12v wallwart reads 12,24vdc and 15vac?

Discussion in 'General Electronics Chat' started by redrooster01, Feb 25, 2016.

  1. redrooster01

    Thread Starter Member

    Jul 15, 2014
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    I'm having problems with my NetComm NP805N Gigabit Wi Fi router not connecting because the configuration changes itself every week or so. Ive recapped it completely and reflowed every dodgy looking solder joint. The router has had the firmware upgraded and Ive changed the crystal resonator as well but it still plays up. It will run flawlessly for over a week at a time and then go weird on me. Would this have anything to do with the power supply having 15vac output? It has the right 12.24vdc output though?
     
  2. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    I don't understand? You are putting 15 VAC into a device that requires 12.4 VDC?
     
  3. redrooster01

    Thread Starter Member

    Jul 15, 2014
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    No! I have a 12vDC wall-wart power-supply that was reading 15vac on my DMM yesterday as well as the correct 12vDC when measuring the DC. Ive since taken the W/W P/S apart and found a dry solder joint on the negative rail and it now seems to be working again fingers crossed?
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    The reading on the AC range of your meter is meaningless unless you know how THAT meter measures AC voltages. Many meters simply rectify the input voltage and scale the average voltage they read after doing so to the pure AC input voltage that would produce that average.
     
  5. redrooster01

    Thread Starter Member

    Jul 15, 2014
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    I have 2 DMMs and an analog meter that measured: 11.8vDC and 30vac. manual DMM: 12.25vDC and 13.30vac. Autoranging DMM:12.25vDC and ac going from 2.70vac to 39vac?
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    Do you see how the AC range measurements are all over the place?

    That's because each meter has it's own way of measuring AC voltage and each is almost certainly making the assumption that the input is a pure sinusoidal waveform and then transforming the actual waveform into one that has a non-zero average component, measuring that average component, and then scaling it according to the transformation that was made. Unless you know both the transformation that the meter is using AND the shape of the actual waveform, the reading from the meters on the AC scale is meaningless.
     
  7. redrooster01

    Thread Starter Member

    Jul 15, 2014
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    Its meaningful in this context because I would've thought its not supposed to be there,whatever the ac voltage is.Is there an acceptable level of ac on a digital devices power supply?
     
  8. WBahn

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    You still aren't grasping what I am saying.

    If you apply a pure 12 V DC input to most AC meters, they will give a reading because of how they go about reading a pure AC signal. The will respond to a DC signal and display a result assuming that what they read came from a pure AC source. So if your input signal is not pure AC, the result only tells you that the input signal is not 0 V at all times. Other than that, it is meaningless (unless you know both the details of the measurement circuit AND the details of the waveform).

    Either get a True-RMS responding meter or look at the output of the walwart on a scope.
     
  9. #12

    Expert

    Nov 30, 2010
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    Let me try this in different words. There is no AC voltage there. The meter is just measuring DC as if it was AC and telling you lies. You told the meter it was AC by turning the knob to AC. The meter is trying to interpret the AC but it is doing it wrong because it isn't AC voltage.
     
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  10. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Why that? Of a different frequency?
     
  11. redrooster01

    Thread Starter Member

    Jul 15, 2014
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    My computer crashed and went to black screen while I was replying and Ive just got it going again.
    so its best to measure voltage with an oscilloscope if you want a precise reading then.
    Why on earth would you think Id change the frequency? I thought it goes without saying that crystal resonators fail and changing them is a matter of course when troubleshooting.
     
  12. Papabravo

    Expert

    Feb 24, 2006
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    No. An oscilloscope will give you an accurate picture of the waveform, but not necessarily an accurate reading of the voltage. You still have to know what you are doing and how the instruments make measurements.

    Where do you get the idea that ceramic resonators fail all the time and you should just change them without any evidence? I'd be more concerned with damaging the board or lifting a trace, but that's just me.
     
  13. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Not on this planet. Statistically I'm sure one failed somewhere sometime in history, but unless you hit the router with a baseball bat, the crystals are fine.

    And - we build rugged electronic systems for the military. The US Navy has a test, part of MIL-STD-901, where they pound on things with an iron sledge hammer. Air Force - explosive decompression. Army - temperature change of 50 degrees C in one hour. Never lost a crystal.

    ak
     
  14. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Simply trying to understand what you did. Buena suerte.
     
  15. Papabravo

    Expert

    Feb 24, 2006
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    You obviously have a set of strongly held beliefs on electronics and troubleshooting. From my vantage point they do not seem to be well grounded in reality. I've never seen a bad crystal or can oscillator in a career lasting half a century.
     
    Last edited: Feb 26, 2016
  16. redrooster01

    Thread Starter Member

    Jul 15, 2014
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    Now I remember why I rarely frequent this site, precisely because of this smarmy culture that has developed whithin it. You may have modernizes the appearance and the ability to upload photos better, etc.. but the nitpicking,condescending,patronizing,sarcastic,insulting behavior continues unabated. I just checked this site on Alexa to see how its going since its been spruced up and its still bad with a total of 1.19 minutes viewing time a day and no other statistics really because there is only 2 people a day coming here on average. So congratulations you've sh*t in your own nest and driven them all away.
     
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  17. #12

    Expert

    Nov 30, 2010
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    All this because crystals are reliable?
    We better shut this site down. The time I spent here in the last two days accounts for over 2 years of site activity.:eek:
     
  18. Evanguy

    Member

    Dec 21, 2014
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    lol, good one man. these people were trying to help, you fail to understand how DMM's read ac, and what a sine wave looks like.
    yes a dmm will read an ac voltage off a 12v battery its because of how it calculates and how it assumes things.

    YOU told the meter it was AC so the meter then believes you its ac and figures you are feeding the DMM a alternating current but it just alternating at a very slow pace like .0000001 hz, so its assumes eventually it will pass 0

    if you told the meter the truth, and set it to dc it will tell you the dc volts.

    go read the posts at the top of this post OR google how a DMM read ac voltage, and how it assumes it will cross 0 even if its DC and it NEVER crosses 0
     
  19. WBahn

    Moderator

    Mar 31, 2012
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    Only 2 people a day coming here?

    Really?

    And you actually believe that?

    There are, at this moment, 55 members online plus an additional 490 guests (and this does not include the 74 bots).

    Just look at your own thread. You started it less than 24 hours ago and you have 17 responses from 7 different people in that time. So how can you possibly believe that there are only 2 people a day visiting this site, on average?

    If you don't like the culture here because it doesn't bow down and worship you, then you are free to leave and never look back. Instead, you throw a little tantrum and blame everyone else for your own insecurities. Look at the bright side, if you leave now there will only be one person a day visiting the site and you get to condemn that person to responding to their own posts -- what an opportunity for you.
     
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  20. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    You should have worked on the system I worked. I have seen plenty. Granted it was a piss poor component design. The crystal was in a ceramic case. The leads went into the case, with little to no strain relief. The crystal was pinched between the leads. A good jolt would cause the crystal to shift and throw off it's rated frequency or cause it to fail altogether.

    I tuned and or repaired bunches of boards at our client at Montreal Bordeaux Prison. I could never figure out why I had to repair so many from this particular customer till I happened to see it happening on one of my regular PMs. I was in the guard office when an alarm came in, the TTY started to print. I started hearing an unusual noise looked over and the equipment case housing our alarm system was swaying like a hula dancer. I yelled "That's why you are sending me so many data receivers! Get that printer off the top of that case"! After that day I got no more data receivers from that client.
     
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