EE Student Power Supply Project - Page 1
Forum section: electronics / projects

author: criegels
author_rank: Contributor
timestamp: April 21, 2014, 09:31:39 am
content: Finally registered for the forums!This thread hopes to persuade some recommendations for my student power supply project.I'm about to start building my first power supply in an upcoming project assignment from my college's EE program. Since we have access to the machine shop's tools and a giant supply of parts, I want to make an effort to turn this into a home bench supply that will not need replacement from a commercial product later in my career.The original schematic calls for a single 5:1 turns 4 amp transformer to build a 0-30V adjustable bench supply. I happen to have a 1:1 toroidal isolation transformer with dual primaries and secondaries, 150VA per secondary winding. Ideally, I would like to series-connect the primaries and build two supplies, one from each secondary. This, I guess, with capacitor input filtering could deliver up to ~80V at ~2A after regulation from each supply. This sounds great, as I would be able to test out high(er) voltage BJTs and mosfets, even tubes.The schematic below is what the students in the program have been assigned to build, although we are fully capable of implementing different designs.Some dubious things I see in the schematic are the negative voltage charge pump circuit, the use of the dated LM741 opamp, zener shunt regulators, and a oddly small R1 across the bridge rectifier.Some challenging aspects I forsee; sourcing drop-in opamp replacements which could handle the large voltage across them, and recalculating the bias points of the involved transistors, increasing ratings of components, etc. I think these challenges are worth overcoming, and perhaps the only way to achieve the higher voltage is by building the supply with discrete components rather than opamps.Perhaps someone has built a similar supply and can hand me some pointers regarding these seemingly dubious aspects of this design and with regard to the challenges of designing the supply for higher voltages.Hope to hear from you, and thank you ahead of time.-CR

author: Bored@Work
author_rank: Super Contributor
timestamp: April 21, 2014, 10:10:08 am
content: Quote from: criegels on April 21, 2014, 09:31:39 amBuy a properly rated, including safety ratings, mains transformer. Don't mess with something you have little or no experience with.QuoteYou did notice that, with the exception of power electronics, there is a massive trend to go to low voltage in electronics? And 80V exceeds some safety standards. So, I think going to 80V is pure wank for a general purpose bench power supply.Oh, and I forgot, good luck in cooling the power transistor to get rid of 160 W max. per channel.QuoteSo? Then use a transformer with an additional secondary winding to generate it.QuoteSo? Replace them. Hint, most modern opamps have low or very low maximum supply ratings. Don't be surprised when in the end you still end up with 741 or some of its similar old cousins.QuoteSo? If you don't like the reference voltage replace it with a better reference source. If you don't like the negative shunt regulator replace it with a 7905, but check the quiescent current first. Hint, if you think you need a high accurate negative opamp power supply, well, you don't. Check the opamp data sheet, stable yes, accurate no.Quote1kOhm over 6800 uF gives you tau = 6.8 s, which is already pretty large, especially if you consider 3 tau or the like. Fortunately, the rest of the circuit and any load connected also bleeds the cap, so you don't have to wait three tau or so until the supply shuts down.May I recommend you build the supply as designed. And if you can't help it, with a few minor modifications, like a better reference voltage source.

author: oldway
author_rank: Super Contributor
timestamp: April 21, 2014, 10:21:28 am
content: I have not analyzed in detail the schematic for lack of time.But first, the 741 supply voltage is too high : +35 V unstabilized and-5VThere should also be a way to reduce the 35V by a by a pre-regulator or by switching taps of the transformer to reduce the power dissipation of the power transistor.With regard to the reference voltage, you may replace the zener diode by a TL431.

author: facumedica
author_rank: Contributor
timestamp: April 21, 2014, 03:17:27 pm
content: Lm324 rocks!A question: the voltage reference is totally needed? Why?Cheers!

author: Bored@Work
author_rank: Super Contributor
timestamp: April 21, 2014, 06:29:02 pm
content: Quote from: facumedica on April 21, 2014, 03:17:27 pmDid you use your brain before you wrote that?

