DIY 0-24V 0-20A Bench PS help - Page 1
Forum section: electronics / projects

author: Silver_Pharaoh
author_rank: Contributor
timestamp: February 25, 2019, 12:12:11 am
content: Hi EveryoneNew to the forums so I'm hoping this is the right place for it (I think it is!)Anyways, I have modified an old APC Back-UPS BK-250 to house one of those LTC3780 buck/boost modules from ebay. Just hooked it into the battery, drill some holes in the back for 2 binding posts, drill two holes in the front for the 2 pots that control the voltage and current.Works great, but anything over 2A kills the battery in a short period of time roughly 30min. (The battery is new btw)So! The goal is to build a new 0-24v adjustable 0-20A adjustable lab bench power supply.I contacted a local transformer company asking about having a toroidal transformer made for me, 24v approx 500VA. (I realize now I may have to bump this up to 28V 35A due to 60% de-rating I read that somewhere)That will power the voltage adjust and current adjust circuit.I found 2 circuits that I was going to make but now I'm not sure if either one is a good one. And that's why I'm here. I'm hoping someone can help me find a good circuit for this buildThis one is a 12V 20A circuit that I am hoping to make fully adjustable CV/CC, but I'm not sure how...http://www.zen22142.zen.co.uk/Circuits/Power/1220.htmThis one is fully adjustable but I read here:https://www.eevblog.com/forum/projects/lm723-and-variable-current-control-technique/(circuit #2) that isn't a very good circuit... This one I was about to build and just add a few more pass transistors.http://electronics-diy.com/30v-10a-variable-bench-power-supply.phpThanks everyone

author: IanB
author_rank: Super Contributor
timestamp: February 25, 2019, 12:21:41 am
content: What do you have on a lab bench that draws 20 A ?This kind of question comes up a lot, and it is IMHO similar to this question: "I don't know anything about rocket science, but how do I build a moon rocket?"Even if you asked an actual rocket scientist, do you think they could give you a good answer?

author: Silver_Pharaoh
author_rank: Contributor
timestamp: February 25, 2019, 12:45:38 am
content: Honestly, nothing. It's more of a "what if" thing. I can absolutely make due with 10A max.An no, I don't think they could, not a single rocket scientist anyway, but agroupof rocket scientists could, and that's why I came here to ask if a group of us here could possibly come up with some ideas or schematics how to go about building this. And if we can't, that's okaythat means I might be asking for something that's impractical and I need to change the scope a bit

author: IanB
author_rank: Super Contributor
timestamp: February 25, 2019, 01:24:07 am
content: So there are generally two ways to build adjustable power supplies. One is switch mode (a switching circuit is used in a variable manner at relatively high frequency to produce the voltage and current required), the other is linear (a normal transformer is followed by rectifiers and smoothing capacitors and then some kind of automatic variable resistance is used to reduce the voltage and current as required).Linear power supplies need big transformers, big pass transistors, usually some kind of relay to tap the transformer at different voltages, and for anything other than small ones some cooling fans to remove the waste heat from the pass transistors. At 10 A it is going to be big, heavy, and have cooling fans.Switching power supplies need some experience to design, prototype and get right. It is not typical to build one as a home project.You could probably buy a power supply new from China or used on eBay for less trouble than it would take to build one.

author: Silver_Pharaoh
author_rank: Contributor
timestamp: February 25, 2019, 01:45:56 am
content: Quote from: IanB on February 25, 2019, 01:24:07 amOkay so that answers what they would call this kind of power supply - linear. I knew about SMPS, but I wasn't sure what the transformer types were called.I was hesitant on the SMPS on ebay as I'm really not sure of the quality... I can find them at 24V 20A for about $50CAD shipped. Are these any good? I was thinking if I bought one, I would try and swap out the components like caps transistors etc with higher quality versions if possible?Another thing that makes me lean towards linear styles is isolation from mains power.But cost for linear is way up there. I found a 900W 24v toroidal transformer on ebay for $70CAD shipped.... Plus all the parts from digikey but I don't see that being too much more.But I like building things from scratch even if it's a bit more costly because I learn a thing or two from it, plus it's funEDIT: Oh, also found this thread here on the above mentioned 30v 10A circuit:https://www.eevblog.com/forum/beginners/30v-10a-power-supply-keeps-blowing-2n3055s/reading through it now.

