Help me design a PSU - Page 1
Forum section: electronics / beginners

author: Efe_114
author_rank: Contributor
timestamp: November 13, 2018, 05:36:54 pm
content: I want to make a small psu that could go up to 15 volts and DOWN TO 0 VOLTS (because i like hard projects) .no current limit needed. i want to make it out off jellybean parts and not copy it from a random website. And most importantly im a beginner(i have been in hobby for 3 years but since 90% of my projects fail i call myself beginner)Can anyone help me about this? Also this is my first post in this forum so forgive my mistakes please

author: wraper
author_rank: Unknown
timestamp: November 13, 2018, 05:53:48 pm
content: Does it really need to go down to zero? If minimum of 1.2V is fine, simplest LM317 based schematics would do the job. Also what maximum current is needed?

author: Efe_114
author_rank: Contributor
timestamp: November 13, 2018, 06:22:03 pm
content: Half an amp or 1 amp will do the job for me. my main purpose of building this is to make a project that i will use . I want to improve my design skills and also improve this circuit over time. I dont want to just lm317 on its own cuz as i said i dont like simple stuff. I want to go a little bit more complicated. And also i want to make it dual tracking if possible.

author: Efe_114
author_rank: Contributor
timestamp: November 13, 2018, 06:24:34 pm
content: Efficiency is not an issue since i will use it on low power.

author: wraper
author_rank: Unknown
timestamp: November 13, 2018, 07:24:52 pm
content: Quote from: Efe_114 on November 13, 2018, 06:22:03 pmThen why you don't need current limit? Generally it's way more useful than going down to 0V. Tracking generally is not very useful and quite complicated to do on isolated channels. Generally it would be better to just have 2 isolated independent channels or 2 separate PSUs rather than having 2 channels with common GND and tracking.

author: wraper
author_rank: Unknown
timestamp: November 13, 2018, 07:33:47 pm
content: Performance wise, if you don't need current limit or high output current or voltage, it will be quite hard to beat LM317, LT108x or similar ICs. By adding a few volt low current negative supply voltage rail you can also make them going down to 0V.

author: Efe_114
author_rank: Contributor
timestamp: November 13, 2018, 08:16:18 pm
content: I decided to use LT1080 before but it isnt available in my country. What if i put a reverse biased 1.25 volt zener diode in output of the Lm317 what will happen? Genius or dumb? (Dumb i guess)I wonder if i can copy inside circuitry of the lt1080... that would be awesome! (i guess)

author: rstofer
author_rank: Super Contributor
timestamp: November 13, 2018, 08:17:51 pm
content: As a place to start:https://electronics.stackexchange.com/questions/244354/making-lm317-output-voltage-adjustable-down-to-0-vGoogle has a lot of LM317 projects

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 13, 2018, 09:50:06 pm
content: Quote from: Efe_114 on November 13, 2018, 08:16:18 pmYou probably mean LT3080, as the LT1080 is something else?https://www.analog.com/en/products/lt1080.htmlhttps://www.analog.com/en/products/lt3080.htmlThe LT3080 would be the simplest solution indeed.Something like the following schematic would probably fit your requirements with discrete parts, and is a close match.You can use another NPN transistor, it just has to be able to handle at least 20V and 1A. I showed a 19 V DC input power supply, so that you could use any 19 V laptop adapter as an input.You can use a 1 Meg linear pot. for R1/R2. The transistor will require proper heatsinking.Do not use a ceramic capacitor at the output as the regulator would tend to oscillate, a tantalum or electrolytic is preferred here.Note that the set voltage is referenced to input voltage for simplicity's sake. If you want a more precise reference, you can always insert a fixed 15V LDO.

author: Cliff Matthews
author_rank: Unknown
timestamp: November 14, 2018, 01:38:08 am
content: Here's one from Banggood (it's about $6). Someone posted a schematic.. Funny drawback of producing a negative rail with a bucket load of cheapo 1N4007's is the 1 amp maximum. Real efficient heater though..

author: FriedMule
author_rank: Frequent Contributor
timestamp: November 14, 2018, 02:21:06 am
content: I would look at some of the videos from Mr. EEV and after that, try to search google for what you decide to make, then put it together and see what happens. If you hit a wall then show your design here on this forum and ask what's wrong.

author: xavier60
author_rank: Super Contributor
timestamp: November 14, 2018, 04:16:04 am
content: This regulator circuit works well for me. It is possible to just make the positive side only.The current limit is about 600ma. It can be set higher.https://www.eevblog.com/forum/beginners/diy-power-supply-129692/msg1784705/#msg1784705

author: james_s
author_rank: Super Contributor
timestamp: November 14, 2018, 05:06:55 am
content: There are lots of linear power supply circuits out there. If you want it to go down to zero, you need a negative bias supply, it doesn't have to deliver much current. Usually there's a separate winding on the transformer for this.

