psu post on LG's behalf

[ghostman11]

hi, i am doing the first part of this post for LG because of safety matters. first things first.. yes we can google, yes we have googled but.... because i am concerned that he is safe i wanted advice here first so we know what to google etc also i would prefer it if he gets answers or at least advice on what to google for from here so he dosnt just go off and build a death trap, second he will be posting and doing the actual build but for all live testing i will be doing it or at least watching him like a hawk!
so the project!
he wanted a lab PSU for christmas but when i saw the price (£200+) i desided to spend it on something else for him, and he realy wants to make a decent accurate bench power supply.

first i will tell you what he wants then what he has and finaly what budget his tight arse dad has let him have

he wants a 3-4 outlet variable supply that can be current limited, voltage range -12 to +25v DC preferably 5A max but 3A if we cant get to 5, he wants it as stable and noise free as he can get and would like it accurate to 3 decimal places! again if we can. he also wants 3 fixed outputs of the following 3.3v 5v 12v -5v -3.3v

his budget is only £30 but that isnt rigid if safety cost more, we also have a huge range of components, i have a fair few power boards from broken scopes and logic analyzer's and ATX psu's so i am hopeing there is a transformer on at least one we can use.

so first question is with the above in mind what would the best type of supply be to build?? (we can then google circuits for this) can we use more than one transformer connected together? what type and rating transformer should i be on the look out for on the boards we have?
any help would be great

i forgot to mention that a digital read out would be good and pot adjustment on one outlet with the other two or three digital set.
like i say i didnt just want to google and end up building a death trap!!
anyway it is now his thread

 

[DerStrom8]

Thanks for posting this Gman. This sounds like the Mother Of All PSUs! I'm afraid I can't help you much, but I am very eager to see what comes of it. I may want to build one myself at some point

I wish you and LG the very best of luck!
Regards,
Matt

 

[Mr RB]

I apologise for suggesting something different to your desired spec, but an ideal "first bench supply" is a dual LM317 supply, with a couple of analogue (moving needle) meters for voltage and current. About 3-30v and 0-1.5A.

The voltage adjust can be a 10 turn pot, which gives a very fine adjustability (and you can hook up a digital multimeter if you need mV accuracy). For current a rotary switch with 6 or 8 ranges, from say 20mA to 1.5A.

Even though this doesn't match your spec for accuracy it is very easy to build, and easy to understand and repair, and quite bulletproof with the current limiting and even the final LM317 voltage regulator has it's own internal safety limiting for over temp etc.

The two meters also gives a lot of safety as you can see the current or voltage change at a glance if the load circuit does something bad like an overcurrent or some current spikes. Digital meters are nowhere near as good for safety and general usefulness, even though they give a finer resolution readout.

Anyway, that's just my opinion.

 

[Jaguarjoe]

Marlin P Jones & Assoc. has 3.5 digit DC meters, 200mv range on sale now for $3.95. Cheaper than any analog meter.

 

[alec_t]

Perhaps use one of those ATX power supplies to prvide the fixed voltage outputs, but build the variable voltage supply?

 

[ghostman11]

hi all, update!!! ive told LG to hold off for a couple of day's, RB has given me an idea. LG has a ATX made PSU and a small regulator on, he wants more oomph because the motors in his robot are 3A he also likes to mess with opamps so wants to use -v BUT after reading RB's post this morning it got me thinking. i might be able to ge my hands on a realy good rack mounted PSU cheaply, it has three outputs adjustable from -15 to about 20v i think they are meant to be very low ripple ect and exstremely high quality. i wont know for a day or so if i can get my hands on one but if i can then it would solves most the build problem, they also have a fairly hefty I output so my thinking is maybe make something with this with voltage refereces or regulators or whatever, but at least the main part is all there and meet the specs plus is very very cheap new and unused, so will let you know if i can get it.
also i talked with him about the display and he thinks a digital one driven by a pic would be pretty easy but personaly i think they is a bit more to it, like maybe he will have to use a voltage ref chip to get a accurate ADC reading and also think about how to make the ADC stable or averaging the readings...... but it's his project so i will let him take over now thanks for the input