author: IanB
author_rank: Super Contributor
timestamp: February 25, 2019, 01:56:54 am
content: Imagine you have a linear 24 V 20 A power supply, and you ask it to deliver 20 A at 6 V. Since the transformer is producing 24 V and you only want 6 V at the output, that means 18 V has to be dropped inside the supply by the pass transistors. That leads to a power dissipation of 18 V x 20 A = 360 W. That's a lot of heat. It would need many power transistors paralleled up on good heat sinks with powerful fans to blow the heat away. You will have created a 360 W room heater.Designers of power supplies get around this somewhat by having a transformer with multiple taps, say 6 V, 12 V, 18 V and 24 V. They use relays to tap the right input voltage according to the output voltage required. But still they tend to have fans because there is always waste heat to be removed.I cannot tell you if a typical SMPS is any good. You mostly get what you pay for. If it is cheap, it is probably designed cheaply and built cheaply.Also bear in mind that it is considerably easier to design a SMPS for a fixed output voltage than it is for a variable output voltage with optional current limiting. It can be done, but it leads to a more complicated design and it is harder to achieve. A laptop power brick for 19 V fixed and 10 A is much easier to build than an adjustable lab supply.

author: james_s
author_rank: Super Contributor
timestamp: February 25, 2019, 07:28:26 am
content: 20A is absolutely enormous for a variable linear power supply. I have one I built using an old circuit from a magazine which does 5A and it takes a huge heatsink. I wound the toroidal transformer myself for it, that was a fun project in itself, it worked perfectly. Still need to build a box for the thing though.

author: beanflying
author_rank: Super Contributor
timestamp: February 25, 2019, 07:43:16 am
content: As someone who has done a bunch of testing of R/C Brushless motors 20A is peanuts in Current for an adjustable power supply. My Manson 15V 60A ran out of legs on a lot of occasions (both voltage and current). It served as one of my bench supplies until I got a 6632B as my main 'nice' option. Generally it kept good regulation and maximum ripple was under the 20mV specified. These are far from an evilbay cheapy.1-32V @ 20A option here just for a lookhttp://www.manson.com.hk/product/hcs-3402-usb/Not everyone wants to run just sensitive micros or needs dual rail current controlled on their benches!If you do need some niceties there is plenty of good quality HP/Agilent gear secondhand in North America to pick from. If you only want one 'nice' supply then a dual or even a triple output is worth looking at. Plenty of time you will want plus and minus rails and even another to drive a micro (three).Linear is preferable due to the low ripple nature of the output and if you can stay away from Korad and similar bottom of the barrel evilbay crud.

author: james_s
author_rank: Super Contributor
timestamp: February 25, 2019, 04:40:00 pm
content: There's no need for a linear PSU for testing brushless RC motors, I use a server PSU for that, it has no trouble delivering the 50A my largest planes require. If I need other voltages I just use a LiPo battery, that's more real world anyway.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 25, 2019, 05:33:12 pm
content: A 20 A power supply is not really a beginner project. Even with only 24 V there are some dangers to it from possibly exploding parts. As a beginner it would be more like start with 0-24 V and 1 or 2 A. Only after that one should think about more than 3 A.The ready made SMPS are not that bad. Changing caps is not a good idea. Changing the transistors is a bad idea unless broken, as the choice is far from easy. However most of the cheap SMPS are fixed voltage only.At this power level one normally prefers a SMPS.If really going linear, one would definitely go with transformer tap switching or a combined SMPS / linear way to reduce the heat production.There are different grades for the regulation: simple regulated supplies with a voltage regulation and a crude current limiting, mainly for protection. Many of the LM723 based circuit are this type.The other group are full lab supplies with a good, fine adjustable current regulation.Finally there are source-meters (SMU) - essentially a very good lab supply with 4 quadrant capability and higher resolution, like a good DMM.

author: FotatoPotato
author_rank: Regular Contributor
timestamp: February 25, 2019, 05:36:57 pm
content: If you want decent efficiency and manageable output ripple then I would go for a mix of a DC-DC switching pre-regulator with a linear front end. Then you can have the switching regulator regulate the output to be 2v higher than the output voltage you want. This will keep the voltage drop over the linear regulator much lower and the heat will be a bit more manageable. The only downsides are that it is probably the most complex and expensive to build

author: beanflying
author_rank: Super Contributor
timestamp: February 25, 2019, 09:27:45 pm
content: Quote from: james_s on February 25, 2019, 04:40:00 pmIf you want to characterize multiple motor winds/props/thrust and get actual data you need fixed voltages and not variable battery voltages.Then go fly them for real world testing as all the static prop/thrust testing goes out the window by a % strap in a logger to see what actually does happen.

