DIY Precision  Lab Power supply 0-30v 0-3A  - Page 1
Forum section: electronics / projects

author: 8of10
author_rank: Newbie
timestamp: March 02, 2016, 02:41:45 pm
content: Hi every bodyIm new here to the EEVBLOG and i just wanted to show a small project that i have been working on . Well it should be a Precision programmable Lab Power Supply.Im making a power supply with i hopthe following specs:0-30V Range0-3A Range could also be more 5A1mV resolution1mA resolutionThe Micro is based opon the Arduino plattform and the board ack's as a form of back end,because on the board it selfthere is no poti, or encoder port.The communication to the front end is done over RS232 with is isolated from the rest.Soon i will receive the pcb and then i finally can start assemblyIf anybody is interested in the eagle files let me knowIf you see something not right please let me know

author: blackdog
author_rank: Frequent Contributor
timestamp: March 02, 2016, 03:10:05 pm
content: Hi 8of10,95% of all your attension should go to the linear part of the schematic.Forget the Arduino or other controler, be sure the ananlog part is stable under al conditions, U, I en switch over between them.If your schematic is OK (maybe some Spice testing) then make a testsetup and find out the best way to wire up your PSU, YES thats important!Your current source (LM334) wil not work on low output voltages,Better is to use a "real" current source powerd from a low negative voltage (from say -5V) and the currend about 50mA, this realy helps the regulation.Find out wat the best capacitor combination is for over the output therminals and the feedback compensation, (spice can help you in the right direction)Read about "Loop Stability" for power supply's.If all is wel, than you can think about computer control ;-)My 2 centKind regarts,Blackdog

author: 8of10
author_rank: Newbie
timestamp: March 02, 2016, 03:21:04 pm
content: Hi BlackdogThank's for your comment . I dont have any experince in using Spice , but i guese i should have a look at it , untill now i have not used anysimulators, only real world bread board simulator.Ill have a look in to a different current source.best regards .

author: Kleinstein
author_rank: Super Contributor
timestamp: March 02, 2016, 05:42:45 pm
content: The circuit shown does not look like it will work well. At the very least a resistor from gate to source is needed, as the MOSFET is voltage controlled, not current controlled. But there are several more poor design decisions, like using so many OPs, some caps at places you don't want them.So looking at simulation is a really good idea, mainly to make the analog part stable.I would have a look at the old HP supplies, with the floating regulator.

author: Mephitus
author_rank: Regular Contributor
timestamp: March 02, 2016, 07:02:51 pm
content: Sorry it's in german. But attached is a heavily modified design based on (from what I understand) an old HP supply. Some of the design choices may help in planning your own.

author: sangvikh
author_rank: Contributor
timestamp: March 02, 2016, 10:12:33 pm
content: Have you watched the videos Dave has made on lab power supply design?He goes through designing almost exactly what you are designing now.I have a plan on building a LT3083 based design pretty soon. Just using 10 turn pots. Something like the one from episode 3 or 4 with a few modifications.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 03, 2016, 03:52:26 pm
content: The usual regulator chips are not a good basis for a lab supply. The chips are made to supply reasonably well behaved load, and usually don't care much about current regulation, except for protection purpose. With the power transistor and reference in the same chip drift tends to be higher and also there is usually not separate sense and current output.

author: dom0
author_rank: Super Contributor
timestamp: March 03, 2016, 07:15:08 pm
content: I agree, the "µSupply" had design goals different from the usual lab supply, it is not a good basis for a 'normal' lab supply. Refer to manuals of older lab supplies of the big manufacturers.

author: 8of10
author_rank: Newbie
timestamp: March 05, 2016, 09:10:26 am
content: Hi every oneFirst i wanted to say thank you to every one who left a comment , i was quite helpful.Im trying my circuit now in LTSPICE and wow , its unstable as s..t, I have done now a few simulationsand slowly im getting there .But just for testing stability how do i bring an error in my feed back loop, orbetter said what kind of error .best regards

author: dom0
author_rank: Super Contributor
timestamp: March 05, 2016, 09:27:07 am
content: For transient analysis you usually just apply a step to the load current, for example with a current source (parallel resistance <1 k?).

