High Voltage High Current Constant Current Power Supply - Page 1
Forum section: electronics / beginners

author: gogicha
author_rank: Contributor
timestamp: November 09, 2023, 02:32:10 pm
content: Hello!I'm trying to DIY a power supply that will provide 10 to 250 mA of constant current to the load that significantly changes it's resistance over time (10%, 20%, maybe even more). My mains is 220V 50Hz, which gives me ~310V after rectification, so ideally it should be able reach a max power of 300 * 0.25 = 75W, driving a load of 1.2k, but if it'll make things easier, cheaper or more precise, I'm okay with 250 * 0.2 = 50W, driving circa the same load. I've been searching for info for a couple of weeks now but everything that I find is very interesting, some with 0.1% or better precision, but alas much lower voltage (<40V), using LM317, LT3080 and the likes. So if anyone can give me any leads or point me to the right direction, I'll greatly appreciate it!

author: CaptDon
author_rank: Super Contributor
timestamp: November 09, 2023, 03:07:43 pm
content: Give us some more information about the load? What is the expected maximum voltage you will ever need? Does the load need to be isolated from the mains voltage? Do you want a 'one knob' current control solution? I had some old Sorenson DCR-600 supplies that could do up to 600vdc and up to 1 amp and had both voltage and current control. Why does the load vary so much?

author: gogicha
author_rank: Contributor
timestamp: November 09, 2023, 05:31:58 pm
content: The load is two electrodes hooked up to the solution, so basically just a resistor, but there's some electrolysis going on, so the resistance increases due to the loss of ions. Also, the current flow is causing it to heat up so the resistance varies with that as well. At some point I might choose to put it in the ice bath and that will also affect the resistance as well. So yeah, the resistance is kind of all over the place.I'm not sure if it needs to be isolated from mains. Why would it? Maximum voltage is right about that 300V mark. I really want to avoid using transformers.As per current control, I want whatever's more precise. AFAIK that's precision pot, but they are very expensive, so maybe I'll have a bunch of trimmer resistors calibrated for 10, 20, ... 250 mA and jumpers to hook up whichever one I'll be needing.

author: jonpaul
author_rank: Super Contributor
timestamp: November 09, 2023, 05:43:40 pm
content: 310V is NOT "high voltage"Electrolysis = rectification310V 25 ma = potentially lethal.310V + water/liquid = VERY DANGEROUSSuggestUse isolation transformer (~ $20..100)ORGet health/life insuranceORgive it upj

author: gogicha
author_rank: Contributor
timestamp: November 09, 2023, 06:28:49 pm
content: How would isolation transformer help?I'll wear safety gloves, pinky promisebtw, will ordinary latex/nitrile gloves suffice or are heavier ones needed?

author: CaptDon
author_rank: Super Contributor
timestamp: November 09, 2023, 09:54:50 pm
content: Yup, these are the kind of questions that scare the shit out of experienced technicians / engineers. Without the isolation transformer your 300VDC is referenced to earth ground insuring that if you are somehow also earth grounded and come into a liquid contact with your project it could well kill you. "How would an isolation transformer help", if you don't know that answer already then buy life insurance and finish documenting your will and next of kin. Do you really want constant current, you probably also need to set a maximum available open circuit no load voltage although maybe you solved that with rectifying the 240 mains (AFTER A 1:1 ISOLATION TRANSFORMER). Are 'regular' nitrile / latex gloves adequate?? Probably not since you are working with liquids which will penetrate the tiniest of holes and combine with your sweat to give a low resistance path to your fatal outcome. Case in point, I have some butyl rubber gloves around 20 mils thick made by Brunswick for the military marked as "Glove Set, Chemical Mil-G-43976B" which have printed right on them "NOT FOR ELECTRICAL WORK OR FIRE FIGHTING" and they are probably 4 times thicker than 'off the shelf' latex or nitrile gloves. 300 volts and liquids require some quality gloves. You can do current regulation with conventional circuits although your pass transistor should probably be rated to perhaps 450VDC to avoid any chance of failure and it would probably be handy to have an analog voltmeter and analog current meter monitoring your load parameters. You will probably also need a smaller transistor also rated to about 450VDC as a driver for the base of the pass transistor. You may find a ready made power supply meeting your needs on Ebay and such.