author: Silver_Pharaoh
author_rank: Contributor
timestamp: February 25, 2019, 10:07:43 pm
content: Quote from: IanB on February 25, 2019, 01:56:54 amMultiple taps sounds like a good idea if I go with a linear design. Shouldn't be too hard to implement no?I wouldn't need a lot of current at lower voltage I don't think, regardless I see the issue with heat dissipation...I had a feeling those ebay SMPS's are cheap... I'll stay away from those.Quote from: beanflying on February 25, 2019, 07:43:16 amI've been looking at these HP/Agilent PS's since you mentioned them, they seem a tad pricey for what I'll be using it for and close to the amperage I'm looking for. I'm mostly just testing solenoids, charging some Li-Ion packs, a mini ZVS driver and some kanthal coils, some lights stuff like that.Quote from: Kleinstein on February 25, 2019, 05:33:12 pmQuote from: FotatoPotato on February 25, 2019, 05:36:57 pmSo this mix of linear and SMPS, I'm not sure how that would work. Like having an SMPS feed from a transformer or...?I have a use for a lower voltage lower amp power supply. I used to make copper sulphate crystals a year or two ago and i recently learn i can use them to copper plate organics like leaves and wood. I'd like to try it but I don't have a small portable low voltage power supply. I'm thinking 6v at maybe 2A max for this. I'd need CC/CV though since they crystals grow best at low voltage/currents.I realize that a 20A supply is not an easy task, I'd still love to build it though. Or build a decent 10-15A power supply instead, one that I can actually use more than 2A for more than a few minutes

author: FotatoPotato
author_rank: Regular Contributor
timestamp: February 26, 2019, 01:15:01 am
content: The mix is rather simple on paper but it is a bit of a pain in the ass to make. Here is an LTSpice simulation of a power supply that I'm currently making. It uses a switching DC-DC buck converter as a tracking pre-regulator set to always be 2v higher than the output of the linear regulators. If you probe the input to the linear regulators, and their output you can see this in action. My design is built to do 0-30v at 0-6A so it's not as powerful as you want but if you add more linear regulators in parallel and change some component values it should work like you want it to.As a side note, R11 sets the output voltage, for every 1kit goes down, the voltage will go down by 1 voltR24 sets the current limit and can be changed to allow for more or less current.If you have any questions let me know and I'd be happy to help

author: rstofer
author_rank: Super Contributor
timestamp: February 26, 2019, 01:31:36 am
content: Quote from: Silver_Pharaoh on February 25, 2019, 10:07:43 pmOf course it's hard!You need some way to build hysteresis into the relay switching. You don't want them chattering because an analog comparator is switching back and forth. You will likely need 3 or 4 steps. That's why, when you look at Dave's Rigol DP832 teardown, the torroidal transformer has so many taps.Around 15:00

author: james_s
author_rank: Super Contributor
timestamp: February 26, 2019, 04:57:59 am
content: Quote from: beanflying on February 25, 2019, 09:27:45 pmThe server PSU provides a fixed voltage equivalent to a 3S battery which is what I use in 90% of my aircraft, so that takes care of that. Beyond that the battery voltage is quite predictable, it sags slightly under load but I can charge up a battery prior to testing a setup and it's pretty consistent. Then of course I can't fly the plane on a bench PSU anyway so do most of the real world testing using the same battery that I'm going to fly. In the 5 years or so I've been building and flying I've never found myself needing to do anything else.

author: beanflying
author_rank: Super Contributor
timestamp: February 26, 2019, 05:10:52 am
content: And in my case I run from 1S to 6S5000's and serial 4S 5000's (8S) on one plane so the 15V 60A is underdone, even my 2 big chargers pull most of that. Hyperion E-Meter for in flight testing and logging of motor data works great so providing there is room to fit it it gets better real results over static.Unfortunately this is the problem with asking for advice on which Device X should I buy we all have different needs. The OP needs more current than some.

author: Silver_Pharaoh
author_rank: Contributor
timestamp: February 27, 2019, 02:01:03 am
content: Quote from: FotatoPotato on February 26, 2019, 01:15:01 amOutput is the outputs of the LTC3081's correct?For some reason everything is simulating EXTREMELY slowly, only a simple power source and resistor seem to simulate fast? Even the 30v 10A circuit linked above I made yesterday and it too under a second to simulate, today, over 5 minutes and not even 5% complete doing a "Transient Analysis" . Your circuit takes about 5 minutes or so to simulate.. Is this some issue with LTSpice XVII?Quote from: rstofer on February 26, 2019, 01:31:36 amWill watch the video either tonight or tomorrow.Honestly my first thought was a few latching switches controlling individual relays for each transformer tap!

