what is RMS voltage?

Discussion in 'General Electronics Chat' started by ozgen92, Dec 25, 2012.

  1. ozgen92

    Thread Starter New Member

    Dec 25, 2012
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    Hey guys, i have a question about the logic of Vrms. I have the same homework with the following thread;

    http://forum.allaboutcircuits.com/showthread.php?t=78351

    basically I have to transfer a duty cycle from a transmitter to receiver with a LD. I'm thinking of using RMS values of these duty cycles because I have to distinguish duty cycles with different D but same peak in the receiver side.

    Therefore I'm going to ask the logic of this whole Vrms thing. In fact can't quite picture how it will work in real life. I don't think that signals will became a some kind of average voltage named Vrms as the frequency increases and I wonder if they do.


    are there an op-amp combination to convert a duty cycle into Vrms?
    or do the non-ideal components can't change their value too fast and behave as they are on average values?
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    Vrms is arrived at by answering the following question:

    I have a time-varying voltage v(t). Imagine applying that voltage, obtained from an ideal voltage source (i.e., no internal resistance) to a resistor R. There will be some average power delivered to this resistor. Now imagine replacing this source with an ideal voltage source V. What would V have to be in order to deliver the same average power to that same resistor? If you work through the math you will discover that the value of V can be obtained by first squaring v(t), then finding the average (mean) value of this squared voltage, and then taking the square-root of that mean value. In short, the root of the mean of the square, or RMS. Hence Vrms is nothing more than the DC voltage level that would deliver the same power, on average, to a purely resistive load as the time-varying signal of interest.
     
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  3. ozgen92

    Thread Starter New Member

    Dec 25, 2012
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    So say we have a alternating voltage of sint which we connected to a LD (laser diode). Than we have to divide Vrms of input with 2 since half of the period won't produce any work. Am I correct?
     
  4. WBahn

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    Remember that Vrms is nothing more than a description (and just a partial description at that) of a waveform independent of how that waveform is being used. It is based on the power that would be dumped into a resistive load, not an LED. If you want a comparable description that is specific to a particular device, say and LED instead of a resistor, then you need to specify that and use the power relation for that device, which is very different from a resistor because the relationship between voltage and current in an LED is significantly different compared to a resistor.
     
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  5. crutschow

    Expert

    Mar 14, 2008
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    The power dissipated in a relatively constant voltage load, such as an LED, is better determined by using the average value of the current waveform rather than the RMS.
     
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  6. ozgen92

    Thread Starter New Member

    Dec 25, 2012
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    So I cant use Vrms for a nonlinear element such as LED since it is a description for ressistive loads only.

    Then what is the brightness of LED is proportional with? would average value of current*voltage of LED would work?

    If the average current is proportional with the 'average brightness', and if I put some LDR in front of the LED in a dark room, can I also approximate such that LDR will hold an average ressistance that depends on the average current on LED?
     
  7. WBahn

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    Mar 31, 2012
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    You can use Vrms if it makes sense to use Vrms for what you are wanting to use it for. Just like peak voltage and average voltage, it is merely a number that describes one aspect of a waveform. The question is whether that number, irrespective of what it represents, is a useful metric for what is of interest to you.

    For a given LED, I suspect that the output power is reasonably linear, at least over some decent range, with input power and input power is reasonably linear with respect to the average forward current through the device.

    LDRs are frequently used in "digital" or "switching" applications where the linearity of the response is not a concern. Hence the data sheets often only spec a dark resistance and an illuminated resistance under a specific level of illumination and little else. LDRs also tend to be pretty slow.

    What you should probably look into is a photodiode or phototransistor. You should also consider a modulation scheme other than a multilevel amplitude one. First off, if you use iR your receiver is going to be bathed in IR from lots of sources and you need to filter it out. A common way to do this is to use a data carrier at about 40kHz and then only accept signals that are near that range. If you want to send multiple data streams with minimal interaction, then choose two frequencies close enough to be in the same bandpass filter but far enough apart so as not to beat excessively. That will depend, among other things, on the data rate you need to achieve.
     
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  8. ozgen92

    Thread Starter New Member

    Dec 25, 2012
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    I can't use a photodiode or photoressistor, only basic things +LD+LDR are allowed. and I'm not sure if modulation scheme is necessary because I will only transfer 10 01 11 or 00. Not anything more complicated (this class is some kind of introduction to circuits). Actually adjusting the frequency may help.

    when you say "LDRs also tend to be pretty slow." do you mean its ressistance changes slowly, compared to the voltage change of the alternating voltage?

    Therefore do you think it may work if i assume that it will have different and constant ressistance values in accord with the Vrms given to the Laser Diode?

    (i will use a LD of a pointer, so it will be the cheapest LD you can find)
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    It's probably time to start tiinkering with some circuits and discovering the answers to the questions yourself. You now have a good start on some of the questions to be exploring the answers to. Make some measurements and let's see what you come up with and then we can discuss ways to refine things.
     
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