author: VEGETA
author_rank: Super Contributor
timestamp: November 14, 2018, 06:43:03 am
content: To make it go down to 0v you would need a negative rail. To make this very easy for you, get ICL7660 charge pump IC. Feed it with say 5v and it will output -5v with low current of course.This will be the negative rail of your op-amp which sets the output voltage... it could be something like +25v positive rail and -5v negative rail which will ensure 0v output.It is better to put more filtering caps on 7660 output since it generates lots of noise, or better yes as I did in my dummy load design.. which is to give 9v to 7660 so it outputs -9v (or -8v with losses) then put TL431 after it which generates 2.5v in my case (you can make it anything via 2 resistors) with very low noise, now add another 100uF cap after it and you'll be done.I hope this helps you.

author: Efe_114
author_rank: Contributor
timestamp: November 14, 2018, 06:49:25 am
content: i have a suitable 48v transformer for the banggodd kit i may buy that but i want to learn. a negative bias center tapped etc. transformer is gonna be hard to find in my country . Yes i meant lt3080 . i had some problem troubleshooting my lm317 clrcuits i dont know if my lm317s are fake cuz 1 of them blew up and released a lot of magic smoke and caught on fire at 0.5 amps 12v input 10v output 1 watts heat dissipationi wonder if i can make a stable build out of 2n2222 and 2n3055 npn power transistor and a jellybean lm358I can make a darlington pair out of 2n2222 and 2n3055 and use it series pass(?) with the vcc(im. NOT good at transistor configuration names)and a classic error amplifier configuration out of lm358 and pot as a referance and output feedback on the another input i think this can go high currents because i will buy 2n3055 in to-3 package or i might stay away from complexity and use lm317 and i wonder if Putting a zener diode reverse biased in Adj pin will work? i dont think so but just want to ask you does minus voltage regulators need minus voltage in the input? If not i may feed -1.25 to the lm317 otherwise i cant because of the availablity problems i mentioned before .sorry for the complex replyI just seen some of these questions got answered while i write this thank you

author: VEGETA
author_rank: Super Contributor
timestamp: November 14, 2018, 07:29:33 am
content: Quote from: Efe_114 on November 14, 2018, 06:49:25 amActually, lm317 is not meant to be a lab variable power supply regulator, but it kinda works and cheap.If you wanna make your own regulator then it is gonna be hard since there is stabilization and so on that you are gonna face like I did previously. I made a floating regulator design but never built it yet.stick to lm317 + 7660 + lm358 + tl431 like I told you and then you will be fine.

author: not1xor1
author_rank: Frequent Contributor
timestamp: November 14, 2018, 08:28:55 am
content: Quote from: Cliff Matthews on November 14, 2018, 01:38:08 amIt is easy to make a much better (more stable, more efficient) PSU with about the same amount of components (dual opamp and power transistors), but if current regulation is not important even a TL431 + darlington would work.A negative rail and another TL431 would allow to start voltage regulation from 0V.It would be wise anyway to add a foldback current limit via an additional BJT and resistor.

author: VEGETA
author_rank: Super Contributor
timestamp: November 14, 2018, 09:18:07 am
content: Quote from: not1xor1 on November 14, 2018, 08:28:55 amThis schematic doesn't seem to have constant current mode. Adding one will make it better since it is already easy schematic. I guess the purpose of Q1 is for short circuit protection, is it correct?I am not a fan of having negative rails from diodes due to the heat and other issues. My initial suggestion was a TL431 + ICL7660 which is probably the best choice.

author: Efe_114
author_rank: Contributor
timestamp: November 14, 2018, 10:34:39 am
content: i think that Q1 is for short circuit protection too. But i didnt understand does this circuit have negative output? Can i build this thing with opamps instead of programmable zeners and 2n2222s as transistorsOr bd138 pnps?

author: VEGETA
author_rank: Super Contributor
timestamp: November 14, 2018, 12:55:47 pm
content: Quote from: Efe_114 on November 14, 2018, 10:34:39 amYes you can build it with LM358 but using tl431 is easier and does the job perfectly without the need for a separate power supply for the op-amp.This circuit doesn't output a negative voltage but it uses a negative voltage to drive the output to 0v, since tl431 also has 1.25v internal reference similar to lm317 and it is the sole reason why it cannot reach 0v. By using a negative rail, you add -1.25v to the already existing 1.25v to be 0v.The newer LT3081 and LT3083 has the capability to reach 0v simply because the internal reference is different. It uses a current source instead of a voltage source... that means if you force a voltage at the adjust pin (like you do with lm317), then that forced voltage is the output because there is no voltage drop to compensate for.You should study the datasheets well before proceeding, and even in using LT3083 there are issues just like Dave's supply didn't work because of circuit oscillation. I think I have seen LT themselves putting a lab power supply reference design based on LT3081 and even that uses a negative rail!regards

author: rstofer
author_rank: Super Contributor
timestamp: November 14, 2018, 02:28:40 pm
content: There is mention of a 48 VAC transformer above and the output, when rectified, is about 62V (plus or minus the filter capacitor and current draw/voltage drop. Nevertheless, a really big number.When the PS is outputting, say, 1A at 1V, the pass transistor(s) drops 62V - 1V and dissipates 61 Watts (more or less). That's also a really big number.That's why real power supplies use transformers with tapped secondaries and some number of relays that kick in at various voltage settings.If you want to output 5V, there is no point in having more than 8v DC going into the pass transistor (more or less). Clearly, not 62V...

