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What output voltage ?

Discussion in 'Electronics & Electrics' started by THE_PROFESSOR, Jan 29, 2015.

  1. THE_PROFESSOR

    THE_PROFESSOR Member

    Joined:
    May 25, 2002
    Messages:
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    Location:
    Sydney
    Hi All..

    I have a device that I use for testing in my day job.

    The said device has blown both it's transformers due to dropping it and creating a short. Anyways I need to identify the output voltage so I can recreate it.

    Both transformers are old Dick Smith M-2860 at max 500ma. They have two centre tapped secondaries.. 15V, 7.5v, 0, 0, 7.5v. 15v. Both primaries are blown.

    I have included a rough diag of the area I need help with.. I need to know what the DC output voltage is. Is it just one voltage..say 24 or 48 or is it two voltages with respect to earth?

    Any help appreciated.

    Cheers

    [​IMG]
    Click to view full size!
     
  2. 2xCPU

    2xCPU Member

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    my god that is ugly.
    I looks like an exam question designed to torture first years.

    ...and your (+/-) of the RH bride are reversed.

    2.
     
  3. RobRoySyd

    RobRoySyd Member

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    I hope that schematic is wrong :)

    Best guess is would be + / - 45V referenced to earth.
     
  4. sTeeLzor

    sTeeLzor Member

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    I would love to help but I cant :)

    I could severely use a whole heap of relearning my basic electronics and powerconverters from uni.

    How did you determine that RobRoy?
     
  5. RobRoySyd

    RobRoySyd Member

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    I think the thing ends up being two 30V secondaries each feeding a bridge rectifier so 30 x 1.4 = 45 roughly. Problem is the schematic makes no sense, why are the two 7.5 V taps connected together? That would cause the magic smoke to come out. I also cannot figure out why the 0V centre tap of the transformer is being used like that.

    It's more likely to be 2x 15V AC transformers feeding two bridge rectifiers to give +24V and - 24V which in turn goes to two linear rectifiers to give a +/- 15V supply for opamps.

    Knowing what the device it's from would give us a better clue.
     
  6. OP
    OP
    THE_PROFESSOR

    THE_PROFESSOR Member

    Joined:
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    Messages:
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    Location:
    Sydney
    Hi All.

    Thanks for the input..
    The right hand bridge is indeed arse about - my bad, wasn't concentrating when I drew it.

    Can confirm that the rest of the diag is exactly as the device in question.
    This is part of a power box that is used to light up a type of telecommunications switcher, so would most likely have -48VDC.
    It was scratch built by one of the guys at work who is on hols at the mo... It has been working fine for years, so not a new device just built and not working.

    Cheers
     
  7. Flat-6

    Flat-6 Member

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    Looks like you have 30VAC feeding each bridge rectifier.

    Are there any large electrolytic capacitors connected to the output of the bridge rectifiers as this will affect the voltage seen at the outputs.

    Basically with no capacitors - The DC voltage you will see on the output of each bridge will be 1.41*VRMS - 1.4 where VRMS is the voltage of the secondary winding setup [30V] and 1.4 is the voltage drop for two diodes in each bridge.

    Each output is relative to earth since each transformer's winding is earthed at one end. Is there an error on the RHS output - Is it meant to be positive or negative? If negative, you will have 60VDC output across the both bridge rectifiers.
     
    Last edited: Jan 29, 2015
  8. 2xCPU

    2xCPU Member

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    That part is ok.
    The two 0V ends are tied together (no problem there). The 7.5's are tied together - odd, but still ok (both secondaries are simply in parallel at this stage).

    From this, you can assume both 15V terminals 'could' be connected with no ill effect. So you could redraw each transformer to only have one CT winding to make it easier to visualise.

    The LH bridge is across the two '0V' terminals. I'd guess the primaries have their L&N's swapped, so this should put 30Vac on the LH bridge.

    The RH bridge is connected to the two 15V terminals - but as above, all 15V terminals are effectively ground, there should be zero volts across the RH bridge and therefore no output.

    2.

    edit: could we have some pics?
     
    Last edited: Jan 29, 2015

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