Design solutions to build a bench supply - Page 1
Forum section: electronics / projects

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 21, 2013, 02:54:06 pm
content: Hello, i am thinking of making a desk psu for my next project which has two seperate rails (individualy voltage and current controled). I have a tonne of spare 7 segment displays so i would like to use them to display the current and voltage for each rail.My requirements are +-0-13.8v (and also be able to get 24v by briding over both rails) up to 5A (i know i probably cant get 5A on negative rails so i does not need to apply for the negative rail but more is better).I would like it to be a precision voltage source so accurate to 0.1V or 0.01V or higher.10 turn pots!Does anyone know of an existing design that mathes this spec? If not i shall begin designing

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 21, 2013, 04:32:12 pm
content: im thinking about using an lt1083 as the positive side regulator as it meets the regulation specification (except for under 1.25V) this is the 7.5A version so that i have a bit of over head on the current draw. i am probably going to use a multi tap transformer as well to reduce the heat dissipationim unsure about the negative side regulation so im going to have a look now

author: Jebnor
author_rank: Regular Contributor
timestamp: July 21, 2013, 04:48:26 pm
content: Quote from: carbon dude oxide on July 21, 2013, 02:54:06 pmI remember reading somewhere this advice to this effect: Make a supply capable of your Voltage/Current regulation specs at 100mA. Then make one at 1A. Then at 2A, then 5A, then 10A. Then you'll know what you're doing.Quote from: carbon dude oxide on July 21, 2013, 02:54:06 pmThere is no reason you can't get 5A on the negative rail. Most regulators have a -ve version. At worst, build 2 isolated power supplies and connect them in series and call the center pin Ground.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: July 21, 2013, 05:11:01 pm
content: The Lt1083 is a linear regulator. Yes, itcanprovide 7.5 amps but you have to consider something about linear regulators. The power dissipated by a linear regulator is equal to the voltage drop across the regulator times the output current. Lets say for easy math, you supply the regulator with 12 volts, and you set it for a 2 volt output. The voltage drop is 10 volts. Now connect your load that draws 7.5 amps.Power dissipated = (Vd) x I = 10 x 7.5 = 75 wattsThink about the heat put out by a 75 watt light bulb. You would need a huge heat sink with fans to get rid of that much heat. It can be done, but cooling isn't going to be simple.You might want to consider a switching design if you need that much current.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 22, 2013, 06:26:53 am
content: I know that the power disipation would be great, that is why i sugested a multi tap transformer to reduce the voltage gap between the output, but that still may not be enough :/ hmmmIl have a loo into the switch mode supply

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 22, 2013, 09:29:55 am
content: Do switch mode controlers allow a negative input voltage and output a negative regulated voltage?

author: IanJ
author_rank: Unknown
timestamp: July 22, 2013, 10:08:55 am
content: Out of interest, I'm building my own linear bench power supply around the LT1083 & LT1033 linear regs (T03) giving +/- 0-30vdc @ 5A. However, I'm paralleling them up and also pre-regulating the voltage using a couple of mosfets (T03).Yes, it's still a linear power supply so there will be some heat to dissapate whatever I do, but so far my bench testing is proving ok.http://www.ianjohnston.com/images/stories/IanJ/PowerSupply/BenchPsu.pdfhttp://www.ianjohnston.com/index.php?option=com_content&view=article&id=92Ian.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 22, 2013, 04:21:24 pm
content: Im jut thinking over here if i have 2 transformers would i have to put them in series or parrallel?

author: free_electron
author_rank: Super Contributor
timestamp: July 22, 2013, 10:49:35 pm
content: Quote from: carbon dude oxide on July 22, 2013, 04:21:24 pmon top of each other.

author: c4757p
author_rank: Super Contributor
timestamp: July 22, 2013, 11:17:23 pm
content: Quote from: carbon dude oxide on July 22, 2013, 09:29:55 amIn theory, yes. I built a 2A/15V negative in - negative out buck regulator once, but I didn't use an off-the-shelf controller. I'm not aware of any that can do that directly, though there probably are a couple.However, if you make two isolated positive power supplies (rather than using a center-tapped input), you can connect supplyA.out to supplyB.gnd, then your final positive output is supplyB.out, negative output is supplyA.gnd, and the junction between them is the ground output.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 23, 2013, 06:53:44 am
content: Quote from: free_electron on July 22, 2013, 10:49:35 pmSo parralel?Quote from: c4757p on July 22, 2013, 11:17:23 pmYes i know i could do that but for some of my future projects i might want to use two different voltages and one of them may need +- voltage so i was thinking having an option to dual track my supply.What i was also thinking is if i design my supply and then duplicate it in the same case i would be able to get the twin supply both capable of doing the same thing.I do have some requirement alterations, due to the fact that making a 5A supply is going to be a little bit more difficult i have decided to reduce the requirement to 3A. I would still like to have the +-0-13.8V supply with current control on all voltage ranges, im thinking of using a couple of LT1085 positive regulators in series to drop thw voltage down in 3 stages, each with a maximum voltge gap of 5V (power disipated would be 15W but a nice beefy heat sink should probably sort that) the 3rd stage would be the regulator that can go down to 0V but still have the 0.1% accuracy and low dropout at 1V or lower. The transformer/transformers would be. 15vrms (21.2v when rectified) for the higher end and maby a second transformer at 6vrms or lower to be switched, via relatly maby, straight onto the last regulator (so when i am doing low power stuff its not outputting in excess of 45W)I would like it controled via micro (preferably AVR) to control the voltage gaps on each regulator, and front display.The current control would need to be the first step after the transformer and smoothing caps to control the current initialy and then the voltage regulators afterwards, i belive dave did a blog about this someware.If this is an alright idea what negative equivilant is there to an lt1085 (3A pos regulator 0.1% tollerance) and what regulator should i use to get 3A but be able to go down to 0v?Side note i could use just standard lm337 (from memory) regulators for the first two stages and only the last regulator would need to output at 0.1%, that would save a bit of money and remove the lm1075's which may be redundant with there accuracy?