author: johansen
author_rank: Super Contributor
timestamp: November 09, 2023, 11:30:42 pm
content: Yeah nothing about this suggest precision is needed.Variac and a rectifier and capacitor is all you need.

author: gogicha
author_rank: Contributor
timestamp: November 10, 2023, 12:44:12 am
content: I'm thinking of hooking up lm317 to the rectifier output, with output and adjust pins connected via 250 ohm pot, which will provide variable constant current. If I set the pot to 12.5 ohms, so the output is 100mA, then add a resistor in series with the load to make it ~2.7k ohms, the voltage across in and out pins of lm317 will be within the limits. Maybe there's a way to limit the voltage across those pins with an op-amp or something?I did at one point think of using a transformer with two outputs, one 300V and the other 5-30V to drive some microcontrollers or DACs, or other control circuitry, but I just couldn't get a grip of the transformer market. These are all the isolation transformers I can find and I would rather put that money into precision components, if of course they weren't totally wrecking the budget in the first place.https://www.mouser.com/c/power/transformers/?primary%20voltage%20rating=240%20VAC&product%20type=Isolation%20TransformersYeah, variacs sound super cool, but same issue:https://uk.rs-online.com/web/c/power-supplies-transformers/transformers/variacs/?pn=3I don't think DIYing a transformer is a good idea, but I might be wrong.

author: gogicha
author_rank: Contributor
timestamp: November 10, 2023, 12:52:57 am
content: Quote from: johansen on November 09, 2023, 11:30:42 pmI don't *need* precision, I *want* it.

author: johansen
author_rank: Super Contributor
timestamp: November 10, 2023, 01:06:22 am
content: A series string of lm 317's each with their own trim pot, and 30 volt protective zener, would limit their power dissipation.At the expense of a short circuit would blow them all up. But if you had 10 of them in series you could block 300 volts and still have a mostly constant current source.

author: gogicha
author_rank: Contributor
timestamp: November 10, 2023, 01:19:51 am
content: Quote from: johansen on November 10, 2023, 01:06:22 amThat's brilliant! I tried connecting 10 of them in parallel using a voltage divider but then stumbled into the now obvious issue of output pins getting connected to the input pins on the same regulators. It didn't occur to me that I could connect them in series, but that doesn't seem to be very adjustable. I am looking into floating lm317 right now and connecting a bunch in series basically makes them float, no?lm317hv you'd only need 6, since it's input can go up to 60V.

author: CaptDon
author_rank: Super Contributor
timestamp: November 10, 2023, 02:34:41 pm
content: An isolation transformer for safety and a small perhaps 24vac output transformer for the control circuit would allow even interfacing to a computer or serial data link and such. Sense the return current on the negative probe circuit, use your LM317 or what ever and do the control with a pass transistor rated at 450vdc on the high side positive rail. That transistor will at worst have to dissapate to full high voltage at your maximum current setpoint in the event of shorted probes. Your guess at around 75 watts seems reasonable. You will also need a 450vdc rated transistor as the go-between from your current sensing regulator on the negative rail to reach up to and control the pass transistor on the positive rail. Putting those regulators in a series string is among the galactically stupid ideas. Dressen Barnes, HP, Sorenson and a ton of other companies make off-the-shelf power supplies that will do exactly what you want to do including safety isolation. They can be had on the surplus market cheaply enough and by the time you waste your time with unsafe circuits, blown up componets, circuits that don't perform to your expectations and piss poor information and ideas of how to wire your project you would be light years ahead to just buy something. The O.P. seems pretty clueless about electronics to begin with. Seems like a disaster looking for a place to land. Be aware that with the probes out of the solution the probe voltage would go to the maximum so you should have the probes immersed FIRST before applying power so the circuit can ramp to your present value. Did anyone mention that? Perhaps a 'reach-around' circuit on your current controller to force a ramp up instead of a ramp down to desired current would be a good idea? Little things like that which no one ever mentions when offering advice to novice experimenters playing with lethal toys.

