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Adjustable AC voltage

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by fezder, May 14, 2012.

  1. fezder

    fezder Well-Known Member

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    Hi, sorry if this thread has been already discussed, could you please direct me to excisting one.
    But, i would like to design, or if thats not possible, buy a variable ac voltage source. I mean that i can adust the voltage, without affecting frequenzy or waveform. Sine wave would be preferred, but other forms are accepted too. The voltage range would be 50 VAC tops, and amperage dont need to be much, i need this device only for recearch purposes for coils/capacitors.
    Or would function generator be best choise?
    Please advice or ask more :)
     
  2. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    I would suggest a 1:1 Mains transformer for safety isolation with a Variac transformer connected to the secondary of the 1:1.

    I am assuming a 50/60Hz source and load, ,, you have not stated the frequency range.

    E.
     
    Last edited: May 14, 2012
  3. fezder

    fezder Well-Known Member

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  4. dave

    Dave New Member

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  5. unclejed613

    unclejed613 Well-Known Member

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    "back in the day" Sony published an application note for one of their audio amplifiers on how to use it with a function generator and a transformer to get 120Vac with variable frequency (with most power transformers, this is limited to 40-100hz or so). here in the US, our power frequency of 60hz is stable.... very stable, but there are parts of the world where the line frequency is a "ballpark" figure, and so Sony came up with this method of testing electrical and electronic devices with a variable AC supply...

    i'll try to clarify this:

    [func gen]---->[amp](24Vac out)-------->[xfmr, 24:120]-------->[outlet box]----->[device under test]
     
    Last edited: May 14, 2012
  6. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    The OP wants to vary voltage not frequency.

    You could use a simple potentiometer or resisters for low power.
     
  7. Boncuk

    Boncuk New Member

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    Hi fezder,

    if you are lucky you might get an ancient variac transformer from a junk yard. Those variacs were used in theatres and cinemas to dim surrounding light. (now done using triacs).

    Cinema variacs are pretty bulky and heavy since they had to serve for some 100Watt of lamp power.

    There are still variable transformers on the market. They are not isolated from mains, but if isolation is required you could use a 1:1 isolation transformer to connect the variac.

    If waveshape is no issue you might use a triac dimmer circuit. They are available for households in well assorted electric shops.

    (Caution! No isolation from mains live!)

    Triac dimmers require a minimum load for proper function - normally about 30W.

    Boncuk
     
  8. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    Yup, my vote goes to a variac. I recently got one (a friend of mine was very generous and gave it to me :D) and I love it! It's just a big variable transformer--that's all it is. You can find them on ebay for about $30-$70 USD (about 23€-55€). Of course, that'd be a very simple, low-current (maybe 5-amp) variac. It'll work for simple projects--might be worth looking into.
     
    Last edited: May 14, 2012
  9. fezder

    fezder Well-Known Member

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    Ok, thanks for info's, gotta start looking for these variacs at scrabyard or somewhere else :) ill try if i can use potentiometer at start. These potentiometers i have are only 1/2w thought, but i dont need much current
     
  10. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    A potentiometer adjusts the current, not the voltage. If you try connecting a potentiometer to that kind of power, you'll fry it instantly (even if it's just a few volts). They're also mainly for DC, not AC. You really need an adjustable transformer, not a potentiometer.
     
  11. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    As you are working with coils, its important what the source impedance of the voltage/Current sources are, if you use a resistive potentiometer its possible that the readings you get from your testing could be misleading.

    When you say not much current, please give a minimum / maximum range of current and voltages.??

    E.
     
  12. fezder

    fezder Well-Known Member

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    well i think about maxinum voltage could be 50v and current is enought at 2 amps maxinum. I just have coils lying at my ''junk'' box that needs to be organized. Its the coils induktance i need to figure out, as it doesnt read on it :D i thought it would be wise to get coils organized that i know what is its induktance at certain parameters. did this help at all?...

    and when i know the induktance, then reaktance can be figured ou too?
     
    Last edited: May 15, 2012
  13. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Looking at those values, I would suggest a Variac and a 1:1 mains transformer rated at 100VA to 150VA
     
  14. WTP Pepper

    WTP Pepper Active Member

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    Wein Bridge Oscillator (Google it) >>> Power amp with volume control (LM386) >>> Reversed mains (240-12VAC) transformer and there you have it.
    Add an AC meter on the output to set the voltage on/off load.
     
  15. unclejed613

    unclejed613 Well-Known Member

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    i usually just use a function generator with a square wave output. couple with a known value cap, connect a scope across the coil, and adjust the FG freq for resonance (i.e i get a sine wave, and max amplitude across the coil).

    with this calculator: http://midnightscience.com/formulas-calculators.html#formulas1 you can find the inductance.

    or if you want a more exact reading. put the scope at the FG side of the cap. resonance will be at the lowest dip in the FG output. this works because most FG outputs have either a 50 or 300 ohm output impedance, and works as a voltage divider with the series resonant circuit (the known cap and the unknown inductor). at resonance, Xl and Xc are 180 degrees out of phase and equal, so they cancel. all that's left over is the ESR of the cap in series with the DCR of the inductor.
     
    Last edited: May 15, 2012
  16. fezder

    fezder Well-Known Member

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    Haha, i newer thought to calculate inductance with that formula uncle, we have so ''many'' formulas at our study book that i totally forgot that one :D but that formula is somewhat familiar, just cant remember all these formulas :)
     
  17. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    A variac is usually used when somewhat higher current AC is needed to power something being tested, like 3 amps or more. They make smaller variacs too though. But certainly a frequency generator with low output impedance is going to be more versatile for testing low power circuits like small inductors and capacitors. If you have to test big inductors then a variac is good, but small signal circuit inductors are best tested with a frequency generator where you have more control over the output signal, for one thing 50 or 60Hz is not a good test frequency for many smaller inductors.
     
  18. unclejed613

    unclejed613 Well-Known Member

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    i was going to paste the formula, from that page, but to find inductance, you need to rearrange it a bit, or use this chart. where Xc and Xl are equal, is resonance.
     
  19. fezder

    fezder Well-Known Member

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    well my coils are pretty small, maybe i should get/buy signal generator then, if 50hz isnt good for testing purposes :( as i have only 50hz
     
  20. unclejed613

    unclejed613 Well-Known Member

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    also, a Gate Dip Meter is another device you can build inexpensively to measure smaller coils. it's a small RF generator with a coil on it. you couple the meter coil to the coil you are measuring (and connect a known capacitor across the coil being tested), and find the resonant frequency.. so you could use the function generator method up to 1Mhz for coils larger than 10nH, and the GDM from 1 to 100Mhz for coils smaller than 10nH.

    also, back before "antenna analyzers" were made inexpensively, GDM's were used for finding the resonant frequency of antennas. they are very handy for many things....
     
    Last edited: May 16, 2012
  21. ChrisP58

    ChrisP58 Well-Known Member

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    One reason to use a function generator, instead of an AC power line derived voltage, is that you can choose the frequency to match the target. Trying to measure small inductors or capacitors with 50 or 60 Hz will be very difficult.
     

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