author: sangvikh
author_rank: Contributor
timestamp: March 05, 2016, 10:30:49 am
content: Quote from: Kleinstein on March 03, 2016, 03:52:26 pmAre there any regulators with external references?Also, wouldn't the internal reference mean that accuracy goes down as current goes up? Not too big of a problem as you usually don't need microamps accuracy at high currents.From what i understand of the video is that you use it strictly as a voltage regulator, and limit the current by dragging down the voltage in constant current mode.What is the better way of doing this? Designing a discrete regulator? Probably a lot of work, but a great learning experience.

author: David97
author_rank: Contributor
timestamp: March 05, 2016, 11:08:28 am
content: this looks awesome. Will have to come back in tomorrow and have a proper look.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 05, 2016, 11:31:03 am
content: There are regulator chips made for an external power device (e.g. µA/TL/LM723 or LT1575) - so the reference and regulator part will not get hot under load. A linear regulator just gets quite hot - something like a 100 K temperature rise for the power device is not uncommon. With a 100 ppm/K reference coupled to the power device this would mean a 1 % change in voltage or current. Some of the LT308x based plans essentially use an external reference to set the voltage, but the amplifier part still gets hot.The proper way is to have the heat of the power transistor(s) separate from the regulator and reference. Usually this means using separate ref chip, OPs and transistors. The LM723 lacks accurate current regulation, the LT1575 is only low voltage.A precision lab supply usually (at low voltage) mean having sense terminals, or at least sense lines directly to the connectors to compensate for a drop there.

author: jpb
author_rank: Super Contributor
timestamp: March 05, 2016, 11:33:32 am
content: Quote from: sangvikh on March 05, 2016, 10:30:49 amThere are a lot of regulators that will take an external reference. For example the LT3081/LT3091 - you can connect an external reference to their set pins and I think you can combine this with using the current measurement output to compensate for cable drop (according to the data sheets). I'm looking to build a low noise fixed supply at present and was planning to use this approach.For a variable supply you then have the issue of varying the reference since the LT3091 for example will just set the output voltage equal to the reference voltage (it acts as a buffer).

author: dom0
author_rank: Super Contributor
timestamp: March 05, 2016, 12:37:24 pm
content: No one makes a "lab supply regulator IC"¹, because there is no demand for such thing; professional designs are "discrete" anyway, and the hobbyist either uses/abuses normal regulators for that, or builds a "discrete" circuit like sane designers do.And it's not like there is anything wrong with that. Even a good "discrete" lab supply only has a dozen or so components more than two LM317s in a row, just that the "discrete" circuit is better in essentially every way.The only thing that is difficult, or at least perceived so, is thermal design. AoE has like four pages on that in the entire book, why? Because there is nothing more than four pages to be said about that for virtually any application.¹ the LM723 doesn't have the characteristic parallel control loops, i.e. it lacks stable, repeatable, accurate current control.</semirant>

author: Audioguru
author_rank: Super Contributor
timestamp: March 05, 2016, 01:57:50 pm
content: I cannot read or see the schematics in the first post. There seems to be 6 tiny little no-contrast schematics on one page.Please post each schematic separately especially the analog one. This is what I see:

author: rdl
author_rank: Super Contributor
timestamp: March 05, 2016, 03:05:28 pm
content: The actual image is about 3600x2500. The forum software is scaling it down. In Firefox, I have to click it once to enlarge it, then right click and select "view image" to see the original.

author: Audioguru
author_rank: Super Contributor
timestamp: March 05, 2016, 04:44:31 pm
content: Thanks, I have Windows 10 and edge or whatever it has. I left-clicked on your PNG and it got larger. Then it had a + and - size window that also makes it as large and as clear as I want. I tried other PNG schematics and got the same.

