simple bench supply question - Page 1
Forum section: electronics / beginners

author: MG1
author_rank: Newbie
timestamp: October 28, 2020, 12:18:00 am
content: Hi there,This is a bit of a hello post as I've been a long time lurker and avid watcher of the Dave's videos. I'm a bit nervous posting this as I'm not sure if it's a stupid questionI'm pretty new to any form of electronics, but am hoping to learn a few things as a help my 8 yo work through some projects in this electronics kit we got for him.https://www.amazon.co.uk/Cambridge-Brainbox-Primary-Electronics-Kit/dp/B001MJ7DB2/ref=sr_1_21?dchild=1&keywords=electronics+kit&qid=1603842250&sr=8-21Anyway, I'm hoping to build a few small projects such as a fume extractor for my soldering station and a 12v thermally regulated fan controller for a power resistor battery discharge bank.But I thought I'd start with a simple bench supply that will provide power to my lipo chargers for RC and also give fixed voltage outputs in 1.5v increments for battery simulations. I did think about just getting one of the DPS3005 modules but I'm a little confused about which ones would be suitable and they are probably more than I need right now.Currently I have 2x 750w HP server PSUs that I have connected in series, floating the ground on the second unit (by isolating the board from the case so that mains supply earthing is not interrupted) in order to output 24v for charging lipos via an external charger.SO, the possibly stupid question is how to get to the required voltage outputs?It feels very excessive to purchase separate buck convertors for each voltage and so I was wondering if I could run a number of linear regulators that utilise the output from regulators up the food chain so to speak.I would probably only need to go up to 9v and then also have a 12v and 24v output.Can I have something like the following (apologies for lack of drawing...I'm not sure where to begin with circuit design software...even for block diagrams)Regulator 1: 12v in 9v outRegulator 2: 9v in 7.5v outRegulator 3: 7.5v in 6v outRegulator 4: 6v in 4.5v outRegulator 5: 4.5v in 3.0v outRegulator 6: 3.0v in 1.5v outMy thinking was that this way, the power dump and heat generated is spread between the regulators, so that when working off the 1.5v output rather than dumping all of the heat from a 12v to 1.5v conversion into a single and possibly substantial heat sink, it is being dissipated by all six regulators and six smaller heat sinks. Yes the power is still approximately the same I guess, maybe a little more because of inefficiencies, but my gut says this should be a better solution.Also, and it may not be something that I need to consider at the moment, but by using linear regulators would I be getting a 'cleaner' lower noise output? Do I need to include a filtering stage before the regulators?I know the DPS unit would be probably easier, but I'm in this to learn a little each time, so I hope that this possibly silly question doesn't offend the regulars and I hope you kind folk will be able to help teach this old dog some new tricks!!Many thanks in advanceMichael

author: ledtester
author_rank: Super Contributor
timestamp: October 28, 2020, 01:05:30 am
content: QuoteThere is something to be said for using an appropriate sized power supply for your application. Do those supplies have any kind of current limiting? 750W is a lot of power to dissipate if something goes wrong.How much current do you need for, say, charging your batteries?The output of almost all buck converters can be changed on the fly. So, if you need 9V why not just set the output of the buck converter to 9V? For charging batteries you don't need the kind of clean power that a linear regulator will give you and it'll be a lot more power efficient (i.e. you'll generate much less heat.)If you need multiple outputs at the same time just get multiple buck converters. There are cheaper alternatives to the DPS3005 (down to just a couple dollars a piece) which might do the job for you -- i.e. search for LM2956 modules.