 

[dr pepper]

I have recently made a psu, with 2 independant o/p's so it can produce + and -.
Drawback is it only has one fixed o/p at 5v and thats not quite finished yet, and the main o/p's are only 1.5a.
And it uses some still available but probably much fronwed upon linear reg chips, but it does have a 2 line display for o/p volatge, current, wattage and heatsink temp.
No good for you though as theres no way your gonna build it for 30 notes.
You might be able to come up with something that uses straightforward meters, you can get digital ones from ebay for around 4 quid.

 

[ghostman11]

the £30 is for extra's we have a massive array of parts already, i only gave him the budget incase i didnt have something he needed. i kind of deal in component clearences from factory's and the like so getting parts is the easy bit. what i was more concerned with was the fact that most on google are eigther ATX based or look a bit iffy and whatever he settles on to build i want it safe. i might even stop being tight and go buy one lol but he is against that and wants the fun from a build. he is also doing a POV clock and a meter based thing. the clock is pretty impressive with what he is trying to come up with, i am realy proud of the effort he is putting in
TBH i would like him to use up some the seven segment displays we have kicking about i am sick of tripping over box's of the bloody things!!

 

[throbscottle]

I was looking for PSU designs and info a while back. Some things I found may also be of use to you:

http://www.electronics-lab.com/projects/power/001/index.html
http://michaelgellis.tripod.com/power2.html
http://english.cxem.net/power/power1.php
http://michaelgellis.tripod.com/power3.html (note interesting rectification)
http://www.analog.com/library/analogDialogue/archives/41-05/ldo.html
http://www.dos4ever.com/flyback/flyback.html
https://www.electro-tech-online.com/threads/dual-output-lab-supply-0-16v-3a.96187/ (really nice, very clever)

There are a few PSU project designs here on ETO, but the links don't seem to be too healthy...

Look on www.instructables.com - there are dozens if not hundreds of designs for psu's people have built, many are re-purposed atx psu's but there are some gems in there too, and plenty of general good ideas.

May be get some ideas here http://www.instructables.com/id/Precision-multimeter-calibration-reference/ (it might be decent if I'd built it properly!)

Also found on Ebay - used KIS 3R33 buck converter modules - cheap and very hackable, use Maxim MP2307 but cost far less than the chip itself. Lots of conversion kits for them on sale too.

I once built a massive bench psu using transformer and other parts from a ex mainframe psu. Weighed a ton. Tracking was via a dual potentiometer that could be switched in (to maintain isolation when not in use) - not accurate, don't do it! Wasn't a good design at all, but I could only use the knowledge I had at the time (in about 1985!)

Hope you find something useful in this lot!

 

[dr pepper]

This is also an excellent design, and more modern, a switcher with variable volts and amps, and it goes down to 0v.
If you build this get the pcb as layout can be tricky.
I've nicked a few ideas from this design.
You would easily be able to fit meters instead of the intended microcontroller 2 line display.

https://www.electro-tech-online.com/custompdfs/2013/01/50-00002.pdf

 

[Mr RB]

It doesn't really matter which one you build first, they should all last for years and the more PSU's around, the better! I have about 4 variable PSUs in general use, and some specialty ones like a regulated 13.8v 12A meterless supply (which you can buy pretty cheap on ebay).

 

[ghostman11]

fantastic info guys thanks alot!! should know soon if my other idea is a go. actualy i am thinking of making a ATX one for myself, i have a cheap bench PSU that overheats lol, main thing both of us lack is -V i know its not used much but would be nice to have it.

 

[Mr RB]

My main 5A bench PSU used to overheat, under constant high current use. I got crappy with it and added a second heatsink to the back to double the heatsinking, and paralleled the big TO-3 transistor on the back with a second one. It's a 1983 model and i think I added the second heatsink and tranny in 1984 or so, and it has worked very well ever since.