author: coromonadalix
author_rank: Super Contributor
timestamp: February 27, 2019, 10:19:20 am
content: The problem(s) is sometime the inrush current when your load kick's in, tried a few psus with motor drills for fun, they run around 1 amp, but surge at 10 amps or higher when kicking lollSo your 20 amps output, should be a 30 or 40 amp maybe ? or a simple 100% current limitation at 100% load, say a full 0 ohms pure short, not adjustable by any means ??Like some dc to ac inverters, Ex: a 3kw model, it will support 3kw inrush current for the appliance startup, but run at full power around 2kw ?? like some xantrex inverters we have here at Canadian Tire

author: Silver_Pharaoh
author_rank: Contributor
timestamp: February 28, 2019, 01:25:19 am
content: Quote from: coromonadalix on February 27, 2019, 10:19:20 am40A seems a bit much... The more I look at schematics the more I think building a 20A circuit is a bit much. Looking at 10 or 15A instead now.I took a spin at modifing a 12V 20A schematic I found. I looked through the example schematics in the LM723 datasheet and I implemented part of the 2-7V adjustment circuit for voltage. I think I got it working, voltage only goes down to 2.8V though I was hoping to go down to 1V. The current seems to go down to 2mA which is sweet.But I don't have enough experience to know if the circuit works or only works by fluke.I've attached it and the models if anyone can look over it.

author: FotatoPotato
author_rank: Regular Contributor
timestamp: February 28, 2019, 02:39:17 am
content: I agree with you that 20A is a bit much. I get the "because I can" mindset. I was there too, and still, kinda am lol. Having so much power at your bench can be destructive and fun but it can be very expensive and actually quite dangerous to DIY. Just imagine if you make mistake and send 20A through a part that shouldn't be getting 20A.... The results will be explosive, to say the least. 15A is a lot but definitely more doable and less expensive. 10A is easy and you can buy cheap bench supplies that do 0-30v 0-10A for like $160 I would look into making something like this:You can very easily make this PSU with all of your requirements, voltage adjust, current adjust and limit, and a 10-15A output. If I'm not mistaken the DC-DC he uses in the video can do up to 10A so you would just need a fitting input supply for it and you are good to go!Good luck and stay safe

author: radiolistener
author_rank: Super Contributor
timestamp: February 28, 2019, 08:15:11 am
content: Quote from: IanB on February 25, 2019, 12:21:41 amusually such power is required for transceivers, they usually require 13.8 V. Using Switching PSU is a bad idea in such case, because it produces high noise and you cannot filter it to avoid interference for receiver. Good switching PSU has good filtering and manual frequency regulator, it can help partially, but the noise still will be an issue. So, linear PSU is much better for such purposesOn the other hand, if there is no requirement for low noise, it's better to use switching PSU, because it's more efficient.

author: poorchava
author_rank: Super Contributor
timestamp: February 28, 2019, 08:16:02 am
content: From someone doing similar stuff profesionally:-ditche the idea of mains frequency transformer, go for an off the shelf SMPS, for example from meanwell.It has PFC, and at 500W power levels, PFC becomes convenient instead of being an unnecessary (yet regulations mandated) pain in the ass. It may also be a bit cheaper than a trafo itself, not to mention much smaller weight.U can get a good price on MeanWell power supplies from many places, and they are decent quality. Avoid super cheap alibaba stuff, as they usually don't meet their specs.After that just get a half-bridge, either discrete (2x FET + driver) or integrated (CSD97396Q4M for example). Smaller stuff can be a bitch to remove heat from, but it's doable. Ideally, u want to select FETs that are at least 50V rated, to avoid damage from switching transients on the bridge output, which is the reason why at 24V the integrated chip i referred to earlier may not be the best option. At that input voltage u can run this thing at 150...200kHz without much issues. Of course lower is also possible, but inductor will get quite large.If you want very low output voltages below 0.5V, get a linear final stage, as with SMPS low output voltages translate to very short on-time, which is problematic. I'd aim for shortest on-time around 100ns, which at 200kHz translates to about 0.48V. Linear regulators tend to be nasty bitches at 20A though.Control circuitry is up to u. Unless you need extreme transient performance and reaction times, digital control is the easiest. There are currently multiple choices when ti comes to MCUs with HRPWM capability. TI DSCs have highest performance of all those, but documentation is hard to navigate and architecture is peculiar - in general they are pain in the ass to develop for, especially when compared to Cortex-M based stuff. ST has one part (STM32F334) that has HRPWM capability and Infineon has some as well in XMC1000 and XMC4000 families. U can get very acceptable performance without delving into all the hard math stuff like z-transforms etc. Simple PI loop works well enough in most cases.Analog circuits are also available of course.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 28, 2019, 05:43:23 pm
content: The final regulation should definitely be analog, not in software.Starting with a ready made SMPS power supply is a good idea, even if it requites an additional SMPS stage - not the most elegant way, but the lower voltage makes it easier to have a high frequency for that stage. The ready made stage also can provide some extra short circuit protection.For beginner it's usually more about 1 A. So better first start low power to understand the control - this tend's to be the difficult part. Chances are it takes more than a few tries anyway. Later going from 1 A to 10 A or 20 A is not such a big step anymore, if one can keep the same type of circuit and does not need extra slow start of similar.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 28, 2019, 06:20:37 pm
content: Quote from: Silver_Pharaoh on February 25, 2019, 01:45:56 amThere are some good reviews of bench power supplies (both linear and SMPS) on YouTube.I would trust reviews from people like Dave and Mike, et.al.IME, you can pretty quickly judge a YouTube reviewer in the first 5 minutes.Clearly if they bumble around and use the wrong terminology and don't know anything more than you do, then it isn't worth watching. Except for the entertainment value and learning from THEIR mistakes.