author: Efe_114
author_rank: Contributor
timestamp: November 14, 2018, 03:31:24 pm
content: Quote from: rstofer on November 14, 2018, 02:28:40 pmthats more than some heaters i cant imagine running it at full power 3A 1v which would create 183 watts heat my aquarium heater is 60 watt

author: Efe_114
author_rank: Contributor
timestamp: November 14, 2018, 03:56:05 pm
content: I dint know how but i made a lm317 circuit go under 1.25 volt without anythingI just connected like this:Pot pins at the outwards go to ground and outputSlider goes to adjVcc goes to VinI wonder how did it work i will probe everywhere and test it under load when (if) i find out whats going on i will post here

author: wraper
author_rank: Unknown
timestamp: November 14, 2018, 06:48:06 pm
content: Quote from: Efe_114 on November 14, 2018, 03:56:05 pmDunno what you made but LM317 won't go below 1.25V without negative voltage on ADJ pin.

author: VEGETA
author_rank: Super Contributor
timestamp: November 14, 2018, 08:02:28 pm
content: Quote from: Efe_114 on November 14, 2018, 03:56:05 pmThat cannot be, please post schematic and pictures if possible.I explained to you why it happens and you could see its internal schematic to see it.QuoteYes because you need to compensate for the +1.25v of its internal reference.

author: not1xor1
author_rank: Frequent Contributor
timestamp: November 15, 2018, 07:43:46 am
content: Quote from: VEGETA on November 14, 2018, 09:18:07 amthe original poster stated:no current limit neededI anyway added a foldback current limit as I think it is the bare minimum for safe operation.The diodes used to produce the negative rail dissipate few mWs because the circuit is just a charge pump.There is absolutely no need for a further IC which might introduce just more noise besides the complexity and the cost.

author: not1xor1
author_rank: Frequent Contributor
timestamp: November 15, 2018, 08:05:08 am
content: Quote from: Efe_114 on November 14, 2018, 10:34:39 amThe negative rail is not meant for outup.It is there just to let the output range from 0V.It does that by bringing the "anode" of the top TL431 to -2.5V.Of course it is possible (and better) to use an opamp to do the same, or even a dual opamp to provide proper current regulation.In my spare time I'm testing (just on LTspice at the moment) a very simple circuit inspired by that designed by NSC during the 70:https://www.eevblog.com/forum/projects/very-low-noise-preregulator-for-benchtop-power-supply/msg1375039/#msg1375039My circuit is based on modern ICs (LT1013) and is simpler than the original.I just want to make some more tests before posting it here.

author: not1xor1
author_rank: Frequent Contributor
timestamp: November 15, 2018, 08:16:02 am
content: Quote from: rstofer on November 14, 2018, 02:28:40 pmIMHO the power is still manageable by using more power BJTs and a big heatsink.The main problem is rather the high voltage which would require a much more complicated PSU design, with a floating supply for the opamp and less common BJTs (i.e. the high voltage version of 2N3055 or something like the 2SC5200).I think a beginner should start with a simpler design and a more manageable voltage and power, i.e. something around 15-20V 1A.

author: rstofer
author_rank: Super Contributor
timestamp: November 15, 2018, 03:14:41 pm
content: Quote from: not1xor1 on November 15, 2018, 08:16:02 amYes, it can be managed but pretty soon you have a space heater - 60W of waste heat is a LOT. Parallel pass transistors with the bases driven by another power transistor is fairly common. It just needs to be considered.

author: Efe_114
author_rank: Contributor
timestamp: November 15, 2018, 04:23:54 pm
content: After changing the lm317 it wasnt going under 1.25 when i plugged the previous one back it was stabilized at the input voltage and didnt have a voltage drop i was feeding 12v and it gave 12 volt out i think something funny is going inside the chip also i ordered them from the most unreliable seller in my country

author: james_s
author_rank: Super Contributor
timestamp: November 15, 2018, 04:49:36 pm
content: It has obviously been fried, the internal pass transistor is shorted. Also if you bought them from a random China source they are probably inferior counterfeit parts.

author: fsr
author_rank: Regular Contributor
timestamp: November 16, 2018, 01:19:47 pm
content: You can make an LM317 go to 0v even without a negative transformer. The trick is to add two diodes between GND and the (-) output of the PSU (so, your outpur is aprox 1.4v above the real GND). The diodes must be able to handle the max output current and the power dissipation. The resistive divider that controls the LM317 output voltage is also referenced to this point.You will need to consider the power dissipation of the LM317 in the worst-case scenario and use a suitable heatsink. The datasheet has a very good explanation on how to calculate this under the "Heatsink Requirements" section.EDIT: i'm sorry, the circuit i posted before won't work, it was just a part of a circuit i did which had a current limit that could go all the way down to 0v, but for the CV control to go down to 0v, you need some way of getting 1.25v below your main diode bridge's negative output. I should have checked the circuit carefully, instead of sketching it and posting.Anyways, if you use the LM317, remember to check the "Heatsink Requirements" section, and forget about using a 48v transformer. The power dissipation for the chip is about (input_voltage - output_voltage) * current. Try to use a transformer/bridge/capacitor combination that will result on a voltage that just goes over the requirements for the LM317 for the output voltage you need (check the dropout voltage), and whatever additional drop that the final circuit could introduce.

