variable voltage power supply general questions

[ghostman11]

hi, lately i been looking for a variable power supply for the bench, at present i have a simple fixed voltage supply that i made from a old ATX PSU from a defunct PC. It's worked well and so far done pretty much all i need. i have 5v 12v and 3.3v.
but as time goes by and i am starting to look into different stuff for logans train project etc, i have often thought a bench supply i could vary would be useful and perhaps one with a negative supply would be great for messing with opamps and the like. so i have a couple of questions just to check i am on the right track.
BTW yes i could go splash some cash and just buy one!! but not much to be learned from that and on the face of it i dont think what i am after should be that hard to do. so what exactly would i like from it???
well for starters i would like to use the ATX PSU as the base of the project
2. keep 3 fixed voltage outputs of 3.3v 5v 12v
3. 3 variable outputs, now realisticaly from what i been reading theese will probally be easiest to do if the fall between 1.5v upto 10v, so i have the LM317t voltage reg in mind for the project wich would also give me a max of around 1.5A (plenty for most what i do)
4. i would like a digital LCD readout of all chanels for voltage so my guess is a pic with lcd with the voltage read via ADC?
5. negative variable voltage on 3 seperate channels, here i need some recomendations on the best voltage reg to use, again voltage would be -1.5v --10v
anything else you can think of that would be pretty simple to add and nice to have?
and most important of all......am i on the right track or heading down a blind alley?
cheers chaps

 

[alec_t]

That all seems doable and on the right track. As you want to keep the 3.3, 5 and 12V supplies fixed that rules out tinkering internally to get the variable outputs. Presumably you already have a fixed -12V supply (at least) as a starting point for your negative voltages?

 

[Reloadron]

I would think about finding yourself a nice center tapped transformer. Something like a 36 volt or 40 volt center tapped secondary at maybe 2 to 3 amps, Then as you mentioned use a simple little LM317 for the positive supply half regulation and use a LM337 like this one for the negative regulation. The LM337 is a negative regulator compliment to the LM317 positive regulator. Place a full wave bridge across the full output of the transformer and the center tap becomes the common for the + and - supplies. Add capacitors and season to taste. Personally I would leave the fixed supplies alone and just build another dual + / - supply.

Ron

 

[ghostman11]

now you got me thinking ron!! actualy no i dont have a -12v supply there is a - supply on a ATX but off the top of my head i cant remember the voltage, actualy i like the idea of a complete build from a transformer, i had kind of thrown the idea away as being too complex, mainly after taking a ATX supply apart and seeing how stuffed full of stuff it was so i assumed a bench supply from scratch was a bit out my league, but as always i am willing to give it a go .
as for internaly tinkering..... now theres is a idea i am deffianately up for!! it dosnt matter about messing the supply up as i have 3-4 of theese kicking about in the junk box. what kind of outputs could i exspect to get from messing with the internals? not too bothered about high current 1.5A tops is plenty and would rarely go much above 750mA to be honest but i thought best to double that. the pic i have in mind will probally be the 18f1330, what size rectifier would i need if i go the way of a complete build?
cheers jason

p.s just checked the pinouts on a ATX and they do indeed have a -12v supply HAPPY DAYS!

 

[ghostman11]

not sure if this is a bit on the skimpy side https://www.sitebox.ltd.uk/product/CM50VA/ccm-panel-transformers-50va/?paid=googleproducts ??? ok i admit i went for price :s

haveing a think about it i have a junked power supply from a logic analyzer i will have a look to see if it says what kind of transformer is in it. that would appeal to the inbuilt tighness i suffer from

 

[Reloadron]

Something to note as to the -12 volts on ATX power supplies, especially older ones is the -12 volt current is often pretty low. Just something worth noting. The transformer linked to was a peculiar one.

Ron

 

[ghostman11]

yeah the -12v is rated at around 500mA but i mainly want negative supply for opamps so i dont spose 0.5A is going to matter too much (hopefully) still taking the logic analyzer supply apart to see what kind of transformer that has

 

[alec_t]

The only tinkering I've done with a spare ATX supply was to substitute a pot for one of the two resistors forming a potential divider for the feedback voltage from the +5V output to the SMPS control input, so that I could reduce all the output voltages. It wouldn't be safe to crank the voltages up, as ratings of components would probably be exceeded.

Edit: My ATX -5V and -12V outputs are each rated at 0.5A.

 

[KeepItSimpleStupid]

Heathkit made a nice tri-suppy at one time. The 5V was fixed. What you could do is have a switch for 1.8, 3.3 and 5 or have all three.

Then they had two other 0-20 V supplies. They had a switch labeled independent/tracking, but it wasn't a tracking supply in the true sense, but it made the voltages change at the same time. Thus you can have the setpoints change at the same time. So, you had one tracking/independent pot and another pot for the other supply.

They rounded it out with a meter and switch that were able to select cuurent or voltage for each supply. Rather simplistic.

 

[ghostman11]

Heathkit made a nice tri-suppy at one time. The 5V was fixed. What you could do is have a switch for 1.8, 3.3 and 5 or have all three.

Then they had two other 0-20 V supplies. They had a switch labeled independent/tracking, but it wasn't a tracking supply in the true sense, but it made the voltages change at the same time. Thus you can have the setpoints change at the same time. So, you had one tracking/independent pot and another pot for the other supply.

They rounded it out with a meter and switch that were able to select cuurent or voltage for each supply. Rather simplistic.

that sounds exactly like what i have in mind, currently hunting for a nice enclosure for it and gather parts up i like the idea of 'tracking' so have added to the wish list