author: Zero999
author_rank: Super Contributor
timestamp: April 21, 2014, 06:45:26 pm
content: Quote from: oldway on April 21, 2014, 10:21:28 amThat depends on the 741. The normal version is only rated to 36V but if you use the uA741Mwhich is rated for a total supply voltage of 44V it should give you plenty of headroom.The NE5532 is probably a good replacement for the 741 but it has a higher roll-off so watch the stability.http://www.ti.com/lit/ds/symlink/ne5532.pdf

author: facumedica
author_rank: Contributor
timestamp: April 21, 2014, 10:19:54 pm
content: Quote from: Bored@Work on April 21, 2014, 06:29:02 pmNope.I didn't even check the schematicTwo questions:Best op-amp for a power supply? Between the common onesWhy to use a voltage reference? Is needed on a adjustable power supply?See ya!

author: criegels
author_rank: Contributor
timestamp: April 21, 2014, 11:47:13 pm
content: Quote from: Bored@Work on April 21, 2014, 10:10:08 amCall me discouraged to say the least.Just a heads up, this project is not currently underway, and I expect it should not be completed until December. Between now and then, I have a lot of learning time on my hands with college being out for summer and all and really want to make my best effort at nailing this design for the purposes I had indicated.You know, I'd really love to be able to power some high voltage BJTs and mosfets to test and build some short signal path amplifiers with high current without paralleling a whole ton of transistors. Converting voltage to current in one stage is a really nice way to achieve that i think!Perhaps a good start would be to offer me the names of some some industry standard power supply models which operate in this range so that I can search up their schematics and get a peek at their design principles.Thanks again,-CR

author: IanB
author_rank: Super Contributor
timestamp: April 22, 2014, 02:55:52 am
content: Quote from: criegels on April 21, 2014, 11:47:13 pmWell, a classic answer would be to look towards Agilent (HP) supplies. For instance the current 6614C that goes up to 100 V. Service manuals seem to be available, which I presume include schematics.

author: NickAmes
author_rank: Contributor
timestamp: April 22, 2014, 03:54:02 am
content: Quote from: IanB on April 22, 2014, 02:55:52 amI'd recommend looking at a non-programmable supply, like the E361XA series. They are quite a bit simpler, as they don't include the circuitry needed to control the supply remotely.In theE361XA service manual, the principles of operation are explained on page 51, and the schematic is on page 60.

author: Bored@Work
author_rank: Super Contributor
timestamp: April 22, 2014, 06:39:08 am
content: Quote from: criegels on April 21, 2014, 11:47:13 pmEhm, that tactic of trying to induce guilt into those not exactly telling you what you want to hear might work at school. It will hardly work in an engineering work environment and it certainly won't work with me. I am not responsible for your motivation. You are old enough to be responsible for it on your own. And coming here, or in any public place, always includes the risk of not hearing what you want to hear. Or in short, don't try to play that gamehttps://www.eevblog.com/forum/chat/advice-thread-humor/msg429013/#msg429013with us.QuoteThen build the power supply according to the schematic next week. And revise it and build a better one the following two weeks. And yet a better one a month ahead. And so on.Debug each one, measure each one, test each one, including fully loading it. And as Dave says, may your circuits not work the first time, so you have to fix it.The thing is, you apparently want to do one single big design, but lacking the experience to do it, including handling dangerous voltages. You might think that simple PS is beneath you. It isn't. Build it, learn from it. Then build a better one.QuoteYou know, I'd really love to be able to sing an opera.I should maybe start to train for it. Otherwise, lack of talent aside, it won't happen.See, that guilt inducing tactic doesn't work with me.

author: miguelvp
author_rank: Super Contributor
timestamp: April 22, 2014, 07:30:18 am
content: Bored@Work is right.So what if you pass this class by taking shortcuts (the easy way out)?The problem is that the more shortcuts you take the farther away you are from knowing how things really work (i.e. looking at other schematics without knowing really what they do just to pass the class), eventually you'll be so far that your only chance to graduate is to keep on taking shortcuts.And if you are able to graduate by taking the easy way out, eventually you'll go on an interview without cheat sheets and you won't get hired. Then you'll end up on a minimum wage job complaining that companies wont hire you because you don't have the experience they seek and it's unfair because you have the degree."But seriously guys, should I dig deeper? I hear you telling me to climb up, but there are too many ropes over my head that you all threw at me and I think I should keep on digging?"In all seriousness, read his reply and follow it. Start early try to understand how everything works on your own with the material they gave you. If you fail at it then you'll have specific questions to ask and keep trying, even if you don't fail make improvements, to the point to make the subject as second nature as you possibly can. Then will come naturally when you are being questioned on that first job interview.Yeah, I'm a dad with kids in college and they don't listen to a word I said, so why should you?At least I didn't give you the story of why professional basketball players get good at it. It's because they are with that b-ball 24/7. Oh wait I did tell you the story as well, darn!