author: VEGETA
author_rank: Super Contributor
timestamp: November 16, 2018, 02:05:38 pm
content: Quote from: fsr on November 16, 2018, 01:19:47 pmthat act itself creates a negative voltage. Remember that voltage is relative, so it is negative relative to ground which is 0. There is no way lm317 could output 0v without negative voltage.In your case, you did it by splitting the rail using diodes, and in my case by creating another rail using 7660. Same result.

author: fsr
author_rank: Regular Contributor
timestamp: November 17, 2018, 03:09:10 am
content: Quote from: VEGETA on November 16, 2018, 02:05:38 pmI mean that he doesn't need a center-tapped transformer, or something like that for this to work, just a voltage drop greater or equal than 1.2v under the negative output terminal, reference the voltage divider to that point and that's it. Bob's your uncleSomething like this (see attachment):EDIT: Sorry, wrong circuit. Check my previous post for details.

author: spec
author_rank: Frequent Contributor
timestamp: November 17, 2018, 05:47:07 am
content: Quote from: Efe_114 on November 13, 2018, 05:36:54 pmHi Efe,The attached schematic may be what you are looking for. It adjusts down to zero volts using the principle (already discussed) of generating a -1.25V line to offset the 1.25V reference voltage of the LM317, but without an additional winding on the transformer or voltage dropping diodes.If you are interested in this approach I can post a practical schematic showing decoupling capacitors and describe the best layout.(4m7 on the schematic is 4.7 milli Farads or 4700 micro Farads)

author: not1xor1
author_rank: Frequent Contributor
timestamp: November 17, 2018, 07:21:40 am
content: Quote from: fsr on November 16, 2018, 01:19:47 pmThat is not a trick but a really dumb solution as, besides the additional 1-2W of wasted power, the output voltage would drift according to the temperature of the diodes and the load current.As I showed in the previous schematic, with just a couple of diodes and capacitors you can get an additional negative rail from the transformer (charge pump) and with a cheap voltage reference and a resistor you can get -1.25V as stable as the inner reference of the LM317.And if the unregulated supply is coming from an AC-DC converter (switching PSU) you may use a 555 or even a cheap CMOS logic IC (which are usually cheaper than a dedicated IC) to get a negative rail via a similar charge pump circuit.

author: not1xor1
author_rank: Frequent Contributor
timestamp: November 17, 2018, 07:28:33 am
content: Quote from: fsr on November 17, 2018, 03:09:10 amThat would not work.You have to connect R1 to Adj and the potentiomenter to the negative of the rect. bridge.It is anyway a bad solution.

author: not1xor1
author_rank: Frequent Contributor
timestamp: November 17, 2018, 07:37:52 am
content: Quote from: spec on November 17, 2018, 05:47:07 amYou can even use a cheaper 1.25V shunt regulator (LMV431, LM385, etc.). Of course you must properly bias it so that the total current of the negative rail, at the worst condition (lowest transformer voltage) should be greater than the minimum shunt regulator voltage + the quiescent current of the LM317 (aproximately 12.5mA in the schematic you attached).

author: spec
author_rank: Frequent Contributor
timestamp: November 17, 2018, 05:59:51 pm
content: Quote from: not1xor1 on November 17, 2018, 07:37:52 amI know that, but you can't get much cheaper and ubiquitous, than an LM337. Besides, it looks nice and symmetrical with two three terminal regulators.

author: not1xor1
author_rank: Frequent Contributor
timestamp: November 18, 2018, 08:07:32 am
content: Quote from: spec on November 17, 2018, 05:59:51 pmYou're right. I just checked. The price difference is less than what I remembered... just 20-30 cents and only if one carefully selects the cheaper shunt regulator.

author: VEGETA
author_rank: Super Contributor
timestamp: November 18, 2018, 08:51:49 am
content: I think you need to see this:https://www.aliexpress.com/item/50PCS-TL431-TO92-TL431A-TO-92-431-new-voltage-regulator-IC-free-shipping/32694971062.htmlI recommend getting it since you can use it in lots of other purposes and it is dirt cheap.Here is ICL7660 if you wanted it too, which I recommend having for lots of different purposes:https://www.aliexpress.com/item/5pcs-ICL7660SCPA-DIP8-ICL7660-DIP-ICL7660S-7660SCPA-new-and-original-free-shipping/32493575139.htmlBoth of them won't cost you 5$.

author: Efe_114
author_rank: Contributor
timestamp: November 28, 2018, 05:37:39 pm
content: I found out something funny was going on in the lm317 later it just shooted up to 12v and i wasnt able to change the output considering it is from the worst supplier locally i think it was broken

author: VEGETA
author_rank: Super Contributor
timestamp: November 29, 2018, 12:48:00 pm
content: Quote from: Efe_114 on November 28, 2018, 05:37:39 pmI guess this is similar to what happened with me, sort of.Probably this is due to limited total output power of the source, it could be say 10W total... So you could have 10V 1A, but when you draw 2A it will drop down to 5V regardless of how high you dial it. This is a well-known phenomena in all industry.This occurs even on industrial supplies. One time we had a diesel generator which worked perfectly but then we connected a huge pump or motor which made the generator's voltage to drop down significantly.Solution: use a better power sources, test it with DC-DC switching modules to verify its capability, then use it with lm317.

