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Electronic Multimeter help required

Discussion in 'General Discussion' started by SexyGreyFox, 12 May 2010.

  1. SexyGreyFox

    Man of Honour

    Joined: 29 Mar 2003

    Posts: 52,134

    Location: Stoke on Trent

    On saturday I looked down at my guitarists power supply to his Line 6 effects pedal and noticed it sparked down by the wires coming out of it.
    Anyway, I seperated the wires from each other and taped them down just to get us through the gig until I could get it home, hacksaw it open and redo the wires.
    It is a 9v 2000ma power supply so when I put it back together I got my multimeter and put it to the + and - on the plug and expected to see a reading of 9v but I didn't (it wasn't even 1) :confused:
    I realised I'd got the very same power supply to my Line 6 Variax and put the multimeter across that one but that didn't read 9v either :confused:
    I tried the repaired power supply with my equipment and it is OK.

    Why aren't I seeing a reading of 9v?
     
  2. Mucky_Pup

    Mobster

    Joined: 21 Jul 2008

    Posts: 4,818

    Location: Hebburn

    Stupid question?

    Meter in AC? Supply in DC? Or doesn't draw load until it's plugged into equipment?
     
  3. Antar Bolaeisk

    Sgarrista

    Joined: 19 Dec 2006

    Posts: 9,279

    Location: Saudi Arabia né Donegal

    Were you wiring the voltmeter in line with the supply?

    You need to measure voltage across a load.

    Current is measured in line with the supply.
     
  4. Fuzz

    Capodecina

    Joined: 3 Aug 2003

    Posts: 15,302

    Location: Cheltenham ;-)

    Test the voltmeter on a known 9v supply like the terminals of a battery for eg..
    I predict user error. :D
     
  5. SexyGreyFox

    Man of Honour

    Joined: 29 Mar 2003

    Posts: 52,134

    Location: Stoke on Trent

    That was the very first thing I did so I could see and be prepared for a 9v reading.

    When I first cut the wires and stripped them I put the multimeter across the two wires first before I put the plugged end back on but again didn't see a 9v reading.
     
  6. Dinomania

    Mobster

    Joined: 21 Apr 2003

    Posts: 4,328

    An ideal voltmeter has infinite resistance, or an open circuit - so that when you connected it between the + and -, there was no actual load added to the circuit, no current flowing, no voltage difference.

    As said above by Antar Bolaeisk you need a load in order to complete the circuit and see a voltage difference across it.


    Ninja Edit: Hmm. Am a bit wrong - if say you had some random load in the circuit and then added the voltmeter in series, your reading would be pretty rubbish. Across power supply poles it ought to be fine..
     
    Last edited: 12 May 2010
  7. Hotwired

    Sgarrista

    Joined: 17 Aug 2009

    Posts: 8,511

    Eh? What kind of multimeter are you using then.

    A voltmeter should give a reading if stuck into the terminals of a battery or the terminals of a power supply because there is a difference between the two and it would also give a reading of the voltage drop across a load.
     
  8. SexyGreyFox

    Man of Honour

    Joined: 29 Mar 2003

    Posts: 52,134

    Location: Stoke on Trent

    If the power supply charges the effects unit as it should then it is putting out 9v 2000ma (!) so surely putting my multimeter across the + and - should be like putting them on a battery + and - !!
    Or are you saying that to test it I would need to open up the effects unit and put the multimeter across a couple of points in there?
     
  9. Dinomania

    Mobster

    Joined: 21 Apr 2003

    Posts: 4,328

    It's possible I'm being a divot. Always possible ;)

    But dmpoole's a smart man so I assumed it can't be his error!

    I take it the power supply was plugged in and everything, and the voltmeter's not recently been dropped onto something hard...
     
  10. SourChipmunk

    Sgarrista

    Joined: 9 Nov 2003

    Posts: 9,477

    Location: The Motor City

    This is the first thing I would have thought also. Surely a normal voltmeter is going to be able to measure the voltage potential between + and - on the tip of the connector. It sounds like the voltmeter is switched to AC, or the measurment unit is too high (it's reading .9v x 10 instead of 9v x 1).
     
  11. SexyGreyFox

    Man of Honour

    Joined: 29 Mar 2003

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    Location: Stoke on Trent

    I'm not good with this stuff so bare with me.
    Surely if I have set it up to read a 9v battery and it shows 8.97v in the display then surely putting it across the + and - on the plug (while the supply is switched on) will also show a reading close to 9v?
     
  12. geuben

    Wise Guy

    Joined: 23 Mar 2006

    Posts: 1,740

    That's what one would expect....could it be that the plug is broken?
     
  13. Dinomania

    Mobster

    Joined: 21 Apr 2003

    Posts: 4,328

    No, wait...

    OK, I'm being a spanner.

    In series with anything else, a voltmeter reading will be garbage. But a complete open circuit with a pure power supply should indeed produce a decent reading.

    Either, your voltmeter is broken.

    Or, there's something else, some element of the circuit, inside the power supply which ends up being in series with the voltmeter when you try to measure the voltage.
     
    Last edited: 12 May 2010
  14. SourChipmunk

    Sgarrista

    Joined: 9 Nov 2003

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    Location: The Motor City

    Either the PS is 9v AC, or you have two weird power supplies. I'd get a fourth opinion from another power supply. :)
     
  15. SexyGreyFox

    Man of Honour

    Joined: 29 Mar 2003

    Posts: 52,134

    Location: Stoke on Trent

    No because I have 2 identical power supplies and both power the effects unit properly.

    1 - test the meter and set it up with a 9v battery
    2 - Plug the power supply into the effects unit to see it's working
    3 - unplug and put the meter across the plug
    4 - do it with the 2nd power supply
    5 - no 9v readings off either
     
  16. geuben

    Wise Guy

    Joined: 23 Mar 2006

    Posts: 1,740

    Any chance of a picture of the power supplies, showing any info written on them.
     
  17. Cybermyk

    Soldato

    Joined: 31 May 2005

    Posts: 6,850

    Location: Peoples Republik of Teesside

    It's an AC-AC adapter, output is 9V ~ 2000mA 18VA I have a couple of them too :)
     
  18. neodude

    Mobster

    Joined: 18 Nov 2007

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    Location: West Lothian

    You need to switch the meter to AC dude. Batteries are DC, the Line 6 PSU is AC. A DC voltmeter will read an AC voltage as close to 0.
     
  19. SexyGreyFox

    Man of Honour

    Joined: 29 Mar 2003

    Posts: 52,134

    Location: Stoke on Trent

    This is my meter.
    When I'm checking batteries I switch it to the left on number 20 and when checking equipment leads I put it on the the symbol between hFE and 200 (continuity).
    There is no switch on it and of course it's a £4.99 job (in fact I got two for £4.99).

    [​IMG]
     
  20. neodude

    Mobster

    Joined: 18 Nov 2007

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    Location: West Lothian

    It needs to be turned to the right. Either the 600 or 200 scale. 200 would be the obvious choice to pick up 9V but at that scale it might not be sensitive enough.