New transformer for millivoltmeter

Discussion in 'General Electronics Chat' started by Treeman, Mar 26, 2015.

  1. Treeman

    Thread Starter Member

    May 22, 2014
    158
    15
    Hi,
    I've just been given a 110v millivolt meter . A MadellCA2171. In UK I need 240v units so I wondered if just changing the transformer is all I need to do?
    On the main board is an L7815 regulator. There is nothing written on the transformer.

    I'm guessing but would a 240/16v transformer be the ticket?
    The current transformer has 2 wires out to the board.

    Do I need au uprated fuse?
    Thanks.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,418
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    Is the transformer internal on the unit?
    Or is it an external power adapter?

    Yes, you can change the transformer. Or you can get a 240/120 step down transformer which might come in handy for similar situations.
     
    Treeman likes this.
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    if you find a 240 volt transformer, you should use a fuse 1/2 the current rating of the 110 volt fuse.
     
    Treeman likes this.
  4. Treeman

    Thread Starter Member

    May 22, 2014
    158
    15
    Ok I have removed the internal unit and this is what I have (see pic).
    2 questions.
    1. Can I use this transformer. The Brown live lead is joined to the Blue and red indicating to me at least this is a 240/110 transformer divided up to get 110v.
    (2nd blue red pair go to fuse) and the two green to board.

    2. Do I need to examine anything on the circuit board to get me the correct voltage output (lm7815 reg on board)? Or is anything close to 15v OK to use.

    Thanks.
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    Good chance you are correct.
    Before you disconnect the red and blue wires, label the wires:

    #1 - RED connected to BROWN
    #2 - RED alone
    #3 - BLUE connected to BROWN
    #4 - BLUE alone

    For testing only:
    Separate the RED and BLUE from the BROWN wire.
    Connect #1 RED to AC LIVE.
    Connect #2 RED to #3 BLUE.
    Connect #4 BLUE to AC NEUTRAL.

    You need to get the phase correct.
    Test the transformer disconnected from the unit to verify your voltages.

    BTW, the mains wiring might be wired incorrectly.

    This is the correct way to connect AC mains circuit:
    The mains LIVE (AC mains brown) MUST be connected to the fuse.
    From the fuse to the POWER ON/OFF switch.
    From the switch to the transformer.
    From the transformer to the NEUTRAL (AC mains blue).
     
  6. Treeman

    Thread Starter Member

    May 22, 2014
    158
    15

    I was wondering about the 'correctly' above - looking at other items around the house they are wired 'incorrectly' (1970's amp, this millivolt meter). Is this a change of wiring regulation or just the safest method?
     
  7. MrChips

    Moderator

    Oct 2, 2009
    12,418
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    I have seen factory built instruments wired with the fuse on one leg of the transformer and the power switch on the other leg.

    Consider the consequences:

    Case #1
    If the fuse is on the LIVE AC MAINS, then when the power switch is OFF the transformer is still LIVE.

    Case #2
    If the power switch is on the LIVE AC MAINS, then if the fuse blows the transformer is still LIVE if the switch is on.

    The correct method is the fuse MUST be the first component that is connected to LIVE AC MAINS. The next component would be the power switch. If either fuse or switch is open LIVE AC MAINS is cutoff from the rest of the circuit.
     
  8. Treeman

    Thread Starter Member

    May 22, 2014
    158
    15
    _IGP7734.JPG Don't worry Mr Chips - I have been thinking of the consequences unlike the Manufacturer! Att. are the completed Pics. It took me ages but was a good experience so thanks for the help; if I hadn't asked then I would have just put it back as before and made it even more dangerous.
    Always good to ask.

    There is no resistance between Earth Pin and chassis. Tick.
    Any other checks for safety?
    Output to board is 20vac. Sound right?

    _IGP7734.JPG
     
  9. MrChips

    Moderator

    Oct 2, 2009
    12,418
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    Wrong.

    The GROUND tab from the AC MAINS connector must be connected to a ground lug fastened to the chassis. You can put the ground lug on one of the bolts used to secure the transformer.
     
  10. Treeman

    Thread Starter Member

    May 22, 2014
    158
    15

    That's what I have done in photo 2 Mr Chips. Photo 1 was to show the transformer wiring. It came up as a fullsize pic and wouldn't thumbnail again. Sorry if that confused.

    What measurements do I need to take to check safety please?

    The unit turns on but the transformer seems a bit stinky (resin?) and warm after a minute or two. Its all unplugged and everyone is still alive. The wife seems unhappy.....
     
    Last edited: Mar 28, 2015
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