AC Confusion

Discussion in 'The Projects Forum' started by polator, Feb 20, 2014.

  1. polator

    Thread Starter New Member

    Feb 20, 2014
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    hi - i still use my old analogue Avo 8 mk6 for my model railway hobby.

    last weekend my led tv died & i assumed it was faulty caps in the psu.

    on ckecking the caps they "looked" ok,

    the psu has a single output voltage of 12vdc that checked out ok with my meter.

    however when i switched the avo to ac i got a reading of 26vac that lead me to the perhaps incorrect conclusion that the psu was faulty.

    as i'm confused can some please tell me if the 26vac reading is just due to the avo circuitry & would a digital meter just show approx 0 vac in the case i've mentioned assuming the psu to be ok?

    many thanks

    PS: dear moderator could you please move as appropriate - i was getting "blank white page" on other.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    No sure why you would be measuring 26vac. That would seem to indicate some type of failure of the supply, perhaps a rectifier or capacitor.
     
  3. polator

    Thread Starter New Member

    Feb 20, 2014
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    hi - thanks for your reply - i connected my analogue Avo to my regulated bench supply set at 5v - meter (set dc) = 5v & meter (set ac) = 10v.

    i'm just wondering if the ac readings on a dc source are peculiar to a analogue m/meter & similar would not occur on a digital meter ie if i connected a digital to my bench supply it would read 0 vac and not 10?

    i don't want to buy a good digital meter if there is no difference.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Absolutely nothing wrong with an "old analogue" meter (I had to look up "Avo 8 mk6") in fact they always give a true reading were a digital meter might show 100's of volts where none exist due to pickup thru the air.

    As far as your unit is concerned, while it depends on exwctly where you are looking 26 VAC riding on top of 12VDC sounds excessive: I can't think of a PSU section that would appear like that.
     
  5. burger2227

    Member

    Feb 3, 2014
    190
    24
    Probably a bad power supply if the TV is totally dead? Any fuses? Many will flash error warnings when you try to power them on. Count the LED blinks perhaps.

    Nowadays there isn't much the average Joe can do with surface mount stuff. That's about the time I quit fixing them.
     
  6. polator

    Thread Starter New Member

    Feb 20, 2014
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    thanks for your reply - as i mentioned i hooked my avo up to my regulated bench supply set to 5v & got 5vdc & 10vac (?)
     
  7. crutschow

    Expert

    Mar 14, 2008
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    Then it would appear your meter is doing something strange on the AC setting. Perhaps you should invest in a cheap digital multimeter.
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    You cannot use a voltmeter (analog or digital) to measure AC ripple on a DC supply voltage.

    When the meter is set to AC VOLTS setting a diode rectifier is used to convert AC to DC. Hence you will still get a reading (though incorrect) when applied to a DC source with the test leads connected the right way.

    To properly observe the supply ripple you need to use an oscilloscope.

    (You could try using a capacitor in series with the test meter to AC couple the ripple to the meter.)
     
  9. inwo

    Well-Known Member

    Nov 7, 2013
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    I don't believe that's correct. :confused: Any decent ac meter will only read the ac component.
    Even my cheapo meter reads "0" across a 9V battery.

    However I'm not familiar with OP's meter.:p
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    My DMM reads 19.2VAC on a 9V battery.
    Did you try reversing the polarity of your leads?
     
  11. alfacliff

    Well-Known Member

    Dec 13, 2013
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    I hve been using dmm's for years to check for ripple on dc supplies. from radio shack, sinclair, hp, fluke, and even the cheap harbor freight dmms, they all seem to be ac coupled, I have never been able to do this with analog meters tho, must be sonething in the rectification and are dc coupled even on ac.
     
  12. polator

    Thread Starter New Member

    Feb 20, 2014
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    thanks for your reply (& also to all the others) - so an analogue meter will not read the ac conponent of a dcv whereas a dmm will - any idea of who makes a good mid-range dmm with a cap tester? - fluke are a bit beyond my budget.
     
  13. MrChips

    Moderator

    Oct 2, 2009
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    I have a B+K Tool Kit 2704B DMM and it measures AC correctly from 10Hz to 8kHz.
    It has 5 scales of capacitance. Also measures Hz and transistor hFE.

    This model has been replaced with 2704C.
     
  14. burger2227

    Member

    Feb 3, 2014
    190
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    I recently got a Fluke on Amazon for about $30. It has a backlight and auto-shutoff.

    I have one meter that seems to jump around a lot because it has Auto-ranging. It too seems to find AC on DC circuits, but the ranges are so small it makes readings difficult. I don't think I'll ever get one of those again.

    As to your TV, you can probably purchase a new power supply module. Or doesn't it really matter?
     
  15. polator

    Thread Starter New Member

    Feb 20, 2014
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    hi thanks for the info however there seems to be a difference of opinion between you & alfacliff regarding the reading of a ac component of a dc source using a dmm - could this possibly be "make/model of meter" dependent?
     
  16. MrChips

    Moderator

    Oct 2, 2009
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    The difference is in the actual meter. Some meters (that I have tested) are DC coupled even when measuring AC.

    Put a 1μF non-polar capacitor in series with the AC meter and see what happens.
     
  17. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
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    Don't let the low price fool you.

    I also have a fluke 87 and many more.
    I use this every day.
    Look at everything it does.

    http://www.ebay.com/itm/VICHY-VC980...403?pt=LH_DefaultDomain_0&hash=item51b70ca1fb

    Paid much more from a kit catalog that I get.
    I should have shopped around.;)
     
  18. polator

    Thread Starter New Member

    Feb 20, 2014
    6
    0
  19. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
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    Yes, that's the one I referred to. Reads 0 volts ac on a 9V battery.

    There should be a USA supplier if there's a rush. That's the first one that a search brought up.

    Here:
    http://www.ebay.com/itm/VICHY-VC980...843?pt=LH_DefaultDomain_0&hash=item485efee2e3

    So many things I like.
    20amp
    freq
    temp
    ind
    cap
    Peak-hold
    auto off
    rubber holster
    nice leads
    diode check
    transistor

    It probably wouldn't survive use in the field though!
     
    Last edited: Feb 21, 2014
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