Power Supply

Discussion in 'General Electronics Chat' started by junee_bal, Oct 23, 2013.

  1. junee_bal

    Thread Starter New Member

    Oct 22, 2013
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    Hi....

    I am in need of designing a 200V 5A power supply with variable voltage (0-200V) and Variable current (0-5A).

    Please help me in designing.
     
  2. MaxHeadRoom

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    Jul 18, 2013
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    The simplest is probably is a Variac with bridge rectifier and smoothing capacitor?
    Max.
     
  3. #12

    Expert

    Nov 30, 2010
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    In 1975, I repaired one of those. The essential part was a pot connected to the back of the Variac, and that pot was adjusted to set the output voltage correctly compared to the AC voltage that was being supplied to the rectifier. Definitely better than trying to build 1000 watts worth of output transistors.

    First part: find a Variac with a pot on the back end of it.
     
  4. MaxHeadRoom

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    Jul 18, 2013
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    What does the pot do?
    Mine all vary from 0-140% using the normal Variac Knob fitted on top?
    Max.
     
  5. SgtWookie

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    Jul 17, 2007
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    I have to ask what you think you need a 1,000 Watt 0V-200V variable power supply for? That's mighty ambitious for your 1st post.

    While Variacs can be quite handy, they generally don't provide isolation from mains power, which makes them hazardous to work with. We much prefer that people use transformers that provide galvanic isolation from mains power.
     
  6. MaxHeadRoom

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    The OP does not state where he is located, if N.A. then the output of the Variac on 120v would not be enough, What I have done is use a Variac on the 120v and then operate it into a 120v/240 isolation transformer, work out fine.
    Max.
     
  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Because he has a boss like my old boss who thinks it's always cheaper to do it in house so he just dumps it onto some poor schlub and says "Make it so!"

    I worked at a power supply design house for many years (Power Ten Inc) and can tell you a 1 kW switcher is not a trivial design project..... you can still find some of the old 1kW rack supplies we made on the used equipment sites, probably sell for a few hundred bucks.

    We did have a 200V/5A model but not a big seller so maybe not a lot of used ones left.
     
  8. #12

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    The pot on the back of the Variac sets the DC voltage of the output regulator at about 3 volts below the peak charge on the filter capacitors.
     
  9. MaxHeadRoom

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    Ok, mine are all basic Variac's, AC in AC out.
    Max.
     
  10. crutschow

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    Mar 14, 2008
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    As #12 noted the pot controls the regulator. This means the output is regulated against line and load changes, unlike just a Variac rectifier-capacitor output. Also the regulator adds a great deal of ripple filtering so you don't need such a big filter capacitor for a given level of ripple, and the output ripple doesn't vary significantly with load.

    Comment to #12: I think you want to set the regulator output voltage in relation the minimum ripple voltage, not the peak charge, so the regulator never drops out of regulation.
     
  11. #12

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    Yeah, I know. The safe place to set the DC voltage just happens to be about 3 volts less than the peak voltage.
    I'm trying to clue people into the idea that the ripple amount and the Vce is going to cost about 3 volts.
     
    Last edited: Oct 24, 2013
  12. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The OP said:

    I took it to mean an actual power supply which has regulated voltage and/or regulated current, not a variac.
     
  13. #12

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    That's one way, but Max brought up the Variac design. The output is just as good, as far as being a clean, adjustable, regulated DC voltage, but you don't need 1000 watts worth of output transistors. You can use an isolation transformer to comply with the terms of service at this site and to get your highest voltage correct.

    The 125 RMS at my house times 1.4 on the Variac results in 238 V peak. If the power line goes down to 105 RMS X 1.4 = 207.8 V peak. (Designing for 105 to 125 line voltage used to be the "standard".) This illustrates the range of excess voltage in the design, and therefore the power that must be dissipated in the output transistors. 4000 uf/amp results in 1.5 V p-p ripple voltage. A FWB of 6 amp silicon diodes costs 1.8 volts. Guess about 1 V loss on the pass transistors and a 200 V output needs 204.3 V peak (before the rectifiers).

    Worst case, 238V - 200V = 38 volts at 5 amps. That's 190 watts on the output transistors instead of 2000. Better than 90% savings in wasted power.

    Then you can decide whether to use less filtering or adjust the maximum Vrms with your isolation transformer to save 3.5 volts x 5A and the wasted power goes down toward 172.5W for a design that will work from 105 RMS to 125 RMS line voltage.

    It gets better if the required output voltage is lower because the power line variations are reduced by the transformer used to get the filtered voltage. For a 5 volt supply, the Vp variation is only about 1.2 volts, and that is why nobody uses a Variac for 5 volt supplies.
     
  14. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    A variac output is not regulated. As the load current increases and decreases, the voltage will change due to loading effects. It may be "good enough" for some applications, but it is not a regulated power supply.
     
  15. #12

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    True, a Variac is not regulated, but a regulator is regulated. The Variac is just a way to feed a regulator so as to decrease the power that the regulator must dispose of.
     
  16. TorontoBob

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    Oct 5, 2013
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  17. MaxHeadRoom

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    Jul 18, 2013
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    A basic Variac, at least the ones I have, are all variable transformers, no rectification or other control?
    It is up to you what you do with the input/output!
    Max.
     
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