author: David Hess
author_rank: Super Contributor
timestamp: December 02, 2018, 12:51:46 am
content: Quote from: not1xor1 on November 15, 2018, 07:43:46 amI took that to mean no adjustable current limit needed. I would still include short circuit protection.I would use a single supply operational amplifier to drive an output transistor or likely Darlington with a Vbe current limiter on the output. This configuration is the same as that of a 723 but the NPN differential amplifier of the 723 cannot operate down to zero volts.SiliconWizard's design is close to what I would do but I would add a reference and Vbe current limiter. (1)Not1xor1's design requires a negative supply. If you are going to do that, then just use a 317 with the modification to operate down to zero volts.(1) Who am I kidding? I would drive the output transistor using the positive supply lead of the operational amplifier and have the current limiting go into the *output* of the operational amplifier.

author: spec
author_rank: Frequent Contributor
timestamp: December 02, 2018, 10:10:07 am
content: Of course, if you want the ultimate in simplicity for a 0V to nV output PSU, go for the LT3080 family of current reference regulators (take care with the layout and capacitors to ensure frequency stability though).https://www.analog.com/media/en/technical-documentation/product-selector-card/LDO.pdfhttps://www.analog.com/media/en/technical-documentation/application-notes/an142f.pdf__________________________________________________________________________________________________________https://www.analog.com/media/en/technical-documentation/data-sheets/3082f.pdfhttps://www.analog.com/media/en/technical-documentation/data-sheets/3085fb.pdfhttps://www.analog.com/media/en/technical-documentation/data-sheets/3080fc.pdfhttps://www.analog.com/media/en/technical-documentation/data-sheets/3081fc.pdfhttps://www.analog.com/media/en/technical-documentation/data-sheets/3083fa.pdf

author: JoeO
author_rank: Frequent Contributor
timestamp: December 02, 2018, 12:13:25 pm
content: Quote from: Efe_114 on November 13, 2018, 08:16:18 pmWhy don't you turn on your country flag so that the people helping you will know what parts are available in your country.

author: spec
author_rank: Frequent Contributor
timestamp: December 02, 2018, 12:55:04 pm
content: Quote from: JoeO on December 02, 2018, 12:13:25 pm

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 03, 2018, 10:10:36 am
content: Quote from: spec on December 02, 2018, 12:55:04 pmUnfortunately polar bears, whose only guilt is being much more clever and handsome than any politician(although seals might disagree), live most of their lives and almost exclusively feed on fast disappearing sea ice (Ursus maritimusdoes meansea bear) and so they are listed as vulnerable.I would rather laugh when there will no longer be a guy needing a blonde dog to hide his empty head.Probably I won't have to wait much as the U.S., although being on top for health expenditures, are one of the worst places (among developed countries) regarding life expectancy and one of the few where it has been decreasing for several consecutive years.And subsidies to coal and oil and most of other absurd overturns of previous EPA regulations are quite unlikely to help.

author: nemail2
author_rank: Regular Contributor
timestamp: December 04, 2018, 12:45:27 am
content: all you need is a DAC, VREF, uController, opamp, a series pass transistor and some resistors and caps. take a look at daves uSupply and then at my PSU. it is not as complicated as it looks like.https://github.com/mamama1/LabPSU_Darlington/blob/master/Hardware/schematics.pdf?raw=true

author: VEGETA
author_rank: Super Contributor
timestamp: December 04, 2018, 07:04:48 am
content: Quote from: nemail2 on December 04, 2018, 12:45:27 amBut don't you need stabilization? are you sure your circuit won't oscillate?

author: nemail2
author_rank: Regular Contributor
timestamp: December 04, 2018, 07:14:15 am
content: sure, there are several caps for stabilization in the circuit. two on the opamps for voltage and current control, an input- and an output-cap.i have built the psu already, nothing oscillates so far. did test it with resistive, capacitive and inductive loads.

author: VEGETA
author_rank: Super Contributor
timestamp: December 04, 2018, 10:35:20 am
content: Quote from: nemail2 on December 04, 2018, 07:14:15 amHow much max voltage and current does it output?Can it work with pre-regulator? down to 1v diff for example?

author: nemail2
author_rank: Regular Contributor
timestamp: December 04, 2018, 10:46:28 am
content: Quote from: VEGETA on December 04, 2018, 10:35:20 amDave did implement a pre-regulator, however he never finished the µSupply (until now, he says from time to time that it IS actually happening).My design does not have a pre-regulator but I might implement that later, maybe.The PSU does 0-12V (potentially 0-20V but you'll have to change the Opamp supply line and pay attention to the linear regulators power dissipation as well as maybe tune some other parts to the higher voltage) and 0-4A. if you change the opamp gain, you'll get out even more amps, however you'd also have to pay attention to heat dissipation, especially without the pre-regulator and at low output voltages.

