Starting out on a lab power supply adventure... - Page 1
Forum section: electronics / projects

author: FenderBender
author_rank: Super Contributor
timestamp: November 17, 2011, 03:35:11 am
content: I was in the thrift store the other day and saw a pretty dang nice Home theater mixer/equalizer box. Dobly certfied and everything. I wasn't really interested in the piece of equipment itself but rather the torroid transformer in side. Price tag was $8. Took it home. Cracked it open. 14V-0-14V @ 4A. Hell yeah.Pretty perfect for the supply I need for my lab.Aight so here's what I've got so far... If you have any ideas, please tell me. Otherwise, I guess this will more or less just be a thread where I'll give you some updates.The goals for this project:-Dual rails (Positive and Negative)-Adjustable Voltage from +/- 1.25-15V-Adjustable Current (0-1A/rail)-Digital Voltage and Current readouts for each rail. 4 displays total.-High quality and ruggedThe volt and ammeters I will be making myself with some good old ICL7107s. Two configured for voltage, two for current.I really want to keep it as physically thin as possible. My bench is only about 2.5 in depth, so my test equipment needs to be pretty shallow....I'll just have to be creative. The transformer can be bolted vertically or something.Also I plan on building this all on proto-board. Don't have the equipment yet for printing my own and it's just a power supply...though the Digital readouts should be fun. 40-pin DIP...yaay....----------------One question: I'm trying to base the current limiting section off of this.http://www.extremecircuits.net/2010/05/adjustable-current-limit-for-dual-power.htmlDoes this circuit's limiting function work as in 0-1A overall...or 0-1A on each rail? Originally I thought it was overall, but after looking at it, it seems that is is per rail...which would be good and means I don't have to modify anything. If it's 1A between the two rails, then I'm going to have to make some changes.Thanks a lot!

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: November 17, 2011, 04:03:39 am
content: Your bench is 75 cm deep ? That should be enough space for even analog scope

author: FenderBender
author_rank: Super Contributor
timestamp: November 17, 2011, 04:34:27 am
content: Well you made me actually go measure it. it's 23inches..so 58cm. Yes I could put a huge old scope on there...but it's only about a meter and a half wide of working space. So it's a small bench.

author: IanB
author_rank: Super Contributor
timestamp: November 17, 2011, 05:19:16 am
content: Do some careful calculations regarding the realistic maximum voltage you can expect from your power supply. See here for example:http://ludens.cl/Electron/Ps20/Ps20.htmlThis is an excellent article by someone else posting on these forums.

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: November 17, 2011, 06:23:07 am
content: Quote from: FenderBender on November 17, 2011, 04:34:27 amOh yes, 60 cm deep is kinda too little. Well enlarge bench or make smaller test gearGood luck with the project

author: JuKu
author_rank: Frequent Contributor
timestamp: November 17, 2011, 08:35:27 am
content: You'll want a (switchable?) 5V/3.3V output anyway, so put that in as well. The only thing missing from my power supply is current limit/readout from the digital side, so I have to first make sure there is no shorts on the 5/3.3V side, then power up the rest. Oh well, next time...

author: phil_jp1
author_rank: Regular Contributor
timestamp: November 17, 2011, 04:41:43 pm
content: It should be really cool project.I guess you'd better build your power supply using some op-amps for voltage and current adjustment and use MOSFETs for power stage instead of transistors. Since you have only 14V-0-14V transformer and want to have pretty high output voltage - with mosfets it would be much easier to get that done.Plus it would be a good idea to use schottky diodes in bridge rectifier, to get smaller voltage drop and lower power dissipation.Here's the video I've done on building linear power supply:http://jumperone.com/2011/08/lm317-adjustable-psu/You might pick something useful from that.Good luck with your project! And keep us posted on the progress.(and more photos, more photos!!!!!)

author: FenderBender
author_rank: Super Contributor
timestamp: November 17, 2011, 10:36:06 pm
content: Quote from: phil_jp1 on November 17, 2011, 04:41:43 pmYeah I've watched your video like 10 times before this. Haha.I don't know about the op-amps and stuff tho. I'm kind of too stupid to figure out how to do it with op-amps at the moment. Any lead on how I could do split supply with voltage and current limiting like that? Any schematics or ideas?