author: gogicha
author_rank: Contributor
timestamp: November 10, 2023, 07:46:18 pm
content: Quote from: CaptDon on November 10, 2023, 02:34:41 pmYeah, at first I just wanted to buy a nice power supply, but after glancing at some thousand dollar price tags, a good ol' FULL BRIDGE RECTIFIER came to mind and I thought to myself, "Hey! I just need a couple of diodes and a cap!", which is fine for a simple electrolysis experiment, but I would like more fine-grained control over some processes and for that more precise and constant current is needed. I've been looking into it for a while now and I just don't get it. I guess achieving constant current is not as simple as hooking up a couple of components and calling it a day, like it is in the case of a rectifier. I still would like to know the physics and inner workings behind those low noise, precise, adjustable constant current commercial power supplies, but I guess I will never be able to DIY one as good as or better, so I'll just stick to doing what I know best.Any recommendations on purchasing power supplies? RK-80H over herehttps://www.matsusada.com/product/dc-power-supplies/benchtop/rk80/caught my eye with it's 320V and 500mA rating with 1mA ripple, also super cute form factor. This PMX250-0.25A is also okay I guesshttps://telonic.co.uk/product/kikusui-pmx250-0-25a-dc-power-supply-0-250v-0-0-25a/but the accuracy is less clear, although the FREE CALIBRATION TRACEABLE TO NIST sticker is alluring. Is there anything on the forum I can read regarding choosing a power supply?

author: Manul
author_rank: Super Contributor
timestamp: November 10, 2023, 08:46:59 pm
content: Well, should not be *very* hard. First simple idea which comes to my mind is this:1. Get two transformers, lets say 230VAC to 12VAC;2. First transformer will provide isolation and give 12VAC which is useful to power low voltage circuitry, like an OpAmp;3. Connect second transformer in step-up to the first transformer output (12VAC), so at the output it gives you back 230VAC, which will be your high voltage isolated supply;4. Now take a powerful high voltage transistor on a good heatsink with fan, use current sense resistor, scale the feedback voltage to get a range and adjustment you need and design some control circuitry with an OpAmp to drive a pass transistor.A big plus is that it provides galvanic isolation from mains, so it's a bit safer then direct rectification, but still not something to be taken lightly, so take care. Another plus is that you can change compliance voltage relatively easily, for example, if you replace a second transformer with 24VAC seondary transformer, you would get around 115VAC at it's output and so on. Because maybe you don't need that 300VDC, maybe 150VDC is enough and it will save you power dissipation in the pass transistor.

author: nachus001
author_rank: Contributor
timestamp: November 10, 2023, 10:37:49 pm
content: Heythis last suggestion was good, with two transformers you'll ged good isolation, and additionally you can rectify the 12VAC connection in order to get low voltage for the control circuitryThe control circuitry can be a low side mosfet current sink like thisYou need to calculate the R4, R13, R9 with values depending on the max current, and the mosfet should be one for 400/600V operating voltagecheersNachus

author: gogicha
author_rank: Contributor
timestamp: November 10, 2023, 11:08:19 pm
content: Quote from: nachus001 on November 10, 2023, 10:37:49 pmHow does one obtain the ISET? Can it be withAD766JNZDAC? And how do you calculate the accuracy and load regulation of that circuit?

author: nachus001
author_rank: Contributor
timestamp: November 11, 2023, 01:12:23 am
content: Hello GogichaIset is a voltage that depends on the value of R4. VR4 will be ILoad X R4, so you'll have to design R4 to fit the current that you need in the loadfor a current of 25mA, an R4=1ohm resistor is more than enough. In that case, 25mA current on the load will be 25mV sensed on R4, so tocommand the circuit to produce a ILoad of 25mA, you'll need 25mv at the positive opamp terminal.With that in mind, (in example) if your d/a converter produces 0-5V output, we may assume 0-25ma range, then R13-R9 can be dimensioned R9= 10R and R13=1K99 (possibly a resistor plus trimmer for max adj, or an 1K90 resistor if you don't mind a couple of mA more full scale)Now, in regards of the circuit regulation, well, the loop is closed on the pass transistor, so it mostly depends on the opamp temperature drift and offsets.I've used that circuit for many applications in the past, it worked like a charm, and i've specified an LM358 as amplifier, but you can use any opamp with better characteristics as far as it has low output impedance and be designed to work stably at gains around 1 and lowercheersNachus