author: radiolistener
author_rank: Super Contributor
timestamp: February 28, 2019, 10:00:15 pm
content: I found this switching power supply on aliexpress:https://www.aliexpress.com/item/CPS-3232-1000W-0-32V-0-32A-High-power-Digital-Adjustable-Laboratory-DC-Power-Supply-220V/32813324978.htmlActually I don't need switching power supply, because I'm working primarily with radio and spectrum measurement, so I need low noise linear power supply. But this switching PSU looks very nice, it is very small and provides up to 32 V at 32 A with regulation. That's just crazy power (1000 W) and can be used for different funny experimentsI read some reviews and at a glance it doing his job. But I still not decided if I really need it.If it has too high noise, I will not be able to use it for some serious purpose and I don't want to buy it just as a toy.Does anyone tried these PSU?

author: coromonadalix
author_rank: Super Contributor
timestamp: March 01, 2019, 10:46:03 am
content: never saw any teardown of the gopher psu's would love to see the insides, what their worth ??just saw a picture of one model

author: poorchava
author_rank: Super Contributor
timestamp: March 01, 2019, 11:01:46 am
content: Quote from: Kleinstein on February 28, 2019, 05:43:23 pmCompany I work for manufactures a 200A/1kW microohm meter and it can measure primary windings of a 25MVA transformer (inductance is in the order of kilohenries). It's a 0.25% accuracy instrument and the current source is 100% digital. I did all of the software and big part of hardware, and actually the extremely inductive loads are the problem. Regulation on resistive-ish loads was not very hard.Quote from: coromonadalix on March 01, 2019, 10:46:03 amCheck out VoltLog on YT. i think he tore one down.

author: Silver_Pharaoh
author_rank: Contributor
timestamp: March 02, 2019, 02:12:22 am
content: Quote from: FotatoPotato on February 28, 2019, 02:39:17 amThat's what I have already, just the battery feeding it isn't strong enoughOtherwise, it works perfectly and I love it!I could buy another and use that (and I might if I can't find a decent circuit or it's not worth the effort to build one) but I was hoping to build a stronger power supply for future projects

author: Silver_Pharaoh
author_rank: Contributor
timestamp: March 02, 2019, 02:40:55 am
content: Quote from: poorchava on February 28, 2019, 08:16:02 amThis seems complex to me, I would need to sit down and really read up on this.So use an SMPS as the back end and have some FET's drive the current, and the front end for voltage and current adjustment I'd use a microcontroller?Old server PSU's from ebay, not a bad choice for a SMPS?

author: tkamiya
author_rank: Super Contributor
timestamp: March 02, 2019, 04:12:30 am
content: You can probably buy used one cheaper than you can make one yourself.Project like this looks easy and inexpensive but reality is anything but! 0 - 24V, 20Amp power supply will have to dissipate up to half a kilowatt in heat. That means large heatsink with lots of transistors. Also the transformer.... It'll be pretty big and heavy. Just those items will run you few hundred dollars. At this kind of capacity, current limiting circuit is necessary, or otherwise you will be burning up your experimentation subject every time you make a little mistake.Try looking up ones from HP. It's not the most beautiful thing in the world but they are rugged, well made, and relatively cheap.Realistically speaking though, do you really need all this? I don't recall last time I drew more than few amps....