author: MosherIV
author_rank: Super Contributor
timestamp: October 28, 2020, 08:45:25 am
content: HiWelcome to the forum.Converting computer psu into some other form of psu is not a good idea.Especially one for charging LiPo.As Ledtester pointed out, these psu will deliver max current/power.I am not sure that just disconnecting the chassis ground floats the psu.If you use linear regulators, the heat generated is the same no matter how many regulators you spread it across, the total voltage drop is the same. Yes, the heat is now spread across a number of regulators but the total is still the same.Each daisy chained regulator will have to pass the same current.One easy option is to use 1 of the psu that you have and use a single LM317 adjustable regulator.It needs 4V above the output voltage you want. Then with 1 resistor (possibly variable) and 2 capacitors, you have a simple psu circuit. The LM317 come in a few variants, TO92 package will allow max 100mA, then the other variants allow 1A going upto 3A.It is the well troden route for beginners and their first psu.As a beginner, do not worry about variable current limit just stick with varible Voltage.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 28, 2020, 12:08:52 pm
content: Fully agree with being careful with a 750W PC power supply.These melt your cables before shutting down.DPS series are buck regulators, so can only get lower output voltages.DPH series can deliver higher output voltages then the input.So if you connect your 750W PC power supply to one of the DPH series regulators, you have a useable power supply to start with.I disagree with:Quote from: MosherIV on October 28, 2020, 08:45:25 amHaving an adjustable current limit is a very useful feature of a power supply.Especially for beginners, and for circuits where you easily make shorts such as on a breadboard.The current limit does not have to be very accurate though.A simple technique I sometimes use is:* Set the voltage of the power supply* Set the current limit to minimum* Connect the circuit under test.* Increase current limit until nominal output voltage is reached.* Increase current limit a bit more by some sane margin (Guess about 20%)LM317 is a very simple and common way for a simple and cheap, yet useful power supply.Use the TO220 variant, the TO92 overheats far to easily.Another option is to use a L200.It is similar to a LM317, but has provisions for a fixed current limit.This fixed current limit can be made variable by measuring the voltage over the shunt resistor with an opamp, and use a variable amplification of the shunt voltage.

author: ledtester
author_rank: Super Contributor
timestamp: October 28, 2020, 03:39:37 pm
content: Instead of a DHS3005 which costs around $30 and doesn't come with an enclosure, consider getting a NPS-1601 for about $50 which has the same voltage and amperage rating and you'll get a complete self-contained power supply.You can find other recommendations discussed in this thread:https://www.eevblog.com/forum/beginners/cheap-power-bench-supply-any-recommendations

author: MG1
author_rank: Newbie
timestamp: October 28, 2020, 06:34:13 pm
content: First of all, thanks for taking the time to reply. It's truly appreciated and has certainly given me some food for thought.Quote from: ledtester on October 28, 2020, 03:39:37 pmThat's an interesting looking unit...and if I can't get to grips with the circuit design and component selection then it gives me a great fall back. But at the moment I'm pretty sure I want this to be a bit of a project and so will avoid too much of an off the shelf package for now.I'm pretty adept at fashioning things out of wood, plastic, aluminium, so I'm not too worried about enclosures.Quote from: ledtester on October 28, 2020, 01:05:30 amThe chargers I'm using take in input voltage of 7-30V and are limited to 18A input with a power limit of 400W.So at 24V these would limit to just over 16A if I was running just one, or would hit the PSU ceiling if I ran two at a time, pulling just over 15.5A each.Which is why I'm using 10 gauge silicone hook-up wire and 6mm lug connectors and inline fuses between PSU and the XT-60 chassis mounted plugs.Quote from: MosherIV on October 28, 2020, 08:45:25 amJust to be clear, I'm not directly hooking up the batteries to the PSU's, I'm using the PSU's to supply 24v to DC chargers that enables me to hit a higher power delivery than the 12V I'd get from one. At 12v I hit the max current input of the charger (18A) and can only deliver 216W power. At 24V I can get the full 400W if needed.This method for floating the ground has been discussed at length on a number of the RC forums and there's quite a repository of tried and tested units. One of the main attractions for RC use is that they are very cheap for the output they give. I got mine here:https://www.bargainhardware.co.uk/hp-660183-001-common-slot-hs-psu-750w-platinum-plus-656363-b21and paid £12 for the pair...they even managed to dig out two with matching part numbers.Here's some information on hacking these if you're interested:https://youtu.be/d7lF1sIgIlAAlthough there's a fair few other examples out there too....just be prepared to cringe a little at the rough and ready approach some have taken. Rest assured, I'm pretty risk adverse, hence why I'm here asking questions!!SO I basically have the two PSUs already providing 24v but what I would like to do is to make the unit multifunctional. I've got independent switches for the two units. They switch on by making a connection between the PSU Present Signal and Power Supply on/off ctrl signal pin outs, which utilise a low voltage signal and therefore don't require hardware rated for high currents.For the bench supply part of what I'm doing I will only be using the one unit....the second one could remain switched off.Quote from: Doctorandus_P on October 28, 2020, 12:08:52 pmThat's all really useful...I've been looking through the spec sheets for the LM317....I think I need to refresh my basic algebra a little, so it may take me a while to figure some of this stuff out and how to apply it for a simple circuit.If I fall apart then I can always fall back on a module I can just wire up, but I do like the appeal of cheap parts + some maths and physics to get a workable solution that I understand the workings of.I appreciate it's not the easiest route to a power supply, but I think it'll get my head in the electronics game a little more and be more rewarding as I can bring my son along on the journey.I think that replies to everyone, apologies if I missed something...I've been squeezing this response in amongst child care until my wife got home as it's half term here!Thanks again and I'll probably get back with some questions if I don't drown in data sheets...