author: Efe_114
author_rank: Contributor
timestamp: December 18, 2018, 08:52:35 pm
content: Quote from: VEGETA on November 14, 2018, 06:43:03 amwhen i was re-reading the replies i re-noticed this comment and i came up with these questions:why do i need negative rail? if im going to use darlington pair and op-amp. i could use a lm358 which could go down to 0 volts with single supply? i didnt understand why do i need to use op amp. dont i need negative supply for lm317 adj pin ? I am confused. which configuration i can use 7660 lm358 tl431 and lm317 in the same time?(as you say in the next reply)

author: nemail2
author_rank: Regular Contributor
timestamp: December 19, 2018, 08:04:05 am
content: Quote from: Efe_114 on December 18, 2018, 08:52:35 pmif you are going to use a darlington and an opamp like LM358 oder OPA2197 (like I do in my design), you won't need a negative rail. if using a DAC to set the voltage, you'd be basically limited by the minimum voltage the DAC can output + opamp gain you'll be using (e.g. DAC can output 5mV * opamp gain of 3, would give you 15mV minimum output voltage). of course if your opamp can't output such a low voltage, you'll be limited by the opamp. but basically you can go pretty low without having a negative rail here (I do that, and it works perfectly).if you are going to use an LM317 you'll need a negative supply if you want to go all the way down to zero to negate the minimum output voltage of about 1,2V of the LM317. but I wouldn't do that, might make the LM317 under certain circumstances.you do need an opamp for the darlington because in this constellation the opamp would be the "controlling device" which in the end sets and maintains the voltage you wanna have. you can apply current to the base of a darlington but how'd you know the voltage it will allow to pass?with an opamp, you feed in the voltage you want into the non inverting terminal of the opamp, the opamp output goes to the darlington base and the darlington emitter goes to the inverting terminal of the opamp. the opamp will output whatever voltage it has to (limited by its supply voltage) to make the inverting and non-inverting inputs the same. so it will do whatever it can so the output voltage of the darlington matches the input voltage at it's non-inverting terminal.because you don't have DACs which can output tens of volts usually, you'll want to put a gain into the opamp circuit to increase the maximum output voltage (e.g. DAC output is 5V max, you want 20V so you have to put in a gain of 4). also, I think only opamps which are "unity gain stable" can be used without any gain resistors. the datasheet will tell you if your opamp is unity-gain stable. for other opamps, you'll need a gain to make them work stable.there is much more into making opamp circuits stable than I understand so here would some real PRO have to kick in but I hope you get an idea of what it is all about.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 19, 2018, 01:53:48 pm
content: Efe,I know it can be a pain to source components when you dont have a digikey/mouser type of vendor in your country, believe me, Im in Brazil and here sucks as well.Now, for the more experienced guys to help you (and I do not include myself in the middle of them). You do need to specify somethings.If you want something around 15V, your 48V transformer probably wont do (or at least wont do with a simple design).Usually, the hard things to get for us are ICs and other specialized things. So you probably have access to general use diodes, resistors and transistors.So with that in mind we need to get another thing straight. You are trying to learn with the project as well as achieve your goals, now, do you know a little bit about transistors and opamps? If you dont building a PS using an opamp to control de voltage and cascading transistors to get the current you need might be the right solution for you. If you dont have access to an opamp using a simple zener that will set up the max Voltage limit and using a pot to vary that voltage (and still using the cascading transistors) might give you what you want.It seems that you can follow and build circuits from a ready made schematic and what you are looking for is learning more about how the things work and starting to customize the circuits for what you need. Unfortunately asking for help here will not give you that. When you ask for help to build a simple PS you will get a ready made schematic and thus not learning what you are actually wanting to lear which is design the circuit yourself.The guys here are incredibly helpful and knowledgable but you kind need to ask the right question.My suggestion is: If you know how to build LM317`s (and other simple voltage regulator ICs) circuits, if you have a good idea how to use transistors and opamps then you need to add more features to your PS to make the project more challenging. If you dont know how to use transistors and opamps I strongly encourage you to built a couple of simple PS`s using them, even if it doesnt have a proper V regulation, and then starting asking questions about how to tweak the circuit to get the current limiting, proper regulation, less noise...

author: Efe_114
author_rank: Contributor
timestamp: December 20, 2018, 06:09:43 pm
content: Quote from: AngraMelo on December 19, 2018, 01:53:48 pmyes, i think due to my poor english i couldnt express myself right. the title shouldnt be "help...." everyone was so helpful and helped me a lot even tough i couldnt pick the title correctly . i know op-amps transistors, ADCs, DACs, MOSFETs,capacitors inductors and more but i am not even close to being proffesional i dont have such experience. yes i cant find special ICs but i can find jellybean op-amps transistors, but everything i can find are fake, generally they are tend to not work i have to buy 5 to get 2 working , I think for the final design i will buy all the components from farnell and wait 30-40 days shipping and pay extreme shipping and probably a whopping-120%-tax (i tried to buy 100 euro hakko 888d and i had to pay 250 euro in total ) i think i will go with 2n3055 darlington pair, a fast op-amp, some stabilizing caps , a CC sink circuitry for constant load.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 20, 2018, 06:15:21 pm
content: Quote from: Efe_114 on December 20, 2018, 06:09:43 pmAwesome! So you have a pretty good idea of what you are doing!! Im sure you will find plenty of help here! Good luck, buddy!!Im in the exact same situation as you, building a couple PSs and having to deal with huge taxes and a lack of good components.If you dont mind me asking, where are you from?

