Small Audio Transformer to amplify low frequency sine wave

Discussion in 'The Projects Forum' started by idok, Sep 16, 2015.

  1. idok

    Thread Starter New Member

    Sep 10, 2015
    5
    0
    Hi All,

    Im building a 5 hz sine wave circuit using op amp 741,capacitors and resistors. the amplitude of the output would be a bit less than 5 volt, now i want to connect the circuit output to electro optic[EO] crystal so it would operate, the crystal works at 600 volt.

    I need to make this device as small as possible, i thought to connect an audio\signal transformer to circuit output in order to amplify to 600 volt(crystal is isolated so no worries about current that anyway is reduced after passing the transformer)

    can someone help me find the step-up transformer suitable to the device specs? or what would be the more important specifications to look for?
    another question, is it possible to use a step down transformer that has high turns value as a step up transformer(use output as input and vice versa)?

    Thanks
     
  2. #12

    Expert

    Nov 30, 2010
    16,354
    6,852
    That would resemble a transformer labeled, "120:1" and capable of working at 5Hz.
    Very unusual. Not likely to find one already built.
    Also consider that the power level will be very, very low because of the LM741 op-amp current capability limitations.
    If the op-amp can deliver 20 ma, your output current would be less than 42 micro-amps.
    And, yes, you can run a transformer "backwards" with no ill effects.
     
  3. recklessrog

    Member

    May 23, 2013
    338
    102



    Also the losses in the transformer without a fair amount of iron at 5Hz seem to make this impractical without increasing the drive to the primary, also as #12 states, a transformer may well be labelled for a particular application, but its "turns ratio" and the type of core and general construction that make them application specific. google transformer/ coil design and you will find it's a fascinating branch of electronics
    which sometimes seems like a black art (black magic?) good luck
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Here's a possible solution using a standard (albeit not small) transformer:
    A standard power transformer can operate at a low frequency as long as the voltage is proportionally reduced.
    For example, a 60Hz transformer can be run a 1/12th its rated voltage at 5Hz.
    Thus a 120Vrms unit could operate at 10Vrms (14.1Vpk) with a 5Hz sinewave.
    The ratio you would need is then 600V/10V = 60:1 which would be a 120v to 7200V transformer.
    Here you can buy 120V to 7500V neon sign transformers (30mA or 60mA) which should work for you (you might also be able to find a used one at a neon sign shop for a low price).
    Thus if the input voltage is amplified to 600/62.5 = 9.6V@5Hz you will get your desired 600V output @ 5Hz.

    Alternately they sell units with 9,000V to 15,000V outputs which would allow a smaller drive voltage (but at a larger current) to get 600V output.
     
    Last edited: Sep 18, 2015
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  5. dannyf

    Well-Known Member

    Sep 13, 2015
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    Very difficult to do at 5Hz.
     
  6. recklessrog

    Member

    May 23, 2013
    338
    102
    Why don't you drive a valve? valves are voltage ampifiers (ECL82 is a triode/pentode or EL84/EL34 pentodes) all these can work at high voltage, then your transformer requirement is far more practical. In fact you could use the primary of a valve output transformer and take your signal directly from the anode as long as you use a suitable capacitor to block the D.C. Although not technically correct, get a decent 240v primary 350/400v secondary mains transformer, use the primary as the anode load and if your transformer has a center tap and plenty of iron in the core, at least you will be able to make a start.
    Do remember that the high voltage, + any smoothing capacitors etc can be a source of fatal electric shocks, work with one hand and be extremely careful. Discharge any hv capacitors by using a (5w 10k) resistor on a pair of insulated probes. I have a pair of multimeter leads connected to 3x2.5k 5w w/w resistors in series in an insulated case that I made about 45 years ago that I use when working on high voltage equipment. Only ever use it for discharging, don't leave it permanently connected or you will have a nice electric fire.

    You could of course use a high voltage MOSFET but these devices tend to blow up unless the circuit is very carefully designed. Valves are far more rugged and can withstand short period abuse without suffering catastrophic failure.
     
    Last edited: Sep 19, 2015
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  7. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hello there,

    There are a few things to consider when using a transformer, and one of them is the excitation current. The excitation current exists even with no load, and it is not necessarily a sine wave so you could see a lot of distortion, if you even can drive it at all with a little 741 op amp.

    My recommendation would be to design a high voltage output stage. That way you can drive it easily with 10ma and get a nice clean output and you dont have to worry about such a low frequency like 5Hz anymore.
    You didnt mention how clean the output had to be though for your application.

    If you still feel better about using a transformer, then simply built a better driver...bridging between the 741 and the input of the transformer...which could take you from say 5v peak to 20v peak or more, maybe 100v peak without too much trouble.

    I might have suggested two transformers, but your power input is so low this is not likely to work very well without a special design.
     
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  8. idok

    Thread Starter New Member

    Sep 10, 2015
    5
    0
    Hi Guys,

    Thanks for all your help.

    What if i change the frequency to 150 HZ, would it be feasible ?

    is there small transformer that can work under the condition above? (the demand for stteping up 5 vpp to 600 vpp remains)

    Thanks again
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,813
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    How much current does the 'EO crystal' require?
    Does it have to be energised with AC, and in particular a sinusoidal waveform?
    What will the end use of the crystal be?
     
  10. idok

    Thread Starter New Member

    Sep 10, 2015
    5
    0
    the crysatal is an isolator(so no current is necessary) and it used for phase shift of sinusoidal wave.

    so AC is required here.
    the crystal would be the load of a circuit and would use as phase shifter for 2 incoming signals while one is for measuring and the other is for control(depends on how the signals would shift)
     
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    In that case I'd do what has been suggested above: use a high voltage valve or semiconductor amplifier. The EO crystal could be AC-coupled as the amplifier load, avoiding the need for a transformer.
     
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