Something interesting with a 9V AC/DC adaptor

Discussion in 'General Electronics Chat' started by Rolland B. Heiss, Feb 24, 2015.

  1. Rolland B. Heiss

    Thread Starter Member

    Feb 4, 2015
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    A few weeks ago I brought home a 9V AC/DC adaptor they were going to throw away at work because the cord was shorted. I took it apart leaving long enough positive and negative wires after cutting the cord in order to experiment with it a bit. The first thing I did back then was create one heck of a bright light inside a glass jar with a graphite rod after plugging it into the wall. Worked great for a few moments before the graphite rod basically blew up! :)

    So anyway, there this thing sat for weeks after that. Today I thought about it again in such a fashion as my thoughts seem to go:

    "When I plug this into a 120V AC outlet it puts out DC at 9V. Hmmm...? What if I pushed DC power into it from the usual output wires? Would it turn into AC? Ok, I'm trying it. Didn't work or do anything so hey Rolland, another stupid idea of yours man! Besides, the transformer is a step down transformer anyway buddy! Yeah, that's true so let's try this... run a 9V DC into the usual AC input area and see what happens! Hmmm... the first time it rose to almost 8V and after disconnecting the battery, well, on the other end of the capacitor it is going from 7.88V to 7.87V and so on every two seconds or so. Let me discharge the capacitor and try again! This time it gives a quick 5V charge. Hmmm. Let me disconnect the wires and try once more. Odd! Once I took the negative lead off of the battery while the positive lead is still connected it is still charging! What happens if I connect the negative lead back up? I connected it back up and the voltage rose some. Interesting? What if I take the negative lead off and rub it on the terminal with the positive connected? Wow! It rises for some reason!"

    This is basically what I witnessed happen today and I'm not sure why. The adaptor contains only a diode bridge, a capacitor and a transformer... apart from the input and output wires of course. What was going on here? Any ideas?
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    It's too hard to follow what you did and what you saw without some kind of a schematic.

    Also, when you say "it" rose to almost 8V, what is "it"? If it's the voltage on a particular node, what node was the voltage referred to?

    And when you say that something is going from 7.88V to 7.87V "and so on every two seconds or so", what is the "so on" mean? That a couple seconds later it went to 7.86V. And then to 7.85V.

    You need to be much more precise with your descriptions.
     
    planeguy67 likes this.
  3. MrChips

    Moderator

    Oct 2, 2009
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    Electrolytic capacitors have a memory effect.
    When you discharge a capacitor the voltage across the capacitor temporarily goes to zero.
    Then the memory effect in the dielectric comes into play. The voltage will gradually creep up. I think that is what you are observing.
     
    planeguy67 likes this.
  4. Rolland B. Heiss

    Thread Starter Member

    Feb 4, 2015
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    I've just created a video but it may take some time to upload it in order to share it. Please keep an eye out for the video once I get it uploaded and help me explain what I perhaps poorly tried to tell everyone I witnessed happen.
     
  5. WBahn

    Moderator

    Mar 31, 2012
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    Someone else will have to help -- I don't have the bandwidth for viewing videos.
     
  6. ScottWang

    Moderator

    Aug 23, 2012
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    No bandwidth?
    Did you use cellphone to get on line?
     
  7. Rolland B. Heiss

    Thread Starter Member

    Feb 4, 2015
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    Ok, the video finally uploaded! Actually it uploaded somewhat rapidly but the processing took longer than the upload which caused a great deal of frustration on this side of the words being typed!

     
    KLillie likes this.
  8. WBahn

    Moderator

    Mar 31, 2012
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    No. I live in the mountains and our only options are dial-up (very slow), satellite (very slow and strict download limits), or microwave (slow and unreliable due to lack of decent line of sight). Using a cellphone is not an option because we barely have cell service at all here and it is very poor (<1G level of service and very unreliable).
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    I was able to see your video (we've had good connection for the past week and are getting a reasonably stable 1 Mb/s) and what is happening is when you rub one of the leads from the battery you are creating an AC current that is getting coupling through the transformer over to the bridge. If you connect the battery and just leave it, the output voltage will probably decay to zero. The decay is slow because you have a filter capacitor discharging through a 10MΩ resistance (your meter). The charge you are getting initially is due to the transient current spike from connecting the battery. That is also why you are getting an increase in voltage as a result of removing the battery lead. Everytime you disconnect and reconnect the lean from the battery you are pumping current into the output circuit -- basically what you have is a crude switch-mode power supply.

    Also, why do you always insist on using dead 9V batteries?
     
    Last edited: Feb 25, 2015
    Rolland B. Heiss likes this.
  10. ScottWang

    Moderator

    Aug 23, 2012
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    I just upgrade the dial up speed, now is upload speed about 2Mb/s and download speed about 6Mb/s, the cost less than US$20 each month, it was also included wifi in the modem, before that I need to using another wifi modem, although I was used dynamic IP, but I can get on line for 3 days and the tele company will break it down(off line), but it doesn't affecting a lot, because I already setup the function for auto dial up when the line was broke(off line).
     
  11. WBahn

    Moderator

    Mar 31, 2012
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    That's not dialup. More than likely that's DSL. We are too far away from the nearest switching station to get DSL, so our dial-up speed is limited to 53kbps at most.
     
  12. ScottWang

    Moderator

    Aug 23, 2012
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    Yes, that is DSL, but it still needs dial-up, although the Tele company called it Optical fiber, but it just change the modem from ADSL to DSL, and it still used the same telephone line as ADSL.
     
  13. WBahn

    Moderator

    Mar 31, 2012
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    DSL is not dial-up. Dial-up uses audio signals over a standard telephone line. DSL is frequency division multiplexed onto a standard telephone line. Most DSL connections are continuously connected, but that's not a requirement.
     
    djsfantasi likes this.
  14. ScottWang

    Moderator

    Aug 23, 2012
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    I'm not sure what kind of situation in your local, but here the DSL is a dial-up, maybe it was as your said continuously connected, but the Tele company used software to break the internet connection automatic, so everytime only could keeping 3 days.
     
  15. Rolland B. Heiss

    Thread Starter Member

    Feb 4, 2015
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    LOL! Because that's about all I can manage to experiment with after the bills and the gasoline and the taxes and the newly imposed penalty for not having health insurance and so on. Heck, one day I might even eat something! ;)
     
  16. darrough

    Member

    Jan 18, 2015
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    In order for a magnetic field to induce a voltage in a nearby wire, it has to be changing. In fact the voltage induced is proportional to the rate of change in the magnetic field.

    Faraday's law of induction: The induced electromotive force in any closed circuit is equal to the negative of the time rate of change of the magnetic flux enclosed by the circuit.

    Connect a wire to a voltmeter. Set a magnet next to it. The meter reads zero. Put the magnet on a axis and spin it. The meter will show a voltage. Spin it faster and the meter will show more voltage.

    The consequence of this is that a transformer has to have alternating current going in and coming out.
     
    Rolland B. Heiss likes this.
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