author: wizard69
author_rank: Super Contributor
timestamp: October 29, 2020, 09:48:23 am
content: Welcome to the forums.Quote from: MG1 on October 28, 2020, 12:18:00 amWell there are two ways to look at this. One is that there are no stupid questions because we are trying to learn something when we ask them. The other is that all questions are the result of stupidity and we need to ask the question to learn something. So from either perspective the end we are seeking is to be smarter.QuoteI'm actually a fan of power supply building to learn electronics. There are two reasons here. First you can learn about basic circuit components and analyses. The other problem is that power supplies are common failure points when it comes to electronics, so it is good to understand them even if you focus on digital systems.However fair warning I do not suggest that a beginner build lipo chargers. The risk of fire is just too high to break into the hobby with. Now it looks like you want to power a separate device that is a charger but even so you really need to understand the risks of working with these batteries.The simplest power supplies are based around three terminal regulators, either linear or switching. Variable voltage supplies are a bit more complex and can be a lot more complex depending upon what you chose to implement. However there is no reason to engineer a supply as there are hundreds of designs floating about and this is what I would suggest build from a respected design. In this case a variable voltage regulated supply.Now as for fixed voltage supplies they can actually be a good idea if they implement standard voltages such as 3.3, 5, or 12 VDC.Quoteif you need a lot of voltage potentials the rational approach is a variable voltage regulated supply. However if you are powering something like a battery charger then build a supply specific to that chargers need.[quote[It feels very excessive to purchase separate buck convertors for each voltage and so I was wondering if I could run a number of linear regulators that utilise the output from regulators up the food chain so to speak.There are lots of things you could or can do but might not be ideal. At some point you are just better off with a variable voltage supply design.Quote12 and 24 VDC supplies are dime a dozen and it probably would make a lot of sense to buy or make these as fixed voltage supplies.QuoteI'd avoid the above at all costs.QuoteOne thing that you will likely learn over time is that you are better off with multiple power supplies. For example if you need to power 12 VDC automotive devices it is best to build a beefy supply just to power those devices. For something like your battery charger i'd just build a unitized charger assembly / tool. I just think you are on the wrong path to achieve a do everything solution. Possibly more importantly do you give up on battery charging when doing other electronics.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 29, 2020, 04:24:29 pm
content: Designing and building power supplies is a quite good way of learning electronics.Start with the simple circuits such as the LM317 and L200 already mentioned.I had a look at octopart, L200 is not as popular / cheap as the LM317. It costs around USD3, but can still be bought.This datasheet has the opamp for the adjustable current limit shown on page 8:https://datasheet.octopart.com/L200CV-STMicroelectronics-datasheet-644473.pdfThen progress to a power supply designed with some opamps, a beefy (darlington) transistor as emitter follower on the output and some extra circuitry for an adjustable current limit.Possibly extend it to a symmetrical version which can deliver positive and negative voltages.Once you've managed to get such circuits to work stable and reliably you'll have learned a lot about the nitty gritty details of opamps, and you have a few usable power supplies.As a beginner, don't try to build a 30V 3A power supply. In practice adjustable power supplies upto 20V and a half to 1A are plenty for most purposes. If you want to build a power supply mainly for opamp like circuits, a few hundred mA is probably already enough.Power supplies are almost like multimeters.You can never have enough of them.