author: Zero999
author_rank: Super Contributor
timestamp: April 22, 2014, 08:58:00 am
content: Quote from: criegels on April 21, 2014, 09:31:39 amThat's not right.A 5:1 transformer running of 120V is a secondary voltage of 24V. The peak voltage will be 34V which isn't enough when the voltage drop on the rectifiers and ripple is taken into account.You could go for a 30V transformer but that'll give you a peak voltage of 42V, possibly much higher when you take into account the load regulation.An alternative could be to stick with the 24V transformer and add a voltage doubler plus a regulator to give you 35V to power the op-amp.

author: IanB
author_rank: Super Contributor
timestamp: April 22, 2014, 09:12:55 am
content: It's not clear what the maximum output current is designed to be, but it may not be a lot. Also, the result may just be that the maximum output voltage is a bit less than 30 V.

author: jim17
author_rank: Newbie
timestamp: April 22, 2014, 02:01:48 pm
content: Hi, what's the purpose of the first opamp with the 5V zener on the negative feedback loop ?

author: rdl
author_rank: Super Contributor
timestamp: April 22, 2014, 03:32:38 pm
content: I would just build the power supply pretty much as is, but I'd select a transformer for no more than 20 volts maximum. There's not much need for more than that these days, and it will keep the specified parts in their comfort zone. Later if you need higher voltage, build a supply just for that. I would switch to a better voltage reference though. Something with less noise and better temperature stability than a zener. There are many options.

author: sacherjj
author_rank: Frequent Contributor
timestamp: April 22, 2014, 06:47:55 pm
content: Quote from: jim17 on April 22, 2014, 02:01:48 pmIt is being used as a voltage reference, to drive the voltage control pot. Sets the output to ~11.2V if I did the math right. Then R18 and R3 allows you to use 0-11.2V to generate 0-30.2V

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 23, 2014, 05:18:36 am
content: i accuse miguelvp of fear mongering.imo you should focus on the project instead of getting into a debate on ethics.this kid is at least making a forum post and suggestions about improving the project. have you even been to engineering school miguelvp?This guy is doing WAY more then most kids... he wants to distinguish himself by improving something. Electronics is a deep field and I bet there is LOADS of theory behind how "everything really works". You will always be dealing with a model that has incomplete parameters. I bet you don't know as much about the stuff you think that you know about miguelvp.I bet you half the class will offer a barely working implementation of this project because they feel that this type of design will be irrelevant to their desired field of work...you think that most college kids would go through the trouble of going on a forum to try to improve a design?they wanna go home and smoke weed, drink alcohol and have sex with pretty women.i bet that every electronics related suggestion made in this thread has inspired creative thought of the topic starter.most kids will be racking their brains trying to figure out how to re-write their friends code just enough so that the project works. reserve your "ethics" for someone presents some piece of shit design and bumps the thread 50 times because the design review is tomorrow and the thing they used their "intelligence gathering skills" (looking over peoples shoulders) to make does not work.I don't blame him for making a topic at all. If you try to learn this stuff yourself for all you know you are focusing on some detail that hardly matters because you saw some random scrap of information and you do not have the skills to assess if the information is useful.

author: miguelvp
author_rank: Super Contributor
timestamp: April 23, 2014, 06:03:00 am
content: You can accuse me of whatever you want, but when you hear something like this.QuoteWe used to call that plagiarism.I just agreed with bored that the kid should build what we was told to do and start early rather than later to understand it. He'll have plenty of time to look how others do it, or once he has it working then improve it.And yes, understanding the principles in school is key. I rather know how to derive trig functions than learn tables or use a calculator, even if I don't have to do it from scratch anymore, knowing how it works gives you a better understanding.For what is worth I did actually entered his circuit in LTSpice, and I'm not even taking his class, just to satisfy my own curiosity. I might even build it, but I gotta redo the LM741 package first (reason being the model available doesn't have the offsets need)

author: miguelvp
author_rank: Super Contributor
timestamp: April 23, 2014, 06:20:49 am
content: Quote from: miguelvp on April 23, 2014, 06:03:00 amWell adding the offset wasn't so hard after all, since the example model has all of it other than pin 1 and 5