author: JohanH
author_rank: Frequent Contributor
timestamp: November 11, 2023, 10:29:53 am
content: If you want really good regulation, take a look at this design:https://linearaudio.nl/t-reg-hv-regulatorhttps://diyaudiostore.com/collections/kits/products/linear-audio-t-reghttps://www.diyaudio.com/community/threads/jans-hv-regulator.344141/

author: Kleinstein
author_rank: Super Contributor
timestamp: November 11, 2023, 02:14:03 pm
content: A first point would be to check if such a high voltage is really needed. Usually electrolysis is working with a rather low voltage, rarely more than some 4 V per cell. It would mainly be with multiple cells in series where one would really need that much voltage.The general setup could be a like a raw power supply of what ever voltage is needed at most and then a constant current load circuit to regulate the current. So with something like relatively large MOSFETs as power devices and some OP-amp or so for the control. For low accuracy a depletion mode FET may work.Beginner + high voltage is not a good combination. AT least for the start use some isolated 12 V or 30 V. Chances are one would not need more anyway.

author: gogicha
author_rank: Contributor
timestamp: November 12, 2023, 12:22:13 pm
content: Why not just hook up PWM controller to the rectified and cap-smoothed output of a transformer, then measure current using sense resistor likeY169010R0000T9Lby measuring voltage across it using some DAC that's ~0.001% accurate, check the result with a microcontroller and adjust the pulse width accordingly to get output current as accurate as the sense resistor, in this case 0.01%?

author: Kleinstein
author_rank: Super Contributor
timestamp: November 12, 2023, 12:39:04 pm
content: I very much doubt one would need high precision. If one has money for very high precsion parts, than would first spend some on a transformer and avoid the really dagerous directly rectified mains.Using PWM and RC smoothing still need a fast switch (e.g. MOSFET) and the waste heat would be released with the resistor. A µC is rather slow to regulate and may miss faster changes in the load.Anyway the first thing would be to know the load a bit better and what voltage is actually needed. Thing get a low easier and safer if one can start with a low voltage like 24 V or 12 V.

author: gogicha
author_rank: Contributor
timestamp: November 12, 2023, 01:02:08 pm
content: One has worked with the load using commercial power supplies routinely and has an understanding of voltage, current and power requirements. Which are typically 0-300V, 0-300mA, 100W.One day said one watched a teardown of said typical commercial power supply and was astonished by how noisy and inaccurate it was, as well as how little seemingly went into its construction / components.One got obsessed by the idea of purchasing or DIYing a power supply that is less noisy and more accurate in hopes of performing the routine procedure more efficiently. Maybe even throw in an arduino / rapberry pi into the DIY version and control the apparatus from the comfort of a python programming language.After googling day and night, scouring various forums and stack exchanges, reading data sheets for various models from various manufacturers one got extremely frustrated with the apparent lack of off the shelf, easy to understand products that can help one in the above quest. As well as rampant tolerances and accuracies of components in the single and double digit percentages above zero, which one finds unacceptable and is starting to doubt accuracy of every measurement ever made using products that utilize said parts.

author: Manul
author_rank: Super Contributor
timestamp: November 12, 2023, 01:06:54 pm
content: Quote from: gogicha on November 12, 2023, 12:22:13 pmSomething of that sort what was suggested in Reply #14 will give you better precision. PWM makes it difficult to measure current, you need smoothing, smothing introduces delays, and there are many other problems, really not the tool for this job. Properly designed analog control loop will give you very good results. Take current sense resistor as high value as possible, good OpAmp (stable offset), and stable voltage reference. That's all you need.