author: rstofer
author_rank: Super Contributor
timestamp: December 21, 2018, 01:19:29 am
content: No matter what marketing says, rail-to-rail op amps can't go to 0V and still source or sink current. They just can't. Think about the active device that would be pulling down toward the 0V rail. It has to have SOME voltage drop.So, instead of having 0V as the lower op amp supply, we use -1V or so.The problem is the spec and that is the death of most of these projects. A) It's very hard to get to 0V. Change the spec to "approach 0V" and accept 1V or 1.5V minimum. B) Relatively high voltage and current spec (particularly current) when there is no possible use for such high values. Somebody sees it written somewhere and suddenly 30V 5A becomes a thing. Sure, the commercial guys can do it but they can get transformers wound any way they want and they can design in switching relays. You just shouldn't drop 60V at 3A across the pass transistor but the math is what it is! Whatever the input capacitor voltage is, minus whatever the output voltage is, times the current, is turned to heat in the pass transistor. Note that the heat goes up as the output voltage goes down!In the days of CMOS, 100 mA is pretty high current. 1A is completely over the top. Now, it's true, I'm not building audio amplifiers for a parade ground but those would have dedicated supplies anyway. Around the digital bench, 1A is a LOT. We also don't need variable voltage, 3.3V, 5V, +-15V covers just about everything.Yes, when you are learning about analog circuits, an adjustable supply is nice. But it doesn't need to go above 15V or above 1A. I might build such a thing with an LM317 and call it a day. BTW, I would pay a lot of attention to Figure 3 of this datasheet.https://www.st.com/resource/en/datasheet/lm217.pdfDon't overlook 5V 2A wall warts. I use them all the time!

author: spec
author_rank: Frequent Contributor
timestamp: December 21, 2018, 03:06:05 am
content: The ultimate barrier to getting true zero volts from a PSU without a negative rail, is leakage current. The output transistors leak current quite a bit and the killer is that the leakage current increases exponentially with junction temperature- take a look at the data sheet for your output devices.Quite often with many PSUs, you are dissipating a lot of power in the output transistors, so the junction temperature is high and leakage current becomes a real problem.But, bearing in mind that a PSU is a power amplifier, there is another aspect. If you have no way of loosing the leakage current, or you have no current drain on the output stage, you get into a no-mans land where the changing characteristics of the output transistors can cause problems.The solution is quite simple though: Either generate a negative supply (2 x caps, 2 x diodes) Or use a power diode and a constant current sink, of around 20mA (power diode, 1 x small signal transistors, 1 x medium power transistor, 2 x resistors)In general, a minimum constant current drain is good news as it also provides some current sinking in addition to current sourcing, which is handy for some loads.Of course, the ultimate is a current sink capability equal to or more than the maximum source output current capability of the PSU (similar output stage to an audio or servo power amplifier). The LT1118 three terminal regulator, does this.https://www.analog.com/media/en/technical-documentation/data-sheets/1118fd.pdf

author: nemail2
author_rank: Regular Contributor
timestamp: December 21, 2018, 09:01:15 am
content: i'm not entirely sure what you guys are talking about. i'm going down to 15 mV output voltage without any negative rail, as mentioned earlier.

author: BravoV
author_rank: Super Contributor
timestamp: December 21, 2018, 09:13:26 am
content: Alternative design , ->General Purpose Power Supply DesignQuoting the designer ..."... I am only looking a very low cost high performance regulator that could be controlled by just plain potentiometers."Read the 1st post carefully.A sneak peek on the spec (highlighted few items that might interest you) :3V to 30V input source voltage range1.5V dropout voltageTrue regulation down to zero volts1A output current with 0 to 1A current limitHigh level of protection from loads.Transient performance similar to the LT3080No-load to full load output voltage regulation better then 0.001% (Wiring and track resistance is the biggest problem)Loads like a battery can be connected to the supply while the supply is off without causing any problems to the supply or the battery.Design based on "garden' components - potential to make itvery cheap.Possibility of a wide range1Asupply using only board mounted devices andno heatsink.Oh yes, no negative rail needed.A sneak peek ...Again, you need to read the whole thread, just a suggestion.

author: Efe_114
author_rank: Contributor
timestamp: December 21, 2018, 04:08:16 pm
content: Quote from: rstofer on December 21, 2018, 01:19:29 amDont get me wrong but i think you misunderstood something i am already planning for 15v 1A. i know 1A is a lot but i will use a power transistor so why not go up to 1A( i wasnt planning more than that anytime) i bought a 48 v transformer but that was for the kit someone posted a picture of.I will buy a lower voltage transformer specifically for this project which i think will be 24 volts or maybe 18 volts but 18 volt ones are pretty rare i will buy a toroidal one if possible. thank you for the suggestion so i will use ICL7660 charge pump IC to generate negative rail (-5 Volts) Also is it needed to make it with 15v/10v taps? I am deciding to use a 2n3055 so i think 15W max wouldnt be a problem for TO3 package. As i said i dont have much experience at all so i am planning all of these in theory So anyone here have more reccomendations to me about problems that will appear on practice?

author: Efe_114
author_rank: Contributor
timestamp: December 21, 2018, 04:16:35 pm
content: Quote from: spec on December 21, 2018, 03:06:05 amyes im planning a 20 mA CC sink circuitry to serve as a minimum load.

author: Efe_114
author_rank: Contributor
timestamp: December 21, 2018, 04:19:50 pm
content: Quote from: BravoV on December 21, 2018, 09:13:26 amwow thanks thats really intresting i will read it. that specs are very nice. especcially theREDones!