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 23, 2013, 07:01:33 pm
content: Ok, i have a design going on, which involved a multi tap transformer, so to switch between the taps i was thinking a relay, but im going through several variants.One is a transistor solution now i have a high side switch for the positive rail but i am having trouble with the negative rail. Here is my schematic at the moment (yes i know its just a reversed copy of the top side but i just drew something out)

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 23, 2013, 07:04:52 pm
content: Ok this is how i think it should beam i correct?

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 23, 2013, 08:25:03 pm
content: also if i am going to use a current sense resistor how would i do it? if i just use a single resistor it would have to dissipate 42W. would i have to parallel 5 10W resistors or is there another way of doing current sensing?

author: mariush
author_rank: Super Contributor
timestamp: July 23, 2013, 09:26:57 pm
content: You can use <1 ohm current sense resistors and amplify the voltage drop using opampsHave a look at how some power supplies are made by checking their service manuals.... you'll easily find some HP service manuals out there, for example.Here's a few schematics I have on my drive right now

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 23, 2013, 09:51:18 pm
content: ok i think i might be calculating something wrongto calculate the power across that sense resistor it would be voltage drop * current. so for a 1 ohm resistor it would be 3 volt drop for 3A so max 9W dissipation correct?

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: July 24, 2013, 05:35:20 am
content: Your calculation is right, but maybe your thought process is a little lacking.You want to use the lowest value resistor possible that gives you a measurable voltage drop. A 3 volt drop would be great if you're trying to read it with the micro's ADC, but this isn't so good for the load.Look at a chip like the TI TMP512. It's designed as a power monitor and has an ADC with a full scale range of 40 – 320 mV specifically for measuring voltage across a shunt resistor. Select a shunt resistor that yields 320 mV at the maximum design current....and switch to the more sensitive ranges for lower currents.Note: The TMP512 can only handle 26 volts max, but you'll get the idea. I believe Microchip has some power monitors that go to a higher voltage.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 24, 2013, 07:47:05 am
content: For that one in particular is 0.106666 ohms so a 0.1ohm load should be fine right? That was when its drawing a ma of 3 amps i would only need to disipate 1W of heat, which would be fine over 10 1 ohm 1/4w resistorsok to get as close to the actual value i can either: have 9 x 1 ohm resistors and 1 x 2.7 ohm resistor in parallel or 3 x 1 ohm resistors and 7 x 1.1 ohm resistors. all have 1% tollerance and they both give 0.106 very closely which one would be better to use?also, i was looking through some of Dave's videos and he used a INA219 for this purpose, woiuld this be a better solution as i dont need to read thermo couples. if so is it also a 40-320mv range?

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 24, 2013, 07:16:00 pm
content: Ok i have a design questionWould it be better to have:1,multi tap transformers with different voltages each with a centre tap (or multiple single transformers with centre taps) then switch between taps with a single regulator.2 have a single center tap transformer and then have a pre regulator (switch mode) and then into a linear regulator, but this needs to be done with both positive and negative rails.3, cascade linear regulators to get the desired output voltage from a single centre tap transformer.4, another way which i cant think of currently and will no doubt remember later

author: IanJ
author_rank: Unknown
timestamp: July 24, 2013, 09:58:45 pm
content: Quote from: carbon dude oxide on July 24, 2013, 07:16:00 pmAll depends on what quality of design you are looking for, what budget you have and what it's target use will be. For instance, my own design uses a single mosfet, zener diode and resistor (per rail) for the pre-regulator (5A).My advice to you is to draw up the various designs and see what sort of component count, complexity and cost you end up with......and from there you can make your final decision. Don't forget also, you can breadboard test the various designs as you draw them up. You WILL need to revise your design from the process of breadboarding. That's electronics!PS. You can always post your designs here, there's plenty good guys here that will help you.Ian.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 24, 2013, 10:03:50 pm
content: its use will be small/large AVR projects bith 5V and 3.3V also some projects with a 12V input for nixie tubes with the avr running (hence dual supply) occasionally to charge a battery.i have been drawing up some shcematics for each of these senarios but i am unsure with thw pre regulation one. for the negative rails.

author: IanJ
author_rank: Unknown
timestamp: July 24, 2013, 10:09:19 pm
content: Quote from: carbon dude oxide on July 24, 2013, 10:03:50 pmLook at the schematic I posted on page 1 and you'll see how I did it with the mosfet etc for the pos & neg rails......and also the current sense for that matter. Many ways to skin a cat, that's just the way I did it.Ian.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 24, 2013, 10:17:37 pm
content: i dont think i understand how your schematic regulates XD i might be missing somehting.is it using an op amp to compair the output voltage to a reffereance set by the pot and diode then the op amp does it's thing and turns the mofet on and off like a linear regulator (ish) to regulate the output voltage?

