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Question about auto-volt adapters for electrical appliances

  1. Apr 25, 2012 #1
    Some appliances have built in auto-volt transformers that enable them to be used in different countries with different voltage standards for power transmission. Do the auto-volt transformers use a coil spring coupled, motor controlled tap that can change the number of turns from the secondary inductor that the load is connected to depending on the voltage fed to the motor or is it possible to use a solid state control on the auto-volt transformer?
     
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  3. Apr 25, 2012 #2

    NascentOxygen

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    What appliances do you have in mind? I'm sure there is no moving part. But I don't know how it is achieved. It could be entirely managed on the secondary, so that the DC output voltage is maintained at a set value. The appliances you have in mind all use DC, don't they? They probably convert the mains AC → DC → high freq AC → transformer → switching DC regulator
     
    Last edited: Apr 25, 2012
  4. Apr 25, 2012 #3

    jim hardy

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    many modern switching supplies can handle such a wide range of input voltage that they dont care what is input voltage so long as it's between about 100-300 volts. They immediately rectify it to DC then step that down to whatever they want with a high frequency switching regulator. Read up on switch mode power supply, SMPS.

    Daughter's 120 volt flatscreen Vizio TV used a 450 volt capacitor in its SMPS. 450/√2 = 318 volts, so it could easily handle 240.
     
  5. Apr 25, 2012 #4
    The problem is not in the voltage threshold of the power supply itself but the appliance it is going to power. The appliance needs a specific low voltage to operate so how does the auto-volt transformer ensure that the output stays fixed at this level even as the voltage is increased?

    There is this type of transistor that supposedly switches off the current from the power supply when current is delivered to the base. It acts like a NOT gate so that when current is delivered to it through the base, there will be no electrical output. It is denoted by a circle in front of the base terminal.

    May it be possible then, to use this transistor, in addition to a conventional transistor, to control the output of the secondary inductor? The NOT transistor will be connected to the one of the terminals of the inductor while the regular transistor will be connected to a tap which is then connected to the center of the inductor.

    The transistors will have resistors connected to their base terminals to modify the threshold voltages so that if a low voltage goes into the transistors, the terminal connected to the NOT transistor will be switched on and the terminal connected to the center tap will be switched off, allowing the full number of turns of the inductor to be used when the source voltage is low.

    But when sufficiently high voltage goes into the transistors, the NOT transistor will switch off the connection to one of the terminals and the regular transistor will switch on the connection to the center tap, causing the high input voltage to be sufficiently lowered to the fixed output voltage by halving the number of turns of the output inductor.

    The circuit diagram is below:

    http://img232.imageshack.us/img232/4265/autovolttransformer.png [Broken]
     
    Last edited by a moderator: May 5, 2017
  6. Apr 25, 2012 #5

    NascentOxygen

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  7. Apr 25, 2012 #6

    jim hardy

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    Last edited by a moderator: May 5, 2017
  8. Apr 25, 2012 #7
    But circuit diagrams for electronic auto-volt adapters show that AC adapters have transformers.

    hvps4.jpg

    Additionally, there are variable voltage AC adapters that use manually operated variable voltage transformers that are controlled by a dial.

    These transformers, whether manual or automatic have a low wattage rating so they are very small.
     
  9. Apr 26, 2012 #8

    NascentOxygen

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    The two-winding transformer is usually a safety measure, it is included to isolate the user side from the mains. An autotransformer (50/60Hz) is unlikely here because it adds cost and weight while affording no isolation, so no safety.
    By a switch, aren't they? I think you'll find these switch an extra number of turns in/out of the primary winding to accommodate the different line voltages--it's still basically a two winding transformer.
    The transformers you see in modern AC adapters are high frequency transformers, and so can be small and light weight while offering comparatively high power transfer. They are integral to the switched-mode power supply.

    That said, I see no reason why it shouldn't be possible to devise a low power tap-changing arrangement to regulate the AC on the secondary of a multi-winding 50/60Hz isolation transformer. MOSFETs would make good bipolar tap-changing switches. It would be costly and require specially wound transformers. Except for niche applications, I doubt that it is done.

    It was not clear whether your original post was a concept idea from your imagination, or based on an actual design you had read about. :smile: Which?
     
  10. Apr 26, 2012 #9

    jim hardy

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  11. Apr 27, 2012 #10
    The circuit is an original design used for the conceptualization of a variable transformer operated by solid state switches.

    These types of variable transformers are mechanically operated. Is there one that is automatic and transistor operated?
     
  12. Apr 27, 2012 #11

    jim hardy

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    I've seen them motor operated.

    And i've seen industrial transformers that have multiple taps with thyristor switches to select among the taps.

    http://www.google.com/url?sa=t&rct=...b01Oc1RoJwbHvJMwQ&sig2=I2b8P9srvxoHfUSwJwD_aA

    But appliances within my experience are much simpler and operate as in post #6.

    see if this link works.
    http://www.google.com/url?sa=t&rct=...J3r2S3-l_KE6zpRFQ&sig2=2iZ8-otIL6u_l9JB_gvTSQ

    It should get you to Solatron's "Power Solutions Desk Reference" .

    They pretty much "wrote the book".
     
    Last edited: Apr 27, 2012
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