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Thread: operating frequency

  1. #21
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    Rob Withey hey how do you play mp3's in dos??

  2. #22
    Raw Wave Rob Withey's Avatar
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    Quote Originally Posted by cirvin
    Rob Withey hey how do you play mp3's in dos??
    Using a modified port of mpg123 (see my sig and webpage).


    Rob
    Old Systems retired due to new car
    New system at design/prototype stage on BeagleBoard.

  3. #23
    Raw Wave Rob Withey's Avatar
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    Quote Originally Posted by Ricky327
    Diodes are semiconductors, they don't have resistance.

    Sure they still have internal resistance. Thats one of the reason that limits the amount of power it can handle. A high power semiconductor do have very low internal resistance when they conduct...so it enable them to pass higher currents
    OK, basic electronics course...

    This is a diodes IV characteristics:

    http://www.st-andrews.ac.uk/~www_pa/...hars/chars.htm

    Notice how the current sky rockets in forward bias. If you want to talk about "effective resistance" the "effective resistance" is going to continually change at the different current flows. The expression "resistance" is pretty much meaningless with a diode as you need to qualify it with current flow. It would be like trying to define your car engine's power from the acceleration driving up a hill and knowing what gear you were in. You'd have to know the gear (to know the revs), and the slope of the hill (so you knew how much power was being burnt in overcoming gravity).

    This is a resistor's IV characteristics:

    http://www.st-andrews.ac.uk/~www_pa/...r/resistor.htm

    Notice how the voltage drop is directly proportional to current flow. The slope of the line is defined by the resistance (V=IR). A resistor is a very efficient device at converting current to voltage, and vice versa. This is useful for many things, such as feedback loops (potential dividers), and current flow measuring systems (such as current limiting on regulators).

    There are plenty of references out there that explain it better than me. I recommend "The Art of Electronics" by Horowitz and Hill. It's a bit chunky, but quite comprehensive.


    HTH

    Rob
    Old Systems retired due to new car
    New system at design/prototype stage on BeagleBoard.

  4. #24
    FLAC DodgeCummins's Avatar
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    http://www.faqs.org/docs/electric/Semi/SEMI_3.html

    Diode ratings
    In addition to forward voltage drop (Vf) and peak inverse voltage (PIV), there are many other ratings of diodes important to circuit design and component selection. Semiconductor manufacturers provide detailed specifications on their products -- diodes included -- in publications known as datasheets.

    Maximum total dissipation = PD, the amount of power (in watts) allowable for the diode to dissipate, given the dissipation (P=IE) of diode current multiplied by diode voltage drop, and also the dissipation (P=I2R) of diode current squared multiplied by bulk resistance. Fundamentally limited by the diode's thermal capacity (ability to tolerate high temperatures).
    http://www.ecmweb.com/ar/electric_ba...ate_devices_3/

    http://www.allaboutcircuits.com/vol_3/chpt_3/3.html

    http://www.geocities.com/waynem48/electric/scrtemp.html

    http://www.process-heating.com/CDA/A...,74558,00.html

    In my 3 years of electronics classes, this is the kind of info that I received. No device has 0 (zero) resistance...even the powerline going to your house has resistance.
    E (voltage drop) = I (current) * R (RESISTANCE)...
    P (power) = I (current) * E (voltage drop)

    That P in watts is in the form of