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 24, 2013, 10:41:04 pm
content: ok i can also see that from that adjustment pot it outputs an analogue reference which then goes into u9b op amp which presumably inverts the analogue reference from positive to negative which then does the same thing as the positive side and regulates using....wait just realised those op amps are for creating a reference voltage to control some linear regulators that have been paralleled. (sorry having a little trouble with the schematic) bt a, i correct in saying this?edit: woo 300th posthowever i don't understand what the large Ptype and Ntype transistors are doing before the linear regulators, they have a feedback from the output voltage so it has something to do with the regulation,does it turn on and off the power going into it to act like a regulator?. i also noticed the adjustable current source afterwards which i am assuming is to keep the regulators stable with its minimum load requirement.

author: IanJ
author_rank: Unknown
timestamp: July 24, 2013, 11:18:28 pm
content: Using the zener diode a voltage is created at the gate of the mosfet which is a set amount (voltage) above the output of the regulator (irrespective of what voltage the output is set to). This means there is always a fixed voltage across the main regulator.Take two examples for the positive rail as an example:-unreg V+ = 36vdcinput to main reg = 17vdcoutput of main reg (as set) = 5vdcunreg V+ = 36vdcinput to main reg = 24vdcoutput of main reg (as set) = 12vdcThe 12v differential in both comes from the 9v zener voltage plus the mosfets Vgs(th). I've specified a 9v zener, however, this can be varied and a nominal value decided based on testing across all voltages, load conditions, and of course what the main regulator chosen wants in terms of drop out voltage.PS. How robust my circuit is i've yet to see.......I don't often get to run mosfets in their linear region.......so far so good though!The current source gives the main regulator a minimum load.The op-amps are all centred around controlling the ADJ pin on the main regulators....i.e. output voltage and constant current setting.See Dave's videos for his power supply....starting with the very first one for much more Pro info!......you'll get a lot of questions answered there.Ian.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 25, 2013, 06:17:25 am
content: Ok, i have not really used zenner diodes before so i dont know exsactly what they do. I hve had a read up on them so they act as a normal diode untill the breakdown voltage where they then allow current to flow the oposite direction they are manufactured to have a set breakdown voltage ao can be used to clap voltages, basic surge protection (over voltage stuff) and can be used to create a regulator (i belive)The one you have in B1 on your schematic im taking a guess that it clamps the 30v down to 2.5v?

author: IanJ
author_rank: Unknown
timestamp: July 25, 2013, 07:39:18 am
content: Quote from: carbon dude oxide on July 25, 2013, 06:17:25 amhttp://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=snvs749dGoogle is your friend.Ian.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 25, 2013, 09:01:22 am
content: i didnt think to google the part numberi googled about zenners thoughin your 2.5V reference its shorting it to ground when its over 2.5V so there is the 15k to limit the current to 2ma, and that is enough to be a voltage reference to the op amps as they don't draw much current at all on the inputs. and because its drawing such a little current its not going to be producing massive amounts of heat even though it drops the regulation 27.5V.that is a nice way of doing ithow stable is it?also i still don't understand how the transistors work before the regulators to pre regulate the voltage

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 26, 2013, 02:30:55 pm
content: Could you explain how rhe transistor pre regulates the voltage please

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: July 28, 2013, 08:09:42 am
content: What is the step recovery diode in A3 used for?http://www.ianjohnston.com/images/stories/IanJ/PowerSupply/BenchPsu.pdf

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: August 19, 2013, 05:22:25 pm
content: hello, i have a been busy so ahve not been able to post for a while, either way i have started to go through the initial input and rectification. i do ahve a few questions about it aswell.this is the initial mains input which goes through a small smoothing cap and fuse, then passed some mov protection. I am unsure which movs t use for this but it is on uk mains voltage (230V). it then goes into a common mode choke to filterout some high frequency noise, then onto the transformer.it is a multi tap transfromer which can output 3.5A on all 3 voltage pairs (6,9 and 12V RMS). each voltage tap goes into a relay which only 1 is connected at a time which is controlled by an AVR, i am unsure if this is the best way to switch between taps, if its not please let me know.the selected voltage output is then fed into a bridge rectifier with smoothing caps. there are also some suppression caps on the rectifier but i am unsure what value to have them at.is this aright so far for a linear supply so that i can keep the heat dissipation as low as possible by switching tap? i will also be making a switch mode supply design which tracks a few volts above a linear regulator.which way would be better?