author: phil_jp1
author_rank: Regular Contributor
timestamp: November 18, 2011, 01:11:58 am
content: Quote from: FenderBender on November 17, 2011, 10:36:06 pmI'm not sure if there's any really simple circuits for dual-output adjustable voltage/current power supplies exists.I just found document on this topic, but it's dual-output psu with adjustable voltage:http://www.ece.ucsb.edu/courses/ECE002/2B_W11Shynk/Lab2.pdfIt's pretty much the simplest schematic for dual-output psu with V adjustment.(the same schematic you can find in LM337 datasheet, but without explanation)But you can easily make one channel with adjustable voltage and current, and the other channel just with adjustable voltage.Download datasheets from all the manufacturers for LM317 here:http://www.datasheetcatalog.com/datasheets_pdf/L/M/3/1/LM317.shtmlAnd for LM337 here:http://www.datasheetcatalog.com/datasheets_pdf/L/M/3/3/LM337.shtml(each datasheet has some unique circuits)In that datasheets you'll find all kinds of LM317-LM337-based power supply circuits. You can combine simple dual-output LM317-LM337 circuit with LM317 adjustable-voltage-and-current circuit for one channel, and you'll get adjustable voltage and current on positive channel, and just adjustable voltage on another.

author: FenderBender
author_rank: Super Contributor
timestamp: November 18, 2011, 01:28:30 am
content: One question about the rectifiers...Is like 60V reverse V too little? I've typically always used like 600-1000V rectifiers because, well...I could. But it seems that in order to get low forward voltages, the max reverse voltage seems to drop also.Another question. In all of the literature I have ever read, they say silicon rectifiers have a set forward voltage of 0.7V but if you take a 1n4007 for instance, it's forward voltage is 1.1V. I don't get it?I was looking at a 5A schottky with 60V reverse and 0.65V forward. They always talk about 0.7Vdrop, but this is only 0.65 and it's a Schottky?What?

author: phil_jp1
author_rank: Regular Contributor
timestamp: November 18, 2011, 01:39:48 am
content: Quote from: FenderBender on November 18, 2011, 01:28:30 amUsually high voltage Schottky diodes have big forward voltage drop.Here's an example of jelly-bean low-voltage Schottky diode: 1N582240V, 3A,Vf = about 0.5V at 3A.Vf @ 1A = 0.4VAnd when it heats up, Vf will drop a little bit more.

author: FenderBender
author_rank: Super Contributor
timestamp: November 18, 2011, 03:48:27 am
content: So is it better to have a Schottky diode with a high forward voltage or a regular rectifier with a lower vDrop?

author: IanB
author_rank: Super Contributor
timestamp: November 18, 2011, 04:20:12 am
content: Quote from: FenderBender on November 18, 2011, 01:28:30 amThe set "forward voltage" of a diode is a fiction. There is no such thing. For the reality, take a look at a datasheet, for example this one:http://www.fairchildsemi.com/ds/1N%2F1N4001.pdfSee the graph of forward voltage vs forward current? That's what you have to pay attention to. Have a look where the mythical "0.7 volts" appears on that graph.

author: FenderBender
author_rank: Super Contributor
timestamp: November 18, 2011, 04:38:04 am
content: Ah that honestly always confused me. I guess I was too lazy to scroll down the datasheet and try to interpret some data.

author: FenderBender
author_rank: Super Contributor
timestamp: November 18, 2011, 04:45:01 am
content: I don't know. I think I might just use a GBU6M or something. The schottkys seem nice, but with such low reverse voltages and other limitations, I think I'd rather use a more traditional bridge. Though you can correct me if I'm wrong.

author: BravoV
author_rank: Super Contributor
timestamp: November 18, 2011, 04:46:37 am
content: Although I have not try this my self, if you're looking for lower forward voltage and higher efficiency in rectifying the ac from the transformer to dc to your linear circuit, as an alternative, you should check this out ->http://www.extremecircuits.net/2010/05/power-mosfet-bridge-rectifier.htmlHave to admit that this is a bit overkill if its just for a low power linear regulator which is not that efficient by it self, not to mention it's complexity and costlier just for replacing the plain old silicon full wave rectifier.

author: FenderBender
author_rank: Super Contributor
timestamp: November 18, 2011, 04:50:10 am
content: Yeah seems interesting, but I'd have to make two of them for a split rail power supply! Yikes...I mean, i guess it might be worth a try! Let see!

author: IanB
author_rank: Super Contributor
timestamp: November 18, 2011, 05:13:07 am
content: Quote from: FenderBender on November 18, 2011, 04:38:04 amThere's no mystery to it. If you plot voltage vs current for a resistor you get a straight line. If you plot voltage vs current for a diode you get a curved line. For both resistors and diodes, if you want to know the voltage drop for a given current you just look it up on the graph. No real difference.

author: phil_jp1
author_rank: Regular Contributor
timestamp: November 18, 2011, 08:04:11 am
content: Quote from: FenderBender on November 18, 2011, 04:45:01 amFor that beefy transformer you have it would be more than enough to use 1N5822 with 40V reverse voltage. Because peak output voltage of that transformer without load would not be higher than 30V anyways. I think it would be about 25V.So, there's no problem in using 40V Schottky diode.Or if you want, of course, you can build an active rectifier

author: FenderBender
author_rank: Super Contributor
timestamp: November 19, 2011, 04:42:29 am
content: Can that active rectifier circuit be used for a center tap transformer?