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 23, 2014, 06:47:33 am
content: Quote from: miguelvp on April 23, 2014, 06:03:00 amare you kidding me dude? this guy is a engineer. he will be paid to make something that works. how is that plagiarism? what do you expect, for him to invent a new solution to a common problem?he will be paid to figure out how to plagiarize something without actually violating a patentman, differential equations class is a bunch of plagiarism then.I am sure that his boss will appreciate him finding a new solution to a problem with there is a common useful method of doing something in an industry. .......dude, my professors TOLD ME to look at how things are done when I was told to design something. there is usually either room for improvement or room for simplification. being able to reduce or amplify a design is ESSENTIAL to being an engineer. I was told to "look at the schematics of xxxx xxx" by people with PHDs from a well received university. They want you to be able to figure out a problem not torture you.If he could figure out what parts of a commercial solution he needs and does not need in order to roll his own solution then power to him.

author: miguelvp
author_rank: Super Contributor
timestamp: April 23, 2014, 06:53:18 am
content: I guess you must be right, based on the number ofyou added.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 23, 2014, 07:19:46 am
content: well I find it hilarious that you are essentially telling someone to build a practical circuit without seeing how practical circuits are made.perhaps he should break into a fab at night and build his silicon too? damn, bob wildar did all the real work for him when he designed that IC he is using...so what are you saying mate, that EE students who watch dave jones tear down and explain something are cheaters?ffs dave jones, the maker of the forum you are posting on says the best way to learn electronics is to build and reverse engineer things.how could i not mock you? your basically writing jokes (cuz who can take what you said seriously) and putting people down while you contribute nothing of use to electronic design.the title of the forum is projects designs and technical stuff, not chime in with some half baked moral opinions and try to make someone anxious and feel like a failureyou come in here like a bully trying to stir shit. if you don't feel like answering technical questions then don't answer them. putting people down does not belong on this forum.

author: miguelvp
author_rank: Super Contributor
timestamp: April 23, 2014, 08:01:33 am
content: The only thing I said is to learn the basics. nothing else. It's a 102 course!

author: IanB
author_rank: Super Contributor
timestamp: April 23, 2014, 02:57:24 pm
content: Quote from: miguelvp on April 23, 2014, 06:03:00 amNo, it's called engineering.Only a fool makes no reference to how others have done things before when embarking on a project.Stop writing this nonsense. It's not helpful at all.

author: richard.cs
author_rank: Super Contributor
timestamp: April 23, 2014, 03:51:06 pm
content: Quote from: IanB on April 23, 2014, 02:57:24 pmI'd agree with this. Looking at previous designs is not about slavishly duplicating something, if he wanted to do that he'd just build the circuit given. Researching existing solutions to a problem is an important part of engineering.The circuit given is quite dated, and may well not even work that well even for its time. Look at a few different designs and try to understand how they each work, what they do differently and try to decide for yourself what the advantages and disadvantages of each are. A higher voltage bench supply is a useful thing to have around, but for many (most?) people a high current 0-15V and a separate lower current 0-100V (or higher for playing with valves) is a lot more useful than running a linear 0-80V supply into low output voltages most of the time. I have a couple of 0-30V supplies at a few amps, they cover most of what I do but I'm currently designing myself a 0-500V (or maybe a bit higher) 100mA supply, and there are certainly situations when a fixed 5V or 3.3V at quite a few amps would be useful too.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 23, 2014, 03:56:03 pm
content: Quote from: miguelvp on April 23, 2014, 06:03:00 amWho is "we"? In my circles we call that learning and research.Is copying a circuit exactly, including component values "plagarism"?Well, it might be a legally-defined infringment of copyright or patent depending on if/how the circuit was protected (if it was).OTOH, if copying circuits from others is so bad, why do vendors freely publish "reference designs"?

