Safe AC for exercises.

Discussion in 'General Electronics Chat' started by hp1729, Feb 14, 2016.

  1. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    Some exercises just can't be done without AC. Since my breadboards are battery powered I need a way to supply AC that is safe and available. The attached circuit was what I came up with. 60 Hz, variable voltage, modest output current.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Nice, but I'm curious as to what exercises you'd be doing, given that the battery supply and push-pull output stage will obviously limit the current you can draw?
     
  3. DickCappels

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    Aug 21, 2008
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    Looks like a good circuit for modest amplitudes and power levels. By the way,

    Incidentally, you can do many AC experiments using a capacitively couple square wave too, and you might find that easier to build..
     
  4. hp1729

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    Diode, rectifier, basic power supply design, triacs. Peak vs RMS AC voltages, amplifiers, transformers ... Yes it is intentionally low and safe for those in first year electronics.
     
  5. hp1729

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    Yes, just for early basic classes. Sometimes you just gotta have a sine wave to talk about sine waves.
     
  6. Alec_t

    AAC Fanatic!

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    Ah, in that case I suggest you need some sort of current-limiting arrangement to prevent your AC supply frying when the students short-circuit something (which they will :)).
     
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  7. hp1729

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    :) good suggestion. Depending on the op amp used, parts are about $1. I keep it build up on a small breadboard like most of my "doodad" circuits. Voltage regulators, adjustable voltage references, and such. All possible to be built in under $1. The dual op amp used is a TL062 but most any dual op amp designed for + and - 15 V will do. Or you could even use two LM741s. What ever you have on hand.
    Buy prime parts? Only by exception. For the most part I buy "grab bag" parts, in bulk or the cheapest possible. Seldom I will buy prime stuff. I gotta have a good reason.
     
  8. hp1729

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    Something like this?
    It makes the cheap, disposable circuit more complicated. If I were selling a marketable product. maybe. There are single chip solutions on the market but I designed this for parts on hand.
    This brings down the output voltage a bit, too.
     
    Last edited: Feb 14, 2016
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Unless I am missing something, what is wrong to use one of the older mains transformer Wall Warts, the DC portion can be removed if necessary.
    Max.
     
  10. Wendy

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    Mar 24, 2008
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    You can buy AC only wall warts.
     
  11. hp1729

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    Very true, but battery powered was a requirement.
     
  12. hp1729

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    Very true. I suggest using what you have available. Battery powered was one requirement.
     
  13. Wendy

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    Mar 24, 2008
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    Do you want 60 Hz? A power audio amp (DIY) will meet most of these requirements.

    I found a circuit in the ebook that is similar to yours, but will have less crossover distortion. Give me a bit and I will post it.
     
  14. Wendy

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    Mar 24, 2008
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    BTW, I have a similar project in work, different emphasis. A AC signal from a single power supply.

    Wein Ocsillaror v2.png

    I have some peanut bulbs, 1.5V at 20ma.

    Be back with the other circuit.
     
  15. hp1729

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    Excellent idea! Just not anything I have on hand.
     
  16. hp1729

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    Also excellent. 440 Hz? A above middle C?
     
  17. Wendy

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    Mar 24, 2008
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    A = 440 Hz

    What most tuners use.

    This is the amp, I designed it to dovetail with the Wien Bridge Oscillator. My thought is you can take whatever works for you out of the design.

    Amp.png

    It should be a really clean AC signal, keep the battery isolated from the rest. If the input signal is near the battery voltage the P-P will be nearly twice the battery voltage.
     
  18. hp1729

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    Super!
     
  19. Wendy

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    Just merged my schematics. I'll be posting this on a separate thread sometime in the future, after I test it.

    Wein Ocsillaror v3.png
     
  20. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    Using my previous oscillator and the suggested output stages the transistors in the Q1 and Q2 positions kept self destructing. I couldn't find what I did wrong after many attempts I modified the outputs to something I knew. Yes the output was larger, as expected.

    The light bulb in the Wein oscillator ... is it the inductance of it that matters or the changing resistance, or what?
     
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