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 21, 2018, 05:06:58 pm
content: Quote from: BravoV on December 21, 2018, 09:13:26 amBravo, what is use of the RC circuits shunting the OPamps output and input? (beginner here)

author: nemail2
author_rank: Regular Contributor
timestamp: December 21, 2018, 07:48:05 pm
content: i have a 18V AC outputting transformator and i'm doing 4A current with a pair of TIP3055/BD139 Darlingtons from 0-12V without an issue. proper heatsink and fan req'd, of course.at 1V output voltage and below it gets quite hot obviously but it has temperature monitoring and emergency shutoff just in case and besides, what hobbyist needs currents like this at such low voltages, anyway. 3.3V or even 2.5V at 4A it does without any issue.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 21, 2018, 09:07:59 pm
content: Quote from: AngraMelo on December 21, 2018, 05:06:58 pmThe RC elements at the OPs are there to set the frequency response of the regulators. So they decide wether the supply works well, is too slow or oscillates because it tries to be too fast. Getting this part right is the real difficulty.Trying to get 1 A with no heat sink would usually requite a switched mode supply, or a lot of transistors like 2N3055 that might work with no heat sink up to maybe 2 w. In a linear power supply there is quite some possible heat produced and not much to change with this.It gets tricky to go down all the way to zero without a negative supply, as the typical BJT based constant current sink only works down to some 10-50 mV if one tries hard. So a negative auxiliary supply, even if only - 100 mV already helps a lot. This could be as little as 2 extra diodes with less voltage drop and a filter cap.

author: Efe_114
author_rank: Contributor
timestamp: December 22, 2018, 03:42:01 pm
content: I just recieved my order and i started experimenting with TIP142 and i found out voltage jumps up when i it hook up to a load. about 0.3 volts for a 1w power Led. and the worst thing is that overshoot doesnt recover! what could be the reason of it? Im using lm358 as a opamp TIP142 as series pass transistor a pot for referance a 100nF ceramic cap between opamp output and -input.

author: fsr
author_rank: Regular Contributor
timestamp: December 22, 2018, 05:02:40 pm
content: I think that analyzing that will require a schematic. I don't get the part about the capacitor feedback. Only a capacitor would mean no DC feedback at all. And a pot for reference? Took from where? Please, show us a schematic.

author: Efe_114
author_rank: Contributor
timestamp: December 22, 2018, 06:52:24 pm
content: schematic at attachment. I couldnt figure out how can i convert kicad to pdf and didnt see the attachment button down there and converted it to jpg and tried with add image function and it didnt work and i finally figured out attachment button. first time sending something as attachment. so it took quite a while

author: fsr
author_rank: Regular Contributor
timestamp: December 22, 2018, 10:58:55 pm
content: Well, you can capture the screen and save it to a PNG file, probably easier than exporting to PDF.Why is that capacitor there? It doesn't seems like it can do much as it's connected. The voltage at the (-) input will be 2 Vbe below the opamp's output, no matter what.Your circuit will buffer whatever voltage you have on the potentiometer's wiper, but if Vcc has noise, or ripple, or goes up or down, so is your output voltage. So, your output can only be regulated if Vcc is well regulated already.At this point, you may want to take a look at how national made it with the LM317. I attached and image with the functional schematic. That's how they did a variable voltage regulator.To get how the LM317 works, just remember that an opamp with negative feedback will try to set it's output voltage to a value that causes it's two inputs to match.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 22, 2018, 11:28:02 pm
content: Quote from: Efe_114 on December 22, 2018, 06:52:24 pmThe minimal circuit would need at least an extra resistor in the connection between the output and the OPs inverting input.

author: Efe_114
author_rank: Contributor
timestamp: December 23, 2018, 10:18:53 am
content: Quote from: fsr on December 22, 2018, 10:58:55 pmbut im not measuring referance voltage im measuring output voltage and i power a load output voltage rises up i dont mean the ripple, the ouput voltage rises.Quote from: Kleinstein on December 22, 2018, 11:28:02 pmi didnt know a resistor is needed. it says only a capacitor in art of electronics. not speccifically "ONLY" but it says a capacitor

author: Efe_114
author_rank: Contributor
timestamp: December 23, 2018, 07:48:45 pm
content: Whatever i tried i wasnt able to fix it. Any help?

author: fsr
author_rank: Regular Contributor
timestamp: December 23, 2018, 09:11:23 pm
content: Measure the wiper of the potentiometer. If the output voltage is rising in that circuit, the only way i can see that happening is if that point rises when connecting a load. That can only happen if Vcc rises when connecting a load, so you can measure Vcc to check that. Where does Vcc come from?