author: armandine2
author_rank: Frequent Contributor
timestamp: November 12, 2023, 01:52:40 pm
content: The Kikusui PMX-A range has a 0-250V 0-0.25A and 0-350V 0-0.2A options

author: gogicha
author_rank: Contributor
timestamp: November 12, 2023, 02:28:58 pm
content: Quote from: armandine2 on November 12, 2023, 01:52:40 pmIt also has ±5mA load regulation, 1mA ripple and ± (0.3 % of setting +0.1 % of rating) setting accuracy.Correct me if I'm wrong, but as I understand, if I set PMX250-0.25 to 100.0mA, with precision resistor as a load, I'll be getting ± (100 * 0.003 + 250 * 0.001), meaning anywhere from 99.45 to 100.55 mA, with ripple included that's anywhere from 98.45 to 101.55. My load is not a precision resistor, but an ionic solution, which is unfortunately undergoing electrolysis and is getting heat up/cooled down, so the resistance is fluctuating, with a rising trend. When it's resistance changes I am to expect up to 5mA change in current, so the accuracy plummets and the output is now in the 93.45 - 106.55 mA range, which I'm not a huge fan of.I further speculate that the provision of both constant current and constant voltage capabilities in commercial power supplies may contribute to a compromise in accuracy.I hope my calculations are wrong and I can just buy the damn thing for like a thousand bucks and get a constant 100mA ± 0.1mA over the course of an hour or two, during which the load resistance is increasing by several tens of percent.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 12, 2023, 03:13:01 pm
content: I just rebuilt a variable laboratory grade linear Lambda PS. I changed the design to use just op amps for control. To design a good lab grade variable PS is very hard. The ones I have seen have two rectified voltages as others have pointed out, and the pos of one supply is linked to the neg of the other supply, making design considerations complicated. You also need a very reliable very stable voltage reference.To design a constant voltage HV PS with minimal adjusts and current limiting is not that hard.What do you wish to build?A continuously Variable Supply say 0- 300 Volts?A PS that is just 300 Volts? Minimally adjustable.A laboratory grade linear PS with Clean output?Or a SMPS with the necessary filtering etc....What is the tolerance of the output can it vary over time one millivolt or ten?How much ripple are you willing to have?The current limiting circuit by Nachus is good and I used this type of circuit in my redesign. This mosfet control is put into my circuit before the final output transistors, so you do not need a big mosfet. Works very well.My Current Sense Resistor is .166 ohms (166 milliohms). You can also use the voltage across the voltage across this resistor to give you a current meter reading.Here are some suggestions and observations:Here is a variable HV PS.https://www.manualslib.com/manual/1043623/Bk-Precision-1602.htmlIf you find the schematics, it may help you. It is an old linear design. It was made for designers of electron tube devices. Big volts and big power.I really do not know what your specific application is. There are electrophoresis PS available used on ebay that may meet your needs. High volts and high power. Relatively inexpensive. I do not know how clean the output is.You are quite correct about industrial PS being noisy, especially the switching ones. They are made for an industrial environment and even if the output was clean everything else in the factory would make interference anyway, so they are not made for a really clean output.

author: armandine2
author_rank: Frequent Contributor
timestamp: November 12, 2023, 03:48:35 pm
content: Quote from: gogicha on November 12, 2023, 02:28:58 pmmaybe, I don't know. The operation of the units is described here:https://telonic.co.uk/jg/wp-content/uploads/2021/05/Kikusui-PMX-UserManual.pdf

author: gogicha
author_rank: Contributor
timestamp: November 12, 2023, 04:57:32 pm
content: Quote from: Wallace Gasiewicz on November 12, 2023, 03:13:01 pmI'm not looking for constant voltage on the output, quite the contrary, voltage must vary because the load resistance is going to, while the current should stay constant. What I do am looking for however, is current that as closely matches the setting as possible and has as low of a noise / ripple as possible (so no SMPS). Adjustment constraints are also fine. I don't need to go all the way down to 0. I can make do with, say 75 - 125 mA or any range of several tens of milliamps, as long as swapping out / switching between or resoldering a couple of components is possible to change the range to 25-75 or 125-175, etc. I mention voltage rating, because afaik it's impossible to achieve 100mA over 3k load with anything less than 300V, although specifying power rating would've been more appropriate.I'm getting mixed signals regarding the difficulty of this project. The current limiting circuit by Nachus doesn't seem very complicated to build and source precision components for, while "To design a good lab grade variable PS is very hard."As I understand the current regulation in Nachus's circuit is primarily dependent on the tolerance of R4 and the accuracy of ISET / DAC. I don't quite understand the roles of R6, R7 and C2 in the circuit, nor how to select op-amp and mosfet for most precision.Electrophoresis is what I'm going to use it for and after watching teardowns of couple of commercial electrophoresis power supplies (seethis threadfor example) and reading data sheets for various models I concluded that most of them are noisy and inaccurate. I'm wondering if using accurate, low noise power supply will result in better band resolution for single nucleotide differentials.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 12, 2023, 05:34:35 pm
content: Heres a classic. The precsion depends on the three scaling resistors and the Vos+drift of the opamps (plus a bit of leakage from Q2).U2 needs a CMV range that includes Vcc or near to it. Ideally U1 can accomdate a CMV close to Vee.A precision, or to your taste and budget, RIRO is ideal. With the values shown, the complance limit goes to about 9V short of the HT supply rail.

