How does it actually work? (noob multimeter question)

Discussion in 'General Electronics Chat' started by jbuddyman, Jun 29, 2016.

  1. jbuddyman

    Thread Starter New Member

    Jun 27, 2016
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    Hi, I am a young hobbyist, and am new to the all about circuits community.

    So I was tearing apart some multi meters and I thought to myself " Since multi-meters can measure AC, then why is there no transformer inside?" I know that transformers are meant to step up OR down AC voltage. Say for an example, I measure mains 120V AC. How is this meter reading that, but when I tear it apart, there is no transformer? Maybe there is, but I just am not seeing it. I am hoping someone can help me figure this out, or even educate me a little bit.


    To all that reply, thanks for being patient with me, and helping me out. I don't know a whole lot! :)
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Analog to Digital Converter (ADC).
     
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  3. jbuddyman

    Thread Starter New Member

    Jun 27, 2016
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    I have heard of ADC'S, but I don't know too much about them. I will read up on them. Thank you so much for answering my question! I like this community. Have an awesome time!:)
     
  4. #12

    Expert

    Nov 30, 2010
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    Transformers can work with AC, but so can resistors, capacitors, and inductors. There is no law that says only one kind of device can work with an AC voltage.
    If you just connect your AC voltage to the right couple of resistors, the voltage will be reduced to the range that an ADC can work with. Resistors are good because they don't change their behavior with frequency changes. Capacitors and inductors do. You can still make a voltage divider with capacitors or inductors, but you immediately have to be aware of what will happen in different frequency ranges. A resistor will sit there like a lump, doing nothing but resisting, clear into the gigahertz range, but capacitors and inductors can get really bad at a few MHz. Self resonance, diminished capacitance, that sort of thing.
     
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  5. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Depending on the sophistication of the instrument, AC can be either rectified first and then digitized.....or digitized directly from the AC with a high speed sampler, and then numerically calculated. There are advantages to both methods.
     
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  6. KL7AJ

    AAC Fanatic!

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  7. jbuddyman

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    Jun 27, 2016
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  8. hp1729

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    Nov 23, 2015
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    ADC? Only if the rest of the meter is a microprocessor. A rectifier circuit. Not just a rectifier but an AC to equivalent RMS converter. An op amp circuit without the loss of a diode.
     
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  9. #12

    Expert

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    It's called a Precision Rectifier. The diode is in the feedback loop and so the op-amp amplifies even the tiniest voltage difference correctly.
     
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  10. ian field

    Distinguished Member

    Oct 27, 2012
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    Some older multimeters did have a transformer - but I think it was for current measurement and only on some of the ranges.

    Analogue meters have a fairly simple rectifier arrangement so its just another DC measurement as far as the movement is concerned. They usually used a germanium diode for its low Vf.

    DMMs have a precision rectifier - on very old ones it was an op-amp and a couple of diodes. Nowadays its all integrated into a dedicated chip along with the ADC and display drive logic.
     
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  11. jbuddyman

    Thread Starter New Member

    Jun 27, 2016
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    Thanks for letting me know that. You and AACFanatic I think nailed it, and made me understand this mystery. I don't use analog meters, even though my dad had one and he let me use it a long time ago. Now all my meters are digital. I knew there were some rectification, and precision resistors involved, but I didn't know what did the stepping down. I was just wondering how the meter can read 120V since that's a pretty high voltage. Thanks everyone, and I welcome everyone's perspective on it. All multimeters are different however, so we should always keep that in mind.
     
  12. ian field

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    Some meters go up to 1000V or even 2500V - Google "multiplier resistance".
     
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  13. crutschow

    Expert

    Mar 14, 2008
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    And, of course, you can add a high voltage probe (voltage divider) to measure voltages in the tens of kV range.
     
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  14. jbuddyman

    Thread Starter New Member

    Jun 27, 2016
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    That's true! I want to say those kind of probes you are explaining are cat 4 or 5 rated probes. Good stuff
     
  15. hp1729

    Well-Known Member

    Nov 23, 2015
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    Exactly!
     
  16. KeepItSimpleStupid

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    Mar 4, 2014
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    The method used in a cheap DVM is generally based on full wave precision rectification and an averager. The averaging circuit has some frequency dependence. The average value is multiplied by a "fudge factor" to make the value of an mathematically averaged sine wave equal to the RMS value of the voltage.

    Here are some offerings of Analog Devices on their TRMS converter chips: http://www.analog.com/en/products/linear-products/rms-to-dc-converters.html

    This http://www.linear.com/product/LT1088 technology isn't used anymore, but was good to 100 MHz.

    One important parameter in some circuits is the crest factor. See: https://en.wikipedia.org/wiki/Crest_factor

    It's too bad that the side doesn't show the "precision rectified average which starts with the flipped negative axis. The true average value of a sine wave is zero. The average value of the abs (sin(x)) is not.

    TRMS (True RMS) maters usually allow you to read just the AC component and the DC+AC component.

    The RMS value of a voltage will be the equivalent DC value needed to satisfy ohms law with a resistor. e.g the same power dissipation is a resistor.

    Oscilloscopes offer AC and DC coupling which probably should be read as AC and AC+DC, but DC means "Direct coupled" in this context.

    RMS means root mean squared and literally, that's what it is. The average of the sqrt of the waveform squared.

    EDIT: Added and corrected stuff since the like by the TS.
     
    Last edited: Jun 30, 2016
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  17. ElectronicMotor

    Member

    May 1, 2016
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    OH Boy !
     
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  18. jbuddyman

    Thread Starter New Member

    Jun 27, 2016
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    Lol
     
  19. WBahn

    Moderator

    Mar 31, 2012
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    That's not an AC issue. After all, the meter can read 120 V (and higher) DC voltages and that doesn't see surprising to you (since you didn't ask about it). Some multimeters (particularly older multimeters) simple convert AC to DC and then use the same circuitry that was used to measure DC voltages, but with an appropriate scaling factor. On an analog meter, the scaling was done by simply printing a difference scale on the meter face.
     
  20. ian field

    Distinguished Member

    Oct 27, 2012
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    It was probably EPE that published a project about a year or so ago.

    Designed for 25kV CRT TV EHT. Housed in a length of plastic waste pipe was a long narrow PCB with carefully arranged resistors.
     
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