voltage will be seen as a deviation in the output voltage.Since this schematic is still suggested as a solution in many forums for electronics, I recently made a video with several practical demonstrations and voltage measurements to provide better understanding of the issue: