AC/DC ground?

Discussion in 'The Projects Forum' started by Nevermored, May 30, 2012.

  1. Nevermored

    Thread Starter New Member

    May 30, 2012
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    I'm working on a project using micro to measure AC voltages (produced by a magnet and coil). I'm using a half-wave rectifier since I just want to see what voltages can be produced. When I connect the (-) lead of the AC to my DC ground, the values being recorded are incorrect.

    As a test I used a function generator to input a sin wave with 2V P2P (with the (+) as the input to my diode and the (-) connected to the common ground) and I was getting voltages from 0 to 1.8. Account for the voltage drop across the diode, I should be getting values from 1-Vf to 1V.

    When test using DC values, they are read correctly, so I'm thinking it's a problem with AC/DC grounding issue. I'd rather not use a full wave rectifier. Is there any way to make this work?

    Thanks!
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Your problem is not AC/DC grounding.
    Your signal that you are attempting to measure is half of a sine wave (0 to 1V) minus the diode voltage drop.

    Both the value of the sine wave voltage and the diode voltage drop are changing.
    The diode voltage drop will begin at 0V at 0V input and will gradually increase exponentially as the current increases.

    What you need is a precision rectifier followed by a peak detect and sample and hold circuit.
     
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  3. Nevermored

    Thread Starter New Member

    May 30, 2012
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    That makes sense. Thank you. Why do I need the peak detect, though? Wouldn't a precision rectifier with a sample and hold circuit do the job? Just curious.

    Any recommendations for parts that I'd be able to pick in-store at Fry's or any other places? I don't really care about the price at this point, expensive is okay as long as it bundles together as many parts as possible. I've never used any parts like this before, so I figure asking a pro would be better than making a mistake in choosing parts. I'm looking at peak voltages of about 30V.

    Thank you so much for the quick reply!
     
  4. MrChips

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    Presumably, you are using an ADC on a micro to determine the amplitude of the sine wave.
    You need to created a constant voltage to get a proper measurement. To to this you will need to charge a capacitor to the peak of the sine wave voltage and then take the measurement.
    After taking the measurement you have to discharge the capacitor with a reset circuit.

    The parts you need are simple and readily available: op-amp, diode, resistors, capacitors, and transistor.

    A circuit diagram would be in order or you can look up precision rectifier and peak detect circuits.
     
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  5. Nevermored

    Thread Starter New Member

    May 30, 2012
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    I'm looking up the precision rectifier, peak detect, and sample and hold circuit. I don't want to spend time building a circuit. Would a RMS-to-DC converter work for this? It seems to be a common enough conversion, I'd think there was an IC for it or something.

    Sorry about spamming you. Thank you so much for your help!
     
    Last edited: May 30, 2012
  6. t06afre

    AAC Fanatic!

    May 11, 2009
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    Maybe or maybe not. It will depend on the specifications and the signal you want to measure. What is the amplitude and frequency of the signal you want to measure
     
  7. MrChips

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    Analog Devices used to manufacture a PKD01. I don't know if it is still available.
    But you said you wanted readily available components. The circuit is simple enough that you can roll your own. Treat it as a learning experience.

    I don't think these are components that you will find at Fry's, but certainly at Radio Shack, Digi-Key, Mouser, Newark and many other electronic suppliers.
     
  8. MrChips

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  9. Nevermored

    Thread Starter New Member

    May 30, 2012
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    It won't exceed 30V, and the average voltage produced will probably be around 0 -10V. I'm browsing some of the RMS-to-DC right now and it seems that 5V is the highest they go. But that wouldn't matter because I can just put a voltage divider before sending the signal to the convertor?

    The frequency is variable too but I can say for sure that it won't be too high. Less than 40Hz.

    The max voltage and frequency I gave are way out there but there IS a very slight chance that it could occur. Most of the time, it would max at 15V and 20Hz.

    Also, do you know of any places that would sell these in-store? I've looked at Jameco, Fry's, Radio Shack. Jameco does stock a few but it's only web exclusive.
     
  10. MrChips

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  11. t06afre

    AAC Fanatic!

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  12. Nevermored

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    May 30, 2012
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    Unfortunately, I'm kind of on a deadline. I dug myself into this hole. I was chatting with an engineer about my project and he told me to use a full-wave rectifier. It made sense to me, so I didn't look it up. I worked on other parts of the project, which are working just fine. So I have about 12 hours to fix this part. If I'm going to be out and about for op-amps, diodes I might as well see if I can get something that won't take time to build. I've worked with all those components before, it's just a time crunch issue. Building it, debugging, ect.
     
  13. t06afre

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  14. Nevermored

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    May 30, 2012
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    I don't understand. Why would I want a buffer stage? I should put in a buffer stage depending on the resistance of the convertor? :S
     
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