power supply and battery issue

Discussion in 'Homework Help' started by dk31, Nov 9, 2014.

  1. dk31

    Thread Starter Member

    Oct 30, 2014
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    Hi all, first post here
    I 've been reading on electronics the last month (ARRL, Platt and the textbooks here) and I was about to try the " relay oscilator" experiment tutorial (Platt 2nd chapter), when all strange things happened one after the other...
    1) I had an old DC 3amp power supply adjustable for 3 to 12volts. When it didn't turned on the relay, i checked volts out and it read 17 volts! Then I turn the volt knob to 9V and it still reads 17 volts. I turn off the supply switch and still 17 volts, verified with two multimeters. Last, I unplugged it from the wall outlet and the volts wouldn't go. Probably a capacitor malfunction but do these symptoms make any sense? (e.g typical indication that the supply or the volt knob is dead)

    2)So I thoutgh lets see if the relay works with a 9V alcaline battery (the relay is a DSY2Y-S212L and is supposed to work at 12V). Didn't work, and when i checked the battery voltage it was 7.5V and counted down at a pace of 0.05V/second. I stopped at 6.9V and checked with another multimeter and it was 7.5V steady. Then I connected both multimeters and they both showed a fall 0.05V/second! Maybe the first multimeter draws some current while measuring voltage, but again does it make any sense? Is this any indication of the multimeter quality?
     
  2. WBahn

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    Mar 31, 2012
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    You second case sounds like it might be a multimeter issue. You might configure the "good" multimeter as a current meter and measure the current drawn by the "bad" meter when measuring the voltage.

    What kind of DC supply are you using. It may require a minimum load in order to regulate. If it was build up from an old PC power supply this was often the case. But a load on it that should draw at least a good fraction of an amp and see what happens.
     
  3. dk31

    Thread Starter Member

    Oct 30, 2014
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    Thanks.
    I checked it and the good multimeter shows an 6mA draw from the (empty) battery. Is that too bad?

    The (funny) supply is of medium size, bigger than a wall charger and smaller than a PC supply. It is rated 3A and it has a knob to select voltage. After the previous post, I light up a small led and voltage came down to 12V. It stayed at 12V for the entire session (leds and relay messing around). So I guess that's all? Just a tiny load and it starts working? The knob must be dead as it doesn't change anything (volt was always 12V after the load and 17V before the load)
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Does the supply have a model number? Manufacturer name?
     
  5. dk31

    Thread Starter Member

    Oct 30, 2014
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    I will check and post back as soon as I get back home
     
  6. WBahn

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    What kind of battery is this?

    It will be hard to draw any conclusions from measurements on a drained battery. You need to use a fresh battery, or at least one with significant life left.
     
  7. dk31

    Thread Starter Member

    Oct 30, 2014
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    It's a drained 9V alkaline battery. When I measured fresh batteries, all was normal with both multimters
     
  8. WBahn

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    If you were starting with a 9V alkaline that was only producing a no load voltage of 7.5V, then all bets are off. That battery is dead and isn't going to be able to put out much current at all, so the current draw that will result in a rapid voltage drop and the one that will result in a slow voltage drop might well be quite close together.

    Even so, a multimeter measuring anything in the single-digit voltage range shouldn't be drawing anywhere near 6 mA. Something is wrong. I should be drawing more like 1 uA to perhaps 10 uA.
     
  9. dk31

    Thread Starter Member

    Oct 30, 2014
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    Thanks - I didn't know that alkalines behave like that.

    The multimeter that draws 6mA is a small one - if size makes any difference. About 5 times smaller in volume than the other multimeter.
     
  10. WBahn

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    What's the make and model of the small meter? We can probably find the specs online. But most multimeters today, even the <$10 ones, have at least 10 MΩ of resistance on the voltage scales. Are you sure it is set to read volts and not millamps?
     
  11. dk31

    Thread Starter Member

    Oct 30, 2014
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    I will check the model and post back, along with the model of the power supply. It's all cheap stuff but I am yet too early in the learning process to justify a Fluke :)
     
  12. JoeJester

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    There are cheap knockoffs whose specifications in the megaohms.
     
  13. dk31

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    Oct 30, 2014
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  14. WBahn

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  15. JoeJester

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  16. WBahn

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  17. dk31

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    Oct 30, 2014
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    So to resume, power supply needs a load to start normal functioning (already veryfied that) and drained battery voltage readings say nothing about multimeter quality.
    Thank you guys, you ve been very helpful.
     
  18. JoeJester

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    I stand corrected.

    Thanks Bill.
     
  19. WBahn

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    That power supply may need a minimum load current to regulate, but not all power supplies do. The specs "should" state a minimum load requirement and these don't, but they only give "key" specs. Most wallwart type supplies don't regulate without some minimum load and since it's cheap Chinese crap, it's not too surprising that they don't give all the information that is needed.

    As for the multimeter, it should not be drawing that kind of current on the voltage setting, so you do need to explore that some more.
     
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