Looking for an advice to build an isolation transformer.

Discussion in 'The Projects Forum' started by kodoocha, Sep 14, 2012.

  1. kodoocha

    Thread Starter New Member

    Jul 13, 2012
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    0
    I have few questions about isolation transformer.

    Can I use a low power rating isolation transformer to test higher rating machine? For example, most desktop PC has around 450W, which is maximum power output. My computer consumes between 20W to 95W.

    The country I live has 220 AC power grid. If I want to build an isolation transformer with two identical power transformers, what should I look for? Below is specification of a power transformer on a supplier. 50 VA is equal to 50W power rating?

    I found information that building an isolation transformer is just to connect the secondary winding of two identical transformers. I haven't find a through building instruction or guide yet.

    If I plan to build an isolation transformer that has 200W power rating, do I only need buy two power transformers of 200VA and few basic components - power on/off switch, output terminal, a case, fuse?


    Thanks in advance.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    3,234
    Can't you just buy a 220V-110V transformer? They are a commonly available and cheaper than using two back-to-back transformers.

    A 50 VA transformer can handle a 50W resistive load. It's less for a load with a power factor less than one. Generally you probably don't want to connect a load with a power rating greater than about 75% of the VA rating.
     
  3. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    The following could be your reference:

    >50 VA is equal to 50W power rating?

    They are the same, just represent in the different kinds of components.

    A few basic components -

    1. AC Power wire.
    2. Power on/off switch.
    3. Power on Indicator. (Neon indicator as : http://shop.rabtron.co.za/catalog/images/R777R.jpg)
    4. output terminal.
    5. A case.
    6. Fuse.
    7. Fuseholder. (https://www.egr.msu.edu/eceshop/Parts_Inventory/images/fuse holder2.jpg)
    8. Transformer x 2.

    Output Require :
    Output Voltage : 110Vac.
    Output Watts : 200W.

    Output Current : 200W/110V = 1.818 A.
    Add 20%~30% of calculation output current for spare.
    Total Output Current : 1.818A x 20%~30% = 2.1816A ~ 2.363A
    Total Output Watts : 110V x (2.1816A ~2.363A) = 240W ~ 260W.
    Porbably you can buy two 220V/110V/2.5A transformers.

    How many current you should buy, depends on how many watts do you want.
     
  4. kodoocha

    Thread Starter New Member

    Jul 13, 2012
    4
    0
    Thanks crutschow.

    I opened up my step down transformer and took photos.

    Specification:
    AC power: 2kVA
    Frequency: 60Hz
    Input voltage: 220V
    Output voltage: 110V
    Secondary current: 18.2A
    Insulation type: E

    Light blue wire is primary power wire to primary coil.
    Brown wire connected to the fuse holder is ground.
    Yellow and naked copper wire is power wire from secondary coil.

    Am I understand correct?

    How to connect two transformers for making isolation transformer?

    Three wires go into the coil

    [​IMG]

    Rear side
    [​IMG]

    [​IMG]
     
    Last edited: Sep 14, 2012
  5. debe

    Well-Known Member

    Sep 21, 2010
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    184
    That transformer is NOT an Isolation T/F, it only has 3 wires its an auto T/F. An isolation T/F has 4 wires, 2primary & 2 secondary.
     
  6. kodoocha

    Thread Starter New Member

    Jul 13, 2012
    4
    0
    Here wiring diagram I just made.

    The power on/off switch has three terminals: T1, T2 on top, T3 is bottom.
    T3 is connected to blue wire of the cable.
    T1 and T2 is connected even power off state.
    When switch is on, T1, T2, T3 are all connected.

    T1 is connected by yellow wire to O1. A coil wire C1 is connected to the O1.
    C1 and yellow wire are soldered on the O1 terminal.

    [​IMG]

    @debe

    You are right. This one is 220V to 110V step-down transformer.
    I like to know whether I can buy two of these for making a isolation transformer.
     
  7. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    Two kinds of Isolation T/F:
    1) 1:1 - 220V,0V to 220V,0V (One T/F)
    2) 2:1 - a)220V,0V to b)110V,0V → c)110V,0V to d)220V.0V (Two T/F)

    * b)110V connect to c)110V
    * b)0V connect to c)0V.
     
  8. debe

    Well-Known Member

    Sep 21, 2010
    946
    184
    This is an Isolation T/F. there should be 2 seperate windings to isolate the primary from the secondary. See test with megger, no resistance (leakage) betwean primary & secondary. No you cant make an isolation T/F out of 2 Auto T/Fs.
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,014
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    An isolation transformer, by definition, has no electrical connections between the primary and secondary.
     
  10. kodoocha

    Thread Starter New Member

    Jul 13, 2012
    4
    0
    @debe

    Thanks. That makes me move on.
     
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