How to make a dual power supply station from a vintage Powerstat

Discussion in 'Power Electronics' started by David Sanchez, Dec 22, 2016.

  1. David Sanchez

    Thread Starter New Member

    Dec 16, 2016
    5
    0
    Hello, I have recently found real joy in making very simple analog electronic audio circuits; and it's getting more complex very raplidly... My father gave me this old Powerstat a while ago and I was thinking It could make a dual power supply with it.

    As for now I'm working on simple projects like stepped tone generators, lfo´s and for my next project I want to make a variable bandpass filter, so I'm in the 5V to 15V range.

    I would specifficaly need +5/-5V, +9/-9V, +12/-12V and +15/-15V, don't know if I would need certain config for each value or if I can use one config for all values and adjust voltage by the main powerstat knob (that would be ideal).

    Te Powerstat specs are the following:
    InV: 120
    OutV: 0 - 140
    A. 10
    KVA: 1.4
    Freq: 50/60

    I´m attaching the powerstat photos and specs plate so you can appreciate it.

    Powerstat01.jpg
    Powerstat02.jpg
    Powerstat03.jpg

    I came across this image from the Make:Analog Sinthsizers book from MFOS´s Ray Wilson

    upload_2016-12-22_19-45-29.png

    I really don't know if this setup would work, I'd really appreciate any help.

    Thanks in advance, Happy Hollydays and/or Christmass

    David
     
  2. crutschow

    Expert

    Mar 14, 2008
    19,523
    5,444
    You need to add an isolation transformer for safety since the Powerstat is an autotransformer that has no isolation between primary and secondary.
    You could use a 120V primary transformer with a 30-40V center-tapped output (15V to 20V from ether winding to the center-tap).
    Then add a bridge rectifier and filter capacitors as shown in you drawing.
    That will give you a variable output voltage of 0Vdc to about 20Vdc to 28Vdc.
    But note that the output voltage is not regulated with a change in load and will have some voltage ripple in the output.
    That may be okay for some applications but could be problematic for sensitive analog circuit.
    For that you may have to add extra filtering or a fixed voltage regulator.
     
  3. ScottWang

    Moderator

    Aug 23, 2012
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  4. David Sanchez

    Thread Starter New Member

    Dec 16, 2016
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    Thank's for the info, both very usefull. I think now is the time to let you know that I'm very ignorant when it comes to electronics, I'm self teaching myself in a very disperse manner, looking for the koledge I need to complete a specific task. That said, I have a 3° grade title, so I can read and reproduce schematics (not desing), my solder skills are getting beter and I'm willing to experiment.

    By now start to guess if this powerstat is going to be usefull at all. Scott's amazing DIY power source feeds from the domestical outlet, so in this case I really wouldn't need the device, but several transformers.

    But crutschow made me think that maybe I can use the powerstat set to give a fixed output of 30V so I can get ±15V and from there go down to ±12V, ±9V and ±5V.

    This project, more than a "fashionable power source" is a necessity, where I live the price of power sources are prohibitive (as in very much way too expensive) and way over my household budget.

    Thaks again for the replies
     
  5. dannyf

    Well-Known Member

    Sep 13, 2015
    2,196
    421
    what really matters is how much current you expect to draw from each voltage range.

    The simplest would be to wire multiple sets of +/- regulators to a common set of rectifier + capacitors.

    and adjust the transformer for designed output voltage.

    Two downside with that approach:

    1) it cannot supply too much current - the limit there is the regulators;
    2) if the transformer is to too high of a value, it can fry the regulators.

    The alternative is to fix the transformer to the highest setting. The downside is that the low voltage regulators will run hotter.
     
  6. David Sanchez

    Thread Starter New Member

    Dec 16, 2016
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    0
    Regarding the current I need, At the moment, I'm working on very simple projects, currently I'm using a 9V battery. Of course I imagine that with increasing complexity and the sum of devices I could need more Amps, but I´m not there yet.

    I sure hope that when I get there I would actually be more confident and have more knowdelge and maybe be able to design a power source myself.

    As for now, I would rely on this forum's member's experience.

    Heat is a big deal here because of the weather, I would need to use heatsinks and fan coolers.

    I tested the powerstat and up to 50V is in the 0.00 acuracy, increasing while it goes up.
     
  7. David Sanchez

    Thread Starter New Member

    Dec 16, 2016
    5
    0
    Decreasing accuracy while Voltag increases
     
  8. #12

    Expert

    Nov 30, 2010
    18,078
    9,616
    You can use the Powerstat to power a transformer, and then use the rectifiers and capacitors to get a dual DC voltage like in the first post. You can get any pair of voltages, but only one pair at a time. It will be sufficient for a while, until you decide to build a permanent, multi-output power supply.
     
  9. David Sanchez

    Thread Starter New Member

    Dec 16, 2016
    5
    0
    All transformers I have seen convert from 120V to the desired Voltage, so if I set the Powerstat to 120V It would be the same as connecting the transformer to the wall outlet.

    Could there be a way to rectify the Voltage coming from the Powerstat at AC 30V and convert it to DC +/-15V?

    I have a bunch of 5V, 9V, 12V and 15V transformers at hand, but for use with op-amps I don't want to rely on virtual ground.

    Any sugestions? thanks
     
  10. dannyf

    Well-Known Member

    Sep 13, 2015
    2,196
    421
    if you are using them for opamps, you don't need much current.

    In that case, see if the transformer you have provides a center tab. if it does, you are golden.

    if not, you may have no choice but to go with a virtual grounding scheme.
     
  11. A Homeschoolers Workbench

    Member

    Jul 26, 2016
    143
    28
    I am building a really basic power supply using a auto transformer, here is my schematic
    power supply.png

    Here is a great video on auto-transformers
    Best of luck
    AIDayley
     
  12. #12

    Expert

    Nov 30, 2010
    18,078
    9,616
    True. Are you saying you can't turn the Powerstat to a lower voltage and get a lower voltage out of whatever transformer you have laying around?
    No. The Powerstat doesn't have a center tap.
    Use the Powerstat to power a pair of 15 volt transformers. Wire their outputs so a center tap exists. Turn the Powerstat to a lower voltage if you want less than +/- 20 volts.
     
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