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No voltage drop

Discussion in 'General Electronics Discussion' started by Rup13, Feb 18, 2014.

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  1. Rup13

    Rup13

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    Feb 18, 2014
    Hi guys,

    I am very new to circuits and have needed to build one for my research. My professor made a schematic and then I made it and had some boards printed off. I put all the components on but when I tested it, I did not get the voltage drop across the power and ground that I should have. I am using a CP-102A-ND power jack (http://www.digikey.com/product-detail/en/PJ-102A/CP-102A-ND/275425) and when it is out of the circuit I get a voltage drop of 12.2 volts. When I solder it to the circuit, I get a floating voltage of about 1 volt. I looked for a voltage drop on every pin in the circuit and can't find anything. The power goes to a DC-DC converter (http://www.digikey.com/product-detail/en/ASA00BB18-L/454-1446-ND/2203670) and the ground is connected to a common ground for the board. One thing that I have noticed, is that if I place the wall wart jack about half way in (so that power is connected but ground is not), I can see a voltage drop from the pin on the board for power and the ground of the wall wart jack. I not really sure what this means or where to get started on trouble shooting this. If anyone has any Ideas, I would really appreciate the help.

    Thanks,
    Stephen
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Hi Stephen and welcome to Electronics Point :)

    It's kind of hard to know what you're talking about. Can you post a schematic, a PCB layout, and a photo of the assembled board?

    Use Go Advanced and click the paper clip tab to get the Upload dialog box.
     
  3. BobK

    BobK

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    Jan 5, 2010
    You are using the term voltage drop incorrectly, which makes it hard to understand what you are saying.

    I think you are saying that you have a power supply that has a voltage of 12.2 V when not powering anything, and when you use it power your board, it measures 1V. If this is the case you very likely have a short on your board.

    Bob
     
  4. Rup13

    Rup13

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    Feb 18, 2014
    Thanks for the quick replies. I don't have the computer with the layout with me but I have the other two pictures. The circle around the resistor and capacitor just mean that they will be IC sockets and you can see them on the board. I just realized that on of the wires on the SMA connector is broken but don't worry about that. I was not using this board for the little bit of trouble shooting I did. It just happened to be the first board I grabbed for the picture. For the board I tested, I put resistors and capacitors in the IC sockets and had everything ready to go. Let me know if anything else is unclear.

    Thanks for your help





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  5. BobK

    BobK

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    Jan 5, 2010
    How about telling us what you measured and why you think it is wrong.

    Bob
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    You're probably using the switched contact of the socket instead of the power contact. Have a look at the data sheet for the socket at http://www.cui.com/product/resource/pj-102a.pdf

    In the bottom left corner, there's a "schematic" showing the three pin numbers. You need to use pins 1 and 2. Pin 3 is the "switched contact". It's used (not much nowadays) when the device has a second power source, such as a battery. When there's no plug in the socket, it makes contact with pin 2 and feeds the battery power to the circuit.

    If that's the mistake you've made, it's easily fixed - just tack a bit of wire between pins 2 and 3.
     
  7. Rup13

    Rup13

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    Feb 18, 2014
    I just double checked the circuit and I have power going to pin 1 and ground to pin 2. I tried soldering a wire between pins 2 and 3 to see if it would help but there was no change. I checked the voltage at the DC-DC converter and it reads the same floating number as at the power jack (about 1 volt). Is there a chance that the converter is causing funny stuff to happen? I have checked about 5-6 of the circuits and they all do the same thing. From this, I'm guessing that its not a problem with one of the components and just a problem with how I wired the boards. Is that a valid assumption?
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    OK, I guessed wrong. I'll throw out some other suggestions.

    Have you connected the "-Vin" pins of the DC-DC converter module to the ground rail? That connection is not shown on the schematic. You need to do that, otherwise it won't do anything.

    When you plug your power adapter (wall wart) into the socket on the board, do you measure a DC voltage across the pins of the socket on the underside of the board? What is the DC voltage from the wall wart?

    Can you upload an image of the PCB layout? Or photos of both sides of an unpopulated PCB?
     
  9. BobK

    BobK

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    Jan 5, 2010
    Floating means not connected to anything. Why are you trying to measure the voltage at a floating terminal? Or are you using the term incorrectly?

    Once again I ask, tell us exactly what you are doing to get this 1V measurement. Not answering questions is not a good way to get help.

    Bob
     
  10. Rup13

    Rup13

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    Feb 18, 2014
    Here are the circuit diagram and layout.
    image.jpg

    image-2.jpg

    If the ground rail is the ground that goes to the wall wart, then yes I have. If thats not right, then I'm not sure. When I measure the voltage of the wall wart, i get 12.2 volts. If I plug the wall wart into the socket on the board and then measure pins 1 and 2 on the bottom of the board, I get the 1 volt floating voltage.

    Bob,

    What I measured is 1 volt (constantly changing) when I put the red terminal of the voltmeter on pin 1 and the black terminal of the voltmeter on pin 2 of the CP-102A-ND jack. The reason I think this is incorrect is because when I make the same measurement with the jack not soldered onto the board, I get 12.2 volts. This does not seem correct because the pins should have the same values on and off the board.
     
  11. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Your fault description sounds like you may be applying the input voltage to the DC-DC converter with the wrong polarity. Often, modules like that one include a reverse-connected diode across the input for reverse polarity protection. When you apply voltage with the wrong polarity, the diode conducts and limits the voltage to around 1V.

    Working backwards from your layout, from the socket pinout I see that your power adapter (wall wart) needs to have a "centre positive" polarity on the plug. In other words, the inner part of the plug, which makes contact with the thick prong in the centre of the socket, must be the positive output of the adapter, and the negative output must connect to the outer metal on the plug, which makes contact with the spring contact in the socket when you insert the plug fully.

    Checking the connections to the DC-DC converter, U13, I think you've neglected to mirror the "bottom view" pinout in the data sheet. The layout you posted seems to be a top view, because the component markings are readable (rather than mirrored). Check the DC-DC connector pinout carefully against the layout. You may have to do some track cutting and bodgying (that's the technical word for it).
     
  12. Rup13

    Rup13

    5
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    Feb 18, 2014
    I think you nailed it. I did not mirror the pin layout so I'm sure that is the problem. Thanks you so much for your help. I never would have realized that this was the problem.

    Stephen
     
  13. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    No problem :)
     
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