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Dual Polarity Adjustable Voltage and Current Lab Power Supply Worklog

Discussion in 'Electronics & Electrics' started by nux, Mar 31, 2006.

  1. nux

    nux Member

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    I am wanting to build a Dual Polarity Adjustable Power Supply, basically something like this:
    http://www.jaycar.com.au/productVie...d2=&pageNumber=&priceMin=&priceMax=&SUBCATID=

    I want to be able to control the two output voltages independently, and ideally be able to limit the current. I would like to be able to use them together to supply say -5v and +5v, with a common ground. Or -5v and +7v to get 5v output and 12v output.

    So the first thing is to find a schematic to use. I havent been able to find anything that will do exactly what I want, but here are what I've found:

    http://www.uoguelph.ca/~antoon/circ/vps.htm
    Single output variable voltage and current.

    http://home.cogeco.ca/~rpaisley4/PSupply.html
    LM317 adjustable voltage.

    http://www.electronics-lab.com/projects/power/003/index.html
    Single output variable voltage and current.

    http://www.mitedu.freeserve.co.uk/Circuits/Power/l200var.html
    Single output variable voltage and current

    http://www.aaroncake.net/circuits/supply3.htm
    Dual output variable voltage

    http://sound.westhost.com/project44.htm
    Dual output (tracking?) variable voltage

    Has anyone done anything like this, or knows of a schematic for what I want? Or should I use one of the single output variable voltage/current ones and make it twice, using a centre tapped transformer to supply each set with positive/negative power?
     
    Last edited: Apr 12, 2006
  2. Dr feelgood

    Dr feelgood Member

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    How many amps do you want it to be able to supply, and how accurate do you want the regulators?
     
  3. OP
    OP
    nux

    nux Member

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    1.5 Amps or more ideally. More is better.
    I am not too fussed with accuracy, within 5% is fine I think.
     
  4. Dr feelgood

    Dr feelgood Member

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    The Lm317 is garunteed to 1.5 Amps so with a good heatsink you probably could draw more for short periods. Also the the 317 has a 1% tolerance so no issue Here.

    I have built a benchtop power supply useing a circuit very similar to the
    http://home.cogeco.ca/~rpaisley4/PSupply.html one, only it was published in ETI many years ago (prob close to 18 now). I found it to be easily adjustable a stable. I used that PSU to test car amplifiers so I know it can handle a sudden draw of current.

    May I suggest getting a couple of $5 DMM and hacking them apart so you can have digital Volt meters built in.

    If you don't want the hassle of adjustable regulators for the -ve rail you could just use a set -5v chip.

    hope this helps
     
  5. Goth

    Goth Grumpy Member

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    I'd be tempted to use an old computer power supply.

    You can take the 12V and 5V, -5V, -12V rails straight out to binding posts since they're commonly used, and then add a LM317 and LM337 on the +/- 12V rails for your adjustable rails.

    Use a dual gang pot for your regulator circuits, and it's crude dual tracking :)

    Using a computer PSU gives you the high voltage side already taken care of, and you get a nice case which is perfect :)
     
  6. OP
    OP
    nux

    nux Member

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    My original idea was to use a computer PSU, but it doesnt provide enough current (~0.8amps) on the -12v line. Also I would like 20v or more on each output. Plus I would like the experience of wiring it up from input to output. And I dont really want dual tracking, I want to set each output seperately.
     
  7. Dr feelgood

    Dr feelgood Member

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    [​IMG]

    This straight from the LM337t datasheet. Just wack a 18-0-18 V transformer and Bridge rectifier and some decent Caps before it.
     
  8. OP
    OP
    nux

    nux Member

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    Sweet, thanks for that. I didnt see one in the LM317 datasheet, didnt think to look in the LM337.

    That one looks the best bet, I guess I just wont have current limiting. I can live with that.
     
  9. P.YO

    P.YO Member

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    If you want to be able to turn that all the way down to 0V, you could hook up 2-3 power diodes in series with to output. You might need to heatsink the diodes, if ur gonna have high current output.
     
    Last edited by a moderator: Jun 16, 2007
  10. dakiller

    dakiller (Oscillating & Impeding)

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    Current limiting won't be hard to add onto that as well, use another LM317 per rail with a fixed resistor of .83ohm (12*10ohm resistors in parallel :lol: ) and a varible resistor of about 1k in series setup like the CCS from that other thread which will give you resolution to limit the current from 1.5A down to 1.2mA
    Just the slight interruption to this is that the pot will need to be able to handle about 1.8W when using 1.5A
     
  11. dakiller

    dakiller (Oscillating & Impeding)

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    Ohh and the lm317/337 combo posted above is ok, but you can do a little better by using a transformer with dual secondaries instead of center tapped, seperate bridge rectifiers and have 2 independent lm317 based regulators per channel so you have 2 floating outputs that you can make them either +/- outputs by putting the positive output of one to the negative of the other or have 2 positive outputs or leave them floating or whatever, giving much more universal uses to it
     
  12. Goth

    Goth Grumpy Member

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    Jaycar's MT2113 Transformer is probably a good candidate.
     
  13. OP
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    nux

    nux Member

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    Yeah I was thinking of that, not sure how that works. I was actually looking at getting that Toroid from Jaycar. Is that suitable to have two independant supplies and outputs? I am just a little unsure on the ground/earthing. I guess I would just earth the chassis from the 240v input, and have a seperate ground for each set of outputs? Then I can use them seperately or together.

    Also, in the previous diagram using the LM317 and LM337, what current is going through the 120 ohm resistor and pot? I worked it out to be well over 0.5 watts when it is outputting 20 volts?
     
  14. dakiller

    dakiller (Oscillating & Impeding)

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    Yea, leave the outputs floating (not earthed) and just tie the case to earth and if you want, add an earth terminal on the front of the psu

    Throught the 120ohm resistor, the voltage across that is fixed 1.25v = 13mW, so the wattage isnt of concern
    through the 2k pot, @20v output, less the 1.25v = 200mW which should be safe for most pots
     
  15. OP
    OP
    nux

    nux Member

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    Cool, so when the pot is at 0 it is outputting about 1.25volts or so? And when the pot is at 2k it is at 20 volts?
     
  16. LethalCorpse

    LethalCorpse Member

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    I'd suggest using multi-tapped 0-30V transformers, with a comparator to switch relays when the output voltage gets close to the next range up or down, so your input voltage is never more than a few volts above the output voltage. That's how the Jaycar PSU you linked to works, keeps the heat down. And it's still got massive heatsinking, I could barely lift the bugger.
     
  17. fad

    fad Member

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    I have the single rail version of this, its very good. You can hear the relays clicking when you go up and down.

    Make sure you get the TO-3 and a big heatsink.
     
  18. OP
    OP
    nux

    nux Member

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    Hmm thats actually not a bad idea, would reduce heat a lot. I would need two 0-30V transformers for dual output though. Any chance you have a schematic as to how it works?

    I'm still deciding whether to bite the bullet and purchase one for ~$250 or make my own and save $100 or so..
     
  19. dakiller

    dakiller (Oscillating & Impeding)

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    Don't look at it as a way to save money, you usually never do on most things, but look at it as you end up with the pride and knoledge of building it yourself with a greater understanding of how it works
     
  20. Goth

    Goth Grumpy Member

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    And you'll be able to repair it!
     

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