author: criegels
author_rank: Contributor
timestamp: April 24, 2014, 08:13:46 am
content: Quote from: NickAmes on April 22, 2014, 03:54:02 amAwesome! I love it and had not considered the (now) quite obvious solution of simply adding another regulated supply for stably powering the opamps and various components without referring to the variable output voltage. A tiny 12V center tapped transformer is super easy to come by.Anyhow, I've consulted my professor about the intended changes and while he was interested to know why I would want such a high voltage supply, he (a pretty old dude) is now enthusiastically reminiscing of 2n3055 power transistors and is encouraging the effort. So, looks like I've now got an in-the-flesh design consultant too, in addition to you all on this forum.Thanks for the help!-CR

author: criegels
author_rank: Contributor
timestamp: May 19, 2014, 07:21:25 am
content: Hey again,I've spent some time this weekend going over the Agilent application note 90B which is all about the technologies in their ac/dc power supplies. Copy of the PDF here:https://www.eevblog.com/forum/beginners/agilent-technologies-dc-power-supply-handbookOf interest is, on page 50, the mention of a regulator (without secondary tap switching) which allows the power supply to achieve regulation within the supply's power envelope while between the regions of constant current or constant voltage. Any clues as to how this regulator can be implemented or where to look for more informationn regarding this type of "extended range" supply would be most appreciated.Thanks again,CR

author: richard.cs
author_rank: Super Contributor
timestamp: May 19, 2014, 12:46:11 pm
content: In the era of that document it would have been a thyristor pre-regulator on either the primary or the secondary of the transformer (on the secondary it might replace two of the diodes in a bridge rectifier but I've seen a single SCR after the rectifier), implimenting phase-angle control to try and keep the voltage across the pass element fixed at maybe 10V. A modern design would use a switching power supply to do the same job, keeping a fixed voltage drop accross the linear regulator.There are a few app notes floating around describing how to impliment the switcher version, in fact I'm sure I stumbled across a linear technology one last week.

author: skrap
author_rank: Contributor
timestamp: May 19, 2014, 12:48:49 pm
content: The magic search keyword is constant power regulator.Here is one example:http://www.maximintegrated.com/app-notes/index.mvp/id/4470Today you could just do it in digital instead. Just put limits on the CVCC references to keep within a certain power budget. For example as the CV setting goes up you just put in a limit to as how high the CC reference is allowed to go.

author: Zone1
author_rank: Newbie
timestamp: May 20, 2014, 10:13:28 pm
content: criegels, If you are really interested in this project you should check outhttp://www.electronics-lab.com/projects/power/001/index.htmlit is the same basic design and it gives you a description of how the PS works. The version listed at electronics-lab has a few minor deficiencies and one of the forumshttp://www.electronics-lab.com/forum/index.php?topic=19066.0there is a 140 page discussion that spans several years offering solid improvements (as well as an improved schematic and parts list). There is lots of discussion from individuals who actually built the circuit.There are some minor parts differences. The electronics-lab forum recommends TLE2141 op amps. Your version still lists a 24V transformer that won't get you 30 V at the output. You can use a 28V transformer and get a solid 30VDC output but then your op-amps will be at the about 45V. The revised version at electronics-lab solves this by using a few diodes to reduce the negative rail to keep the op amps within 44V. The single output transistor (2N6284 ) while rated for 20A and high voltage, will not likely be able to handle the power of a decent sized transformer. Your version uses a 0.15 Ohm current sensing resistor while the other site uses a 0.47 Ohm. The 0.47 ohm resistor gives more control but not good for high outputs. The 0.15 waste less power and may be more sensitive.You need to review the voltage divider on the current control op amp (pin 3) to see if R20 (36K) and 2k pot combination that sits between a voltage of 11.2V and -5V will give an adequate voltage range to the op amp.Overall the PS is a good circuit.

author: megajocke
author_rank: Regular Contributor
timestamp: May 20, 2014, 11:27:40 pm
content: Quote from: criegels on May 19, 2014, 07:21:25 amHP made a few series of laboratory power supplies based on flyback converters, and the maximum achievable power output (limited by transformer, transistor and rectifier ratings) tends to be quite constant over a wide range of voltage/current. There's a good description from page 11 onwards in this document:http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1981-08.pdfThe optimist would describe the 6024A as an "extended range" 200 W supply, allowing the maximum output power of 200 W (or 1000 W for the large model) to be provided at any voltage between 20V and 60V.The pessimist would, on the other hand, describe it as a crippled 60 V 10 A power supply that can not provide the 600 W you expect but has an additional limit: output power may not exceed 200 W.Component size/cost tends to fall somewhere between that of a square-output-characteristic 200 W and 600 W unit so both the optimist and pessimist have a point.