author: coromonadalix
author_rank: Super Contributor
timestamp: March 02, 2019, 11:44:14 am
content: It depends of the needs, sometime i have to put tons of amps at 5-12 volts, sometimes higher voltages around 80 volts, the perfect psu would some kind of smps or ac phase controlled one.But the size get biiiig, i have a Kepco ate 75-8 "classic linear" who weights 52 pounds on my desk, and a kikusui ate 75-5 (ac phase control, buck boost topology) who weight 22-25 pounds, half the previous one.Had Lambda 5v at 120 amps, pretty rock solid and compact, very old, 68hc705 motorolla cpu in it, but the fan was so damn noisy, could not controll it, and the airflow was needed to work, i could not drop the speedunless having built a variable speed fan controller reacting to the amps output.Myself i would like : say 120vdc at 10-12 amps psu kinda smps based, (it push the ac home main socket to the max 1200watts), reliable, compact, can or could resist inductives and capacitives loads, it take really good designs for that : like xantrex / sorensen but they are rackmount models, and i totally hate that format, the newest Kikusui models are very well made, but in the end all priceyBut you pay for them only onceI have 2x big toroids sitting duck for 3 years now 4x 30vac at 4x 5 amps each, and 4x very beautiful heatsinks who does a funnel once put together, sure they would make a nice psu say : add secondaries in serie for higher voltages at 5 amps, add them in parallel for higher current, serial parallel management etc ... but im noob in programming somekind of an arduino based controller / psu who could do all the switching. the 2 x-forers themselves weight over 20 poundsSome hp psu's models does that EX: 200watts at 10 volts 20 amps, 200 watts 20v 10 amps, 200 watts 40v 5 amps, but they are hell to dismantle and repair, but schematics exists.You have some very good 1200w smps with variable voltage and current with beautiful displays, 400-500$ usd plus a 100-140$ usd in shipping, thats a lot to tackle, and no schematics if they blow.

author: radiolistener
author_rank: Super Contributor
timestamp: March 02, 2019, 06:12:20 pm
content: Quote from: coromonadalix on March 01, 2019, 10:46:03 am

author: coromonadalix
author_rank: Super Contributor
timestamp: March 02, 2019, 07:09:21 pm
content: wow thanks, they look bigger in photos loll, they seems well done ??a 32v 32 amps at around 170$ usd seems okay ??my tests wold be inductive and capacitives loads to see how it reacts

author: Silver_Pharaoh
author_rank: Contributor
timestamp: March 02, 2019, 09:22:08 pm
content: Indeed they do seem decently built.Other than one on Amazon and one on Ebay, where does one find these?I didn't see any listed at Gopherelectronics.com (main site?)

author: poorchava
author_rank: Super Contributor
timestamp: March 02, 2019, 09:22:54 pm
content: Quote from: Silver_Pharaoh on March 02, 2019, 02:40:55 amI'll try to break it down a bit. I'd build a 2-stage SMPS. First stage is AC/DC producing some reasonable DC voltage rail. It is lighter and even in 1-off can be cheaper than a mains transformer. Building one yourself is not economically feasible, ans it's also not easy and dangerous since mains is involved. An old server PSU might be a good choice if you can find one with suitable output voltage. Servers often use 48V supply rails. to get 24V output you'd need something a bit above that, so ~26V. Most SMPS have some form of fine adjustment so this might suffice.Now u need to regulate that voltage down. A synchronous buck converter is simply 2 mosfets driven by complementary signals connecting the output to either VBUS or GND, so if you drive that half-bridge with a PWM, you basically get the same PWM on the output, but with amplitude = VBUS. You filter that wiith an inductor and capacitors and you get DC proportional to the PWM. So if your suty cycle of pwm driving the half-bridge is 50%, you'll get a 50% VBUS PWM, which after wiltering will give you DC equal to VBUS*50%. That's it.You'll need a half-bridge bootstrap driver to drive those fets, but those are very standard parts. There are some shenanigans like minimum on- and off- times, dead time, bootstrap recharge time etc, but those can be googled for. In the end you need to provide a PWM waveform to the driver, either complementary, or the driver manages that automatically and you only need to supply single ended PWM. This can be generated by an analog pwm controller, like TL494 (although it's an ancient and low frequency chip) or something digital. The limitation is, that digitally generated PWM has minimal on-time resolution in best case equal to period of microcontroller clock signal. Assuming a 200kHz PWM and an already very fast STM32H7 series MCU (400MHz) you'll get 2000 timer counts per PWM period at 200kHz. This means that your resolution is limited to about 12mV at VBUS=24V. This equals to 10.97 bits of resolution. This may be enough for stable operation and good precision. To achieve ~12bits, you'd need about 800MHz clock, so not doable. There are microcontrollers on the parket that have a HRPWM (HR = high resolution) technology, which allows for fine pwm adjustment using additional asynchronous delay lines. For example TMS320 series chips which run at 90MHz, but they can have 180ps pwm resolution which would normally require a 5.5GHz clock. In this case it would allow for 14.74 bits of resolution or 0,0036%PWM can be adjusted base on current output voltage/current/power/moon phase. Actually in many cases even a really dumb +/- regulator works ok. The algorithm is something like "if output voltage is less than set point increase pwm, otherwise decrease pwm" or in CC/CV mode "while voltage and current are below limit increase pwm, in case either of those exceeds limit, decrease pwm". Yes, you can implement stuff like P, PI, P^2, feedforward and such, but a dumb solution often works too