author: rstofer
author_rank: Super Contributor
timestamp: October 29, 2020, 05:54:42 pm
content: While researching DIY power supplies, pay attention to the number of projects that failed. The ubiquitous 30V 3A Triple Output supply is darn hard to engineer! Even subsets are difficult.I don't know the details but Dave's project went off the rails before it was complete.The problem is usually in the current limit loop from what I have read.Do I want to discourage you? Probably... You can buy battery chargers all over the Internet and there is no point in building such a thing. For projects, wall warts work fine as do Alkaline batteries. I bought a Rigol DP832 PS a while back but for the preceding 50 years I got by without such a thing.Lacking a general purpose supply worked for me because my interests are in digital and all I need is 5V or, more recently, 3.3V. I can easily build these supplies with 3 terminal regulators. It is also nice to have +-15V and perhaps +-12V.Most of the stuff I play with today is still powered by either USB or wall warts. I do use my bench supply when I bring up new uC projects just to limit the current in case there is an 'oopsie'.These Power Bricks can be pretty handy when current requirements are modest which is most of the time.https://store.digilentinc.com/powerbricks-breadboardable-dual-output-usb-power-supplies/If you want to build a PS, great! But start modest with 3 terminal regulators - even an LM317 adjustable regulator can create a decent supply.Make sure you know where the heat is going to be dissipated in a linear power supply. The problem isn't 30V at 3A, it's at 1V and 3A. More of the transformer voltage is being dissipated as heat. That's why commercial supplies have multiple secondary taps.You might find this video interestingThere are some follow on videos including one where a capacitor fractured and the board caught on fire. The design was changed to fix the problem.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: October 30, 2020, 03:44:10 pm
content: You better have this current limit for your kid.Are you trying to save 75 euro ?Look this thing is cheap : Peaktech 6225A, only my display was broken and they repaired the warranty away in china.Maybe you are more lucky then me, big chance because i already had the broken one, they not all broken i assume.

author: MG1
author_rank: Newbie
timestamp: October 31, 2020, 01:24:46 am
content: Thanks Whiz and all...Yes, Lipo's are quite scary when you read about their propensity to ignite. I almost talked myself out of getting back into RC cars when I read the number of incidents that the FAA have logged in the US. However, there's also a heck of a lot of them in everyday use that don't spontaneously combust. In fact as far as those used in RC are concerned, it's almost always down to user abuse due to mechanical damage, charging above 1C charge rate, charging to over voltage etc...basically folk looking to cut corners. The cells tend to give a warning of failure, changes in internal resistance, uneven discharge or the more obvious puffing and it's a little like the table saw stories that begin with 'I wanted to get it finished and I need to make one more cut...' it's the situations where we choose to ignore the evidence in front of us that puts us at risk. Never charge unattended, never charge a damaged or puffed battery and never up the amps because you're in a rush etc. I also personally avoid cheap no name units....but that may be unfair to some OEMs.I don't mind the expressions of concern though, because I take safety seriously and you don't know me. I'd much rather folk with experience tell me again to be safe and reign in my expectations than have my wife reply to forum posts telling folk here I blew myself up!!For what it's worth, I'm a bit of a nerd and have a spreadsheet that logs the family lipos etc...they have a label on each of them and I note down the charge times, balance delta, internal resistance of each cell along with any notes about physical condition.So hopefully you'll not worry too much about me charging these using a purpose bought DC charger that I'm feeding from the Server PSUs.As for the bench supply, I've been watching quite a bit of youtube!!Quote from: rstofer on October 29, 2020, 05:54:42 pmYes I saw these and a the fire repair...in fact it was the responsiveness of the designer that made me consider one of their smaller units as a possibility for including in the general purpose 24v box...I could have a variable unit for electronics and use chassis mounted XT60s for attaching my chargers.I started off watching Dave's built from scratch unit, but at one point, (maybe pt4?) he suddenly jumped into a waaay more complex design that left me floundering.Quote from: rstofer on October 29, 2020, 05:54:42 pmThose look neat. I can appreciate the simplicity of using those. But at $50 for the four it just feels like I could get a large box of bits and pieces to play with for the money!But I'm coming around to having a separate 12v supply and simply using the LM317 or similar to provide a variable voltage output with current limitation... I'm looking atI'm going to have a little time to try and read up over the weekend. I've found the educational material on the forum and am slowly processing stuff. I'll try and get back with some progress and hopefully some more realistic aims.I had fun with my son today making some basic lightbulb circuits, going over polarity and the rudimentary differences between series and parallel...obviously he just wants to build a burglar alarm!! Weird that when faced with a few basic components this seems to be a universal aspiration?!Anyway, hope that clarifies the LiPo worries...Have a great weekend