author: on1
author_rank: Contributor
timestamp: May 21, 2014, 04:50:31 am
content: Noob question here, how does the bottom op amp set the current limit? I feel like I am missing something obvious.Thanks

author: skrap
author_rank: Contributor
timestamp: May 21, 2014, 06:14:59 am
content: Quote from: on1 on May 21, 2014, 04:50:31 amU1 creates a stable voltage that is used as base to create the reference voltage for U2 and U3.A voltage divider is created R220 (36k) and the 2k current pot that will scale down the voltage given by U1 (Tp.1). This creates the reference voltage that U3 vill try its best to recreate at the negative input (pin 2).Now what might be confusing is that the line in the middle marked GND isn't at the same potential as BOB. The current coming back in to the negative output terminal will pass through the R24 resistor and drop a small amount of voltage. U3 senses this voltage drop through pin 2 via the R7 resistor.The opamp is only allowed to manipulated the voltage through the voltage regulator U2 and it can only lower the voltage reference due to the diode D6. So if the voltage drop over R24 becomes larger than the voltage sensed at pin 3 on U3 the opamp will start to sink (negative voltage on it's output) and current will flow thorugh D6 lowering the voltage reference seen by U2 on pin 3.So to sum it all up U3 sets the current by sensing the voltage drop over the shunt resistor R24. When it's larger than the reference set by the current pot it will lower the reference voltage that U2 sees by sinking through the D6 diode. U2 will adjust the output according to the new reference.

author: Zone1
author_rank: Newbie
timestamp: May 21, 2014, 01:09:27 pm
content: Zone1 wroteQuoteskrap wroteQuoteskrap you are correct, I incorrectly stated that the bottom of the voltage divider would be -5V but it will be close to zero. This means the current range would be about 0-3.9 Amps ((11.2V x 2/38) / 0.15 ohms). If the power supply had a transformer rated for 4 amps and the output was near zero then the single power transistor 2N6284 would have to dissipate approx 35V x 4A = 140 watts. This would be difficult to do even with a large heatsink.

author: on1
author_rank: Contributor
timestamp: May 21, 2014, 02:15:19 pm
content: Thanks!This is the first time I've seen the current sense on the return side of things... In other designs I've seen the current sense circuit use an in-line shunt resistor on the high side of the output rather than the return path -- interesting way to do it.

author: skrap
author_rank: Contributor
timestamp: May 21, 2014, 02:44:08 pm
content: @on1If you are interested in the differences and reasons for choosing high side or low side sensing I can recommend this app note:http://cds.linear.com/docs/en/application-note/an105.pdfMany vendors have some really nice information about the different aspects of it and AN105 is by no means complete.Also somewhat related is this article by Bob on error budgets which make an important point with a current sensing circuit. The figures referenced in the article is in a link at the very bottom.http://electronicdesign.com/power/whats-all-error-budget-stuff-anyhow@criegels Sorry for going a bit of topic. I won't interrupt your thread anymore.

author: criegels
author_rank: Contributor
timestamp: November 24, 2014, 04:43:24 am
content: Alright, I've got my chassis built and circuit boards soldered up, everything is falling into place beautifully. I've taken the overwhelming advise to build a lower voltage power supply, so I have taken a transformer with dual 12VRMS secondaries to build dual 14VDC supplies within the same chassis so that they may be utilized in series/parallel as in a typical dual supply. In the following attached schematic, I have used a ground reference simply for simplicity of schematic testing, but the actual supply will be floating.My hope is that someone will look over the design and make some critiques regarding safe operation and turn-on/turn-off circuit protections.Cheers, Thanks ahead of time!

author: criegels
author_rank: Contributor
timestamp: December 07, 2014, 07:04:36 am
content: Update on this project, the power supply is now functional!Some changes; limited the output to 12V for some regulator headroom, limited the current to 5A to reduce the 2n6284's junction temperature (30Watts seems conservative, with heatsink "C" pictured below).My trouble now is in designing a current foldback regulating circuit which would become active at a VRC*ICpower of 30W. My current sense resistor is 0.01ohms, and I'm unsure where to begin since the max 0.01ohms*5.0amps=.05V drop prohibits the simple foldback circuit on p.181 of Art of Electronics.Some suggestions on where to look or begin would be very appreciated.(also attached is the multisim .ms13 project file for your perusal)EDIT; The foldback curve should be;Vout= 15VRCMAX-30W/ICbetween output voltages 0 to 9V.Constant 5A between voltage ranges from 9 to 12V. (this Constant current part of the regulator is already functional)