Found a very good lab supply design. - Page 1
Forum section: electronics / projects

author: FenderBender
author_rank: Super Contributor
timestamp: April 23, 2012, 07:30:30 pm
content: I ran across this maybe 2 years ago, shrugged it off because it didn't have any LM317s and therefore was impossible.Now that I look at it, it's really not that bad of a circuit to build at all and it has a a lot of the features that you'd want in a good bench supply.http://electronics-diy.presstoday.net/2011/07/bench-power-supply-circuit.htmlTell me what you think!Seems like a winner to me.

author: george graves
author_rank: Super Contributor
timestamp: April 23, 2012, 08:29:08 pm
content: subscribed

author: Unknown
author_rank: Unknown
timestamp: April 23, 2012, 08:47:48 pm
content: no criticism but some questionsfirst of all I wonder what software was used for the schematic. Magazines used to be full with these and sure they look nice.second..As far as I get it the R17 is the current sense resistor, but what does the R15 do then?and lastly..the voltage reference for the entire circuit is the LM7815 but it also carries all the current. Is this a good topology?

author: FenderBender
author_rank: Super Contributor
timestamp: April 23, 2012, 09:01:30 pm
content: Quote from: Electronics DIY on April 23, 2012, 09:01:30 pmWhatever that means...And.Quote from: Electronics DIY on April 23, 2012, 09:01:30 pmSo I guess there is some purpose though it would probably work okay without it. This designer does seem to understand electronics so I won't doubt him too much.

author: free_electron
author_rank: Super Contributor
timestamp: April 23, 2012, 09:27:49 pm
content: Quote from: HAL-42b on April 23, 2012, 08:47:48 pmlooks like an elektor symbol sceme. they used McCad for a long time + ultiboard to do PCb. they switched to Altium in 2004 and recreated that style library in Altium as well. i know there's eople that have done this for eagle as well.as for the design ... what a hairball..the reason this thing has a tendency to oscillate is that he is using an n-mos as a top side driver.Give more output voltage -> Vgs diminishes until the opamp can catch up thus the mos conducts less, opamp overshootsin other words : the opamp and the mosfet are fighting each other constantly in the regulation loop.how does he fight it ? by adding AC feedback to dampen the opamp.

author: Unknown
author_rank: Unknown
timestamp: April 23, 2012, 10:10:31 pm
content: Quote from: free_electron on April 23, 2012, 09:27:49 pmThanks, now I know what I want.Quote from: FenderBender on April 23, 2012, 09:01:30 pmErr...pins 4 and 11 are connectedbeforethe regulator. It may* work but what he did is not what he says he did.* at this point I should check the PSRR of the LM324 but I am lazy and it does not matter really, why not just connect it after the regulator as it says in the text (which I should have red first) After all the ripple on the unregulated side is guaranteed to go up as load goes up.

author: EEVblog
author_rank: Unknown
timestamp: April 23, 2012, 10:58:06 pm
content: Quote from: HAL-42b on April 23, 2012, 08:47:48 pmSilicon Chip redraws all schematics in their desktop publishing program to make them look nice.The side effect of that is that errors can and do creep in from the translation.Dave.

author: FenderBender
author_rank: Super Contributor
timestamp: April 24, 2012, 12:03:57 am
content: Quote from: free_electron on April 23, 2012, 09:27:49 pmSo the designs a no-go? Or do you like the fact that it has AC feedback?

author: Rufus
author_rank: Super Contributor
timestamp: April 24, 2012, 12:53:55 am
content: Quote from: HAL-42b on April 23, 2012, 10:10:31 pmBecause the inputs of the current sensing op-amp would be outside its common mode range.Quote from: FenderBender on April 24, 2012, 12:03:57 amIt isn't a question of liking anything, frequency compensation is required or it will just honk. You have an unavoidable and variable pole from capacitance on the output. You have another from R21 and the MOSFET gate capacitance, more inside the op-amp and another 3 from C4, C5, and C6 by design.The overall combination might give something that is stable with reasonable transient response and tolerant of load capacitance, or the designer might just have fiddled about to get something that works most of the time.I can't tell from looking at it, stuff like this is complicated. I would probably play around with the output stage in spice to see how well it works.

author: amspire
author_rank: Super Contributor
timestamp: April 24, 2012, 02:33:56 am
content: Quote from: FenderBender on April 23, 2012, 07:30:30 pmNot a fan of this design. I would give it 5/10 as they have done a lot of things well. I do think someone has worked on this design on the bench until it behaved reasonably. Thumbs up for that.The 100n caps in the feedback means that to get this design stable, they used the sledgehammer approach rather then properly compensating the feedback circuit. That is probably the main reason they had to use the 7815 regulator, as the supply rejection will be pretty bad. A 5V peak to peak ripple from the mains rectifier circuit would put about 10mV to 100mV ripple in the output without the LM7815.I would say someone has done a lot of trial and error to get it stable, but they didn't really understand how to properly analyze the feedback loop, so they couldn't optimize it. Instead they have added bigger and bigger caps until everything seemed to be stable. Adding extra phase shift with C5 is usually the last thing you want to do, but in this case, I think it is needed to cancel out some of the side-effects of using big caps in the opamp feedback.Given the fact that above 1KHz, the loop gain is limited to less then 5, I am not worried by the MOSFET characteristics too much.I think MOSFETs are inferior to transistors in lab supply designs unless you have a heavy duty gate driver for the constant current circuit. LM324s do not qualify. Again, I think that the LM7815 is doing the main work handling the initial transient effect of a sudden overload. Without it, I think you would see a hefty currrent spike out of the MOSFET if you suddenly applied an overload. It is not hard to eliminate this kind of spike with power transistors, but it is hard with MOSFETs.So overall, this supply will work adequately (assuming it is stable) thanks to the use of the LM7815 regulator. Better then a LM317 based lab supply.The opamp supplies need to connect to the 24V, not the 15V in case you are wondering. No doubt about it at all. Make sure the worse case peak voltage to the LM324 is less then 32 Volts.If you are interested, I am currently entering my first pass of my Mark III General Purpose Supply design into KiCad, so I should be posting that in a day or two. It is an evolving design, but I think it has some interesting features.Richard.

author: FenderBender
author_rank: Super Contributor
timestamp: April 24, 2012, 03:10:01 am
content: Oh right! It's been a while since I looked at the General Purpose PSU thread. Thanks for reminding me of that...And then there is that thing Dave is doing...

author: robrenz
author_rank: Super Contributor
timestamp: April 24, 2012, 03:50:06 am
content: FenderBender,I realize you are looking for a design, not a built PSU so may want to look at the specificatons andschematicsof the PowerDesigns precision supplies. saturation made me aware of these supplies and we both bought several from ebay. As purchased performance of these vintage supplies is simply amazing.saturations post that caught my eye.https://www.eevblog.com/forum/general-chat/what-is-a-good-power-supply/msg101788/#msg101788saturations post with the links to the manuals/schematicshttps://www.eevblog.com/forum/beginners/looking-for-goodinexpensive-bench-power-supply/msg103871/#msg103871We kind of hijacked the original thread so it was renamed and moved. PowerDesigns coverage starts herehttps://www.eevblog.com/forum/beginners/looking-for-goodinexpensive-bench-power-supply/msg103401/#msg103401

author: Unknown
author_rank: Unknown
timestamp: April 24, 2012, 03:58:00 am
content: Quote from: amspire on April 24, 2012, 02:33:56 amWe certainly are Richard. That thread is probably the best read on the forum.