author: rstofer
author_rank: Super Contributor
timestamp: October 31, 2020, 10:20:03 pm
content: When you're reading up on the LM317, pay attention to the part where, ordinarily, the output can not reach down to 0V. You need to provide some kind of negative rail to get all the way to 0V.Getting to 0V may, or may not, matter to you.Here is one approach and it results in a pretty nice supply that can produce both + and - output. Just the thing for op amps circuits using dual rail amps. The output can be 0..60V or -30..30V.https://www.eleccircuit.com/0-60-volt-dc-variable-power-supply-using-lm317lm337/The PCB layout is given and it's just a single-sided PCB.There are many other similar projects but I could get interested in this one. It seems pretty handy.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 31, 2020, 10:45:57 pm
content: Personally I do not find it very important to go below the 1.25V reference outpt for an LM317 power supply. Not many circuits do anything significant at such low voltages, and adds components and reduces accuracy of the power supply.But for a symmetrical power supply you already have a negative voltage available which makes it an easy mod. For better (temperature) stability using an TL431 instead of 2 diodes in series would be better, and it does not cost much.A useful upgrade would be to add an opamp and a switch so the negative supply can track the positive supply.But sites like that have 50+ schematics of different power supplies. Just studying the schematics and analysing how they work can give you insight.It's probably well worth building a few of them on breadboards just to verify your thoughts on how they work and get some experience with building and debugging "stuff".

author: rstofer
author_rank: Super Contributor
timestamp: October 31, 2020, 11:17:58 pm
content: Of course, one could just buy a kit with a similar design (at least in the US, I don't know what's available in the UK):https://www.jameco.com/z/JE215-Jameco-Kitpro-Dual-Output-Adjustable-Linear-Regulated-Power-Supply-Kit_20626.htmlIt would save a lot of fooling around.I bought one of these for messing around with op amps at -15..+15VMostly, I use my Rigol DP832

author: MG1
author_rank: Newbie
timestamp: November 14, 2020, 03:52:32 pm
content: I must apologise for not getting back. Like is full of ups and downs at the moment and a whole load of stuff got in the way of me making any further progress until now.That said, I have been doing some reading and getting to grips with some fundamentals, although nothing too useful for PSU design yet!So what I have decided to do is to bit the bullet and get myself on of the smaller integrated units that will provide a variable bench supply suitable for my immediate needs and allow some experimentation and learning.I'm going to place a components order with Mouser for a selection of passives, ICs and regulators etc to build some of the learning circuits in the book 'Make: Electronics' which I picked up cheaply. And also get the bits for a CMOY headphone amp, something I'd intended to build ages ago but didnt.That said, whilst perusing other things, I did come up with this gem of a supply design which made me chuckle because it's not a million miles off what I was trying to achieve with my naïve cascading linear supplies.More details here:https://www.digikey.com/en/articles/combining-a-switching-converter-with-multiple-ldos-in-the-same-package-increases-design-optionsAnyway, I just wanted to say thanks again for all your input and let you know I'm on the case with a simple module build now, but will be delving deeper into powersupply design, both for a DIY lab supply and for audio projects.