author: timb
author_rank: Super Contributor
timestamp: March 06, 2016, 02:15:59 am
content: Check out the LT1970A Op-Amp. It's a precision power amp that can do 36V@500mA. The nice thing is it has two separate supply inputs, one pair for the output stage and another pair for the rest of the chip. If you hang a pair of FETs and resistors off the output stage you can boost the current.I'd change that around a bit, make it non-inverting Av = 6 and hook a precision dual 5V DAC off the + input and the Vsnk + Vsrc terminals. Use +32V and -2V power supplies so it can swing from 30V to ground.Now you've got a precision power supply with voltage and current control with only an op-amp, two FETs and a DAC. If you want to go really shmick, add a difference amplifier into the feedback loop; use it for a kelvin connection at the load. Now you've got a four terminal power supply!(I'm building a 4-Quadrant power supply/amplifier out of the LT1970A myself. It's a very nice chip.)

author: 8of10
author_rank: Newbie
timestamp: March 08, 2016, 12:00:40 am
content: Thank you very much for all the informationSo now after a lot of breadboarding and simulations in spice i have now updated my circuit.First of all before you might say why i'm using so many OP-AMP'S or why im not using a current sensing amp or a other pre configured devices is that :1- I want to build a precision power supply only with through hole parts , no surface mount, just because ..2- The Analog part should be built only with standard building Blocks (Transistors,Passives,Op-Amps )3- It's a learning experience , because until now i mostly only delt with Digital IC's and Micro's .4- I just want to see how far ,or how Precise i can make it (More is Better)or should i say less .5- And its a lot of FunSo here it is the new circuit , i hope some one might find it interesting or can leave a comment if they se something wrong, or if you have some thing i could improve .Best Regards

author: Warhawk
author_rank: Frequent Contributor
timestamp: March 08, 2016, 07:10:08 am
content: Regarding the LT1970 - I built a module based on this :http://www.epanorama.net/sff/Power%20Electronics/Power_Supplies-Linear/Four%20Quadrant%20Power%20Supply.pdfWorks very well...

author: Kleinstein
author_rank: Super Contributor
timestamp: March 08, 2016, 01:57:46 pm
content: The way current is sensed is rather problematic: the dividers have to bee really stable - so even the 0.1% resistors are not good here. The voltage at the shunt is small (e.g. less than 200 mV) and the minimum shift from the divider would be something like 5 volts. It's much better to use the extra supply that is used for the gate drive anyway and than amplify the voltage at the shunt directly. There are also OPs that work at the positive supply, even if these OPs are not that accurate the result would be better.The caps to smooth the current signal are also problematic, as noise from parts after this would be amplified a lot. Also the phase shift from this tends to cause instability with a low pass function at more than one point.It's generally a bad idea to have a capacitor from the inverting input of the OP to GND. It's not helping much, but can cause a lot of trouble.The gate driving circuit should have some kind of protection against to much gate voltage, also to much negative gate voltage is a problem.

author: 8of10
author_rank: Newbie
timestamp: March 10, 2016, 01:17:28 am
content: So now i hope the analog part is ok ,I added input protection , and protection against to much gate voltage at the power MOSFET.all simulations checked out ok , now i want to bread Boards it.Best Regards

author: Kleinstein
author_rank: Super Contributor
timestamp: March 10, 2016, 04:51:38 pm
content: The current sense part looks better, but could be still rather slow. The extra OP used for the "common mode compensation" is not really needed - that part does not look like doing more than a simple resistor can do. Most of the resistors don't need to be that accurate - they just give a single scale factor, that likely gets adjusted anyway, as shunts are hardly better than 1% tolerance.There are still two capacitor from inverting inputs of OP to ground - they are usually not a good idea and not helpful. There are also 2 of the very small (10 pf) caps to ground that have essentially no effect and if so more like a negative one.The constant current sink is rather complicated. One OP is usually enough.The power mosfet does not look like it is rated for linear operation, especially with something like 40 V and more than 100 mA. Even a more suitable IRFP250 is likely not powerful enough, or would need very good cooling. Something like a 30 V 3 A supply is usually two much for a single power device without preregualtion or at least tap switching.I don't think the current setup will be stable with capacitive loads. How does the simulated output impedance look like ?