author: flyrod
author_rank: Contributor
timestamp: March 04, 2019, 09:44:53 pm
content: I have one of these 30v 30a bench supplies that has been very useful for a wide variety of projects:https://www.amazon.com/dp/B00PWSDH8C/There are a lot of high quality server and telecom power supplies available on ebay for very cheap. I'm surprised there are not a lot of projects here that re-purpose them or their components. They are much higher quality than the chineze stuff and usually less expensive. The only drawback is that they're usually not very adjustable or current limiting. I bought a 2000w 48v supply and stuck a couple cheap adjustable buck converters on it, and it seems to work well. Some cheap panel meters show voltage/amps. Here are the buck converters I used:https://www.ebay.com/itm/273509350150

author: Doctorandus_P
author_rank: Super Contributor
timestamp: March 05, 2019, 07:19:32 am
content: Rockets are easy.You take an enourmous amount of liquid fuel and oxygen and suspend it in midair. Then you wrap 3 layers of aluminum foil aroundi it to stop it from leaking. After that you stick a funnel through the aluminum foil and you light the liquid which is flowing through it.It's really that easy. Any 4-year old can do it.

author: poorchava
author_rank: Super Contributor
timestamp: March 05, 2019, 08:30:06 am
content: Quote from: Doctorandus_P on March 05, 2019, 07:19:32 amNo need to be sarcastic about it. The reality is, that smps converters and digital control being very complex and difficult is not always the case. Yes, there are difficult cases, when your're shootig for extreme performance and getting the best possible transient response or efficiency or power density can be really challenging, but to get it to work well enough for bench use is much, much easier.Besides, do you think I didn't blow up ma fair share of FETs trying new stuff out? IMO there is no other way to learn. Pay hard, go pro. You won't become one building RGB blinkenlights with Arduino.

author: najrao
author_rank: Regular Contributor
timestamp: March 05, 2019, 09:12:06 am
content: Flyrod, I have modified a particular 2kW 54V telecom 'rectifier', with just bits of additional circuitry, to make up a CC/CV PS.Being an early design of this manufacturer, it used one large multilayer board, making circuit tracing just that bit easier. Still, not for the faint hearted, nor if you are in a hurry. I just identified nodes for injecting analogue feedback signals, and added may be one or two 431's, as I remember. I kept no records, but it should not be too difficult to reconstruct the crime.My purpose was not making a lab grade PS, and I did not bother with ripple and noise targets. These won't be any worse than what the base unit (the rectifier) had. It was born to to charge large lead acid batteries, and never worked without them being connected.Still, my results were good to say the least: 0 to 56V, 0 to 40A, settable by coarse and fine pots, the only limits to resolution. As I remember, the crossover from CC to CV and vice-versa was perfectly sharp too. I only had a single 4 digit meter, so I multiplexed it by a rocker switch to show one thing at a time.If there is any interest, I can work up a diagram, but in good time -- have a few things on hand!