author: gogicha
author_rank: Contributor
timestamp: November 12, 2023, 07:26:50 pm
content: Quote from: Terry Bites on November 12, 2023, 05:34:35 pmThis looks awful lot more complicated than Nachus's circuit. How do I tell the advantages / disadvantages of each?I'm taking baby steps in learning to read circuits. There'sthis documentfrom TI which describes Nachus's circuit using some easy to understand formulas (except the fourth one, it's a bit harder to grasp). Although it too doesn't mention anything about the roles of R(ISO) R(FB) and C(F).According toDAC1120Edata sheetit can output up to 2*Vref, so if used in conjunction withREF102CUcan output up to 20V.By formula 1 of the above document, Rset = Vdac,max / Iout,max. I am willing to have Iout,max = 200mA, so that Rset = 100and I can useY1453100R000V9L.By formula 2, Rload = (Vcc - Iset,max * Rset) / Iset,max. Vcc is going to be rectified 220-240AC, so ~300V conservatively. Hence Rload = (300 - 0.2 * 100) / 0.2 = 1400, which is fine.Minimum resistance of load is going to be around 400ohms.By formula 3, Pdiss = Vcc * Iset,max - Iset,max ^ 2 * (Rload,min + Rset) = 300 * 0.2 - 0.2 ^ 2 * (400 + 100) = 40W, soIRF820PBFshould suffice.I've no idea how to obtain DAC TUE, so I just calculate RSS of "linearity error", "unipolar offset error and gain error", "bipolar zero offset error" and "gain error", while having no idea what half of those terms mean, obtaining sqrt(0.000_015 ^ 2 + 0.000_060 ^ 2 + 0.000_015 ^ 2 + 0.000_150 ^ 2) = ~162 ppm, let's round that up to 0.02%.Accuracy of Vref is 0.025% per data sheet.By formula 4, using the Vos of the same op-amp, output TUE(%FSR) = sqrt(TUEdac ^ 2 + (Vos,amplifier / FSR * 100) ^ 2 + Tolrset ^ 2 + (Accuracy of Vref) ^ 2) = sqrt(0.02 ^ 2 + (0.3/20_000 * 100) ^ 2 + 0.005 ^ 2 + 0.025 ^ 2) = ~0.033%, which is < 0.05%.Assuming my reasoning and calculations are correct, if I set DAC to output 10V, I'm going to get 100mA with 0.05% accuracy and finally be happy.Please correct me if I'm wrong. I feel like I am, but can't quite put a finger to where exactly.Additionally, Vcc is not a stable 300V, it's a bit more, but with several volts worth of ripple as a result of rectification. What effect is that going have on the circuit?