author: phil_jp1
author_rank: Regular Contributor
timestamp: November 19, 2011, 09:47:54 am
content: Quote from: FenderBender on November 19, 2011, 04:42:29 amYes, it should work. But the problem with active rectifier is that you need an additional power supply to supply juice to op-amps (or comparators).

author: FenderBender
author_rank: Super Contributor
timestamp: November 19, 2011, 05:51:37 pm
content: Really? According to that project, it says that the nature of the mosfets allow for it to start up and run off the main transformer...Am I wrong?

author: phil_jp1
author_rank: Regular Contributor
timestamp: November 19, 2011, 06:15:03 pm
content: Quote from: FenderBender on November 19, 2011, 05:51:37 pmYeah, you right. Sorry my bad! I didn't paid much attention to details.So, you've decided to build an active rectifier?

author: johnmx
author_rank: Frequent Contributor
timestamp: November 19, 2011, 09:05:05 pm
content: I use that active bridge circuit in my homemade bench power supply. The mains transformer outputs 18Vac and I can get 20Vdc@3A maximum regulated output. I wanted 20V maximum and this is only possible with that circuit.The bridge transistors don’t dissipate almost any heat, at least I cannot feel any with my fingers. But it’s very important to use ultra-low on-resistance transistors, which can be expensive. I use the IRFZ44ZPbF and IRF4905PbF.Pay attention to the resistors for the dividers. It didn’t work well with 1% resistors, so I changed to 0.1% and it worked pretty well. You can check this with an oscilloscope.Don’t connect the rectifier without a fuse on the input!!

author: Unknown
author_rank: Unknown
timestamp: November 19, 2011, 11:10:24 pm
content: Is active rectification really worth it for ~15V/1A? Seems overkill to me, both in complexity and parts costs. A normal rectifier would cost pennies and wouldn't need any cooling either. At best you win 2V, or ~10%.

author: phil_jp1
author_rank: Regular Contributor
timestamp: November 19, 2011, 11:29:43 pm
content: Quote from: alm on November 19, 2011, 11:10:24 pmSure it would be much easier. As I said earlier, he can use 1N5822 3A Schottky diode, which has 0.35V forward voltage drop under 1A load and 25 degrees C. And even less when it heats up.So, if to use Schottky rectifier - the voltage drop would be about 0.7V @ 1A.But active rectifier is more interesting

author: Unknown
author_rank: Unknown
timestamp: November 19, 2011, 11:47:04 pm
content: Quote from: phil_jp1 on November 19, 2011, 11:29:43 pmNo argument there.

author: FenderBender
author_rank: Super Contributor
timestamp: November 20, 2011, 07:25:05 pm
content: Well I think I might try Active rectifier because it's not THAT complicated, and I don't really have a strict budget. Might as well try it out...

author: FenderBender
author_rank: Super Contributor
timestamp: November 20, 2011, 07:26:01 pm
content: I estimate that it will probably cost $7-8 vs @2 for a standard bridge. I mean...ehh...

author: FenderBender
author_rank: Super Contributor
timestamp: December 11, 2011, 09:18:59 pm
content: I haven't posted on this thread in a while, which is most likely due to me not having much time to work on this project. Yesterday, I decided to make an order from Digikey so I could atleast start prototyping, or making a mockup.I decided to go with a design similar to one I found in the Linear LT1033 datasheet which is for a dual rail power supply with a common ground. I've decided to use an LT1033 and LT1085...which are quite similar to the LM317 and 337 but many people have stated the Linear parts to be superior in several aspects..including price.I was originally planning on using perfboard for construction but i am planning on etching my own board for the first time..so we'll see. Hopefully that could help prolong its life. Speaking of which, parts I have purchased are very good quality and can't wait to see them all together in one unit. I can't believe that about 15% of the project cost is going towards a potentiometer...Also, I will be making a single digital voltmeter which will be hooked up to the positive rail. Since it is dual tracking, I wil asume that the negative voltage is the same or very similar. Instead of having digital ammeters, I think I will use some panel analog meters. They should serve the purpose well enough...If I really need a precise measurement, I can use my multimeter. For the fun of it, I'm going to stick some LEDs in the analog meters so they are illuminated. Colors undecided yet. =p.Current limiting is undecided as of now. 8 could have some sort of prestage LM317 thing with a rotary switch to switch in resistors. I know it's not very elegant, but it's also quite solid and there would be very few issues if any. Ideas for that are appreciated.Hopefuly pictures soon!