author: najrao
author_rank: Regular Contributor
timestamp: March 06, 2019, 04:34:37 am
content: Coromandalix asked for details of my 'hack'.It was years ago, and not even a back of napkin sketch remains. I will do my best.First, I painstakingly identified op amp inputs which (1) are fed by whatever processor on the board, (2) are in the analogue signal paths and directly act upon the output voltage and current separately. I noticed that there is at least one more circuit, perhaps for OV protection and setting. There are some 12 amps (3 quads, all 324), and it took days to locate the valid nodes.What with 4 layers and myriad vias, imagination boggles soon; mine soonest, but I persisted with the beeping DMM till done.Next, I found that both the Vset and Iset points are at ZERO volt at the op amp inputs when their control loops are in steady state -- perhaps in the summing mode of several signals. Their swing for full excursion of the setting is but a couple of hundred millivolts. Any external bias can be used to modify the settings: but you do need + and - stable variable millivolts.The on board 'SB' smps produces all control supplies: +/- 12V as well as 5V for the processor. What remains to do is to set up dividers and include coarse and fine pots as appropriate. Choose the right scaling to set the end points of the pots. Some extra biasing from the pre-existing current shunt and also from the output voltage itself was needed, but cannot remember the details now.I cannot say whether what I have done manipulated the reference or feedback - as I have used the summing nodes. ( I had said that I used 431's, but this is not so).It is clear that this method may degrade the accuracy of regulation, particularly towards zero setting. In the limited testing I have done, the 4 digit meter did not show any error upon perturbation, down to zero setting.From what I have gleaned, the power stage in this rectifier operates resonant mode at some 250kHz, and I had feared serious stability problems at far away settings. Surprisingly there were none, but then I have done no transient response tests.As a bonus, I added a mosfet 'down- programmer' circuit to help with the unavoidable 1.5mF output capacitor.I have many more telecom rectifiers, including 4kW 54 volters. They are ever more compact, use daughter boards with very small IC packages, 0603 smd resistors, etc. Extremely difficult to hack.

author: Silver_Pharaoh
author_rank: Contributor
timestamp: March 07, 2019, 12:23:47 am
content: Interesting hack on the telecom stuff!That sounds like it would be the final power supply I'd ever need!I haven't had much time lately to tinker, but I am having success with a modified schematic for 12.5A power supply. If I can get it working nicely then I should just need a way to power it.Haven't heard back from the torroid company.. So I might go with SMPS back end like most are recommending instead of the original torroid linear idea.

author: najrao
author_rank: Regular Contributor
timestamp: March 07, 2019, 03:42:10 am
content: Glad you are still at it, Silver_Pharaoh.Here are a couple more I did using about the same hack:1. This started life as a projector lamp driver, Italian by birth. Rated 35V 14A 350W by the maker, tests good higher. The dual edge-wise moving coil meter worked in a power station somewhere. The four CC and CV pots had done their stint in a Tek oscilloscope. This unit does 40V 14A and over 350W and is something of a workhorse for me.2. Was a HP ps, extremely well made, but would work only with specific computer. Using a dedicated programmer, you could set CC, CV, OC, OV and what else. Alas, I had no programmer, and so. The meters (also HP) came from a surplus store in England (Chiltmead of Reading) I used to visit some decades ago. Needed a separate SB supply, which I made up from a laptop brick. 35V 5.5A.The moral of the story is -- simple hacks work!

author: aleksis1977
author_rank: Newbie
timestamp: June 10, 2022, 01:30:11 pm
content: Hello! Where do the black and brown wires go?Why external current transformer?Maybe there is a diagram?What voltages are used for regulation?Thank you!

author: najrao
author_rank: Regular Contributor
timestamp: June 10, 2022, 02:37:51 pm
content: Aleksis 1977, I wish I could answer you properly, but alas, it was quite a few years ago, even well before I wrote my last mail to Coromandalix. Memory, while quite good for so many decades, is probably attenuating pretty rapidly now. And I never kept any records either.There certainly was no external current transformer. Please read my previous post carefully to answer the other questions. This mod cannot be done without understanding each step.I am sorry if I am not enough helpful. And please ignore this if your questions were meant for someone else.najrao

author: wizard69
author_rank: Super Contributor
timestamp: June 12, 2022, 10:14:54 pm
content: Quote from: Silver_Pharaoh on February 25, 2019, 12:45:38 amEven 10 amps is a lot. There are two issues with such supplies, they dissipate a lot power for one. Two they can be dagerous in the sense that even 10 amps can melt things on a test bench.QuoteUnfortunately we don't know your level of ability nor what this will be used for. You could be building a supply for a home metal plating setup for all we know.QuoteWell never say can't! However don't just jump into the river not knowing how swift the current is.I'm not telling you to not build the supply, what I'm suggesting is to consider what your needs are. The better they are relayed the more likely somebody might have experience. Back in my youth, more decades than I want to admit, some of my first experimentation in electronics involved 3 terminal regulators, resulting in a few supplies so I understand the draw. Most of the time I actually had a use for the little supplies. What I didn't have was the resources to build a 20 amp supply.In any event a good resource for stuff like this is ARRL as Ham radio operators are often using or building low voltage high current supplies. One book is: ARRL 5010 - RSGB's Power Supplies Explained. However I seem to remember a few other articles and or publications.