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 12, 2023, 08:33:18 pm
content: It sounded like you were looking for an electrophoresis unit.I worked in a research immunology laboratory years ago and we purposefully did not use high volts. As I recall lower voltages were used.Buffer medium was made by us. Agar gel was made by us. Electrophoresis was done overnight in a sealed bath unit.Lower voltages and longer times made the resulting protein lines much more stable and even. We also did various diffusion tests that were not electrophoresis.Besides that we were afraid the high volts would tear apart some proteins.Modern electrophoresis units are made to give results with their kits of medium in a shorter period of time, but they are looking only for a few better defined entities. We were looking for anything new.Back to the supply:If you want to control the current by using the system of measuring the voltage difference of the output current sense resistor, where do you plan to put the control mosFET? Before or after the output transistors? I put my mosFET in three stages of amplification before the output. At this point the current thru the mosFET is not high so heating of this component is minimal. The op amp should be relatively unaffected by heating because it just has to supply the very high impedance of the mosFET, very little power.The Measuring current resistor must have excellent characteristics in the temp range it will endure. ALL the current goes thru it. Lower ohm values result in less heating and make resistor selection easier. Again my current sense resistor is 166 milliohms. Not much power dissipation and hence very little heating. The exact ohm resistance of the current sense is not that important, what matters is it's temp stability.If you are going to use the dual supply, one for output and one for "control" which I think you have to do because the op amps require a stable smaller voltage...I think the real difficulty in design is referencing your control voltage to the output voltage. Voltage is a potential difference, if you start with two separate supplies, you must somehow link them without changing the voltages in the lines. You must also somehow be able to use a reference stable voltage to compare these current control voltages to.Let me change one thing I said to this: Designing a variable voltage, variable current laboratory grade supply is hard. By this I mean a supply that will change voltages from 0 to whatever your high limit is and limit the current from 0 to whatever, at the same time.About your ripple, a pi capacitor resistor capacitor network would help that and if you have a really big choke. a capacitor choke capacitor network is better. There are formulas for these things. I do not recall them however.

author: gogicha
author_rank: Contributor
timestamp: November 13, 2023, 12:25:37 am
content: AS-1T250transformer has two 250V and two 6.3V windings. If we connect first two in center-tapped configuration and second two in a way that gives 12.6V, rectify 250V center-tap with two diodes and 12.6V with a bridge, we should get ~350VDC@0.4A max and ~17VDC@3A max outputs respectively. Smaller output will drive the voltage reference, DAC, op-amp, ADC reading the voltage across the sense resistor and microcontroller that adjusts DAC according to said readings. Larger output will drive the load.Quote from: Wallace Gasiewicz on November 12, 2023, 08:33:18 pmWhy not just tie the grounds together?Quote from: Wallace Gasiewicz on November 12, 2023, 08:33:18 pmSo those ripple specs in data sheets are the result of poor filtering of rectifier output(s)? Does the control circuit also require significant filtering or can higher ripple be tolerated there? As in it won't translate to ripple across the load.Quote from: Wallace Gasiewicz on November 12, 2023, 08:33:18 pmI just followed the formulas given in the TI document to arrive at the number. Also, higher resistance resistors might generate more power, but they are also more readily available with tighter tolerances, for example, the one I linked has a tolerance of 0.005% and a temperature coefficient 0.2ppm/C. It's wattage of 0.6W also aligns well with the requirement of max 200mA, I ^ 2 * R = 0.2 ^ 2 * 100 = 0.4W.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 13, 2023, 12:44:40 pm
content: About linking the neg of the lower PS to the pos of the higher supply------I did it because all of the schematics do it this way, I think their reason is that the current sensor voltage that would appear on the op amp would be the whole output voltage which is too much.About filtering: The regulator on the smaller voltage line should get rid of ripple.About the sensing resistor: I have seen a lot of different values. I used the one from the Lambda schematic, yours has a good temp coefficient.The voltage across the resistor will vary with the load, if everything else stays the same.If the external load is zero, all the voltage will appear across sensing resistor. does not matter what the ohm value would be. Your anticipated load is very high, mine can be much lower.Assume a sensing resistor of 1 ohm and a external load of 9 ohms. Assume a voltage of 30 V. The total load is 10 ohms. you get 3 Amps. 30 Watts1/10 of that is dissipated in the sense resistor. This is just an example. 30 Watts is very high for a Lab Grade Variable PS.If the external load wound be 1 ohm then total load would be 2 ohms and 1/2 of that would be dissipated in the current sense resistor.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 14, 2023, 11:03:30 am
content: The circuit shown has very high precision and low drift. The current setting is not dependent on the MOSFET's loosely defined characteristics- and loose the are!You can also set the current without having to fiddle with values on the HV parts.So a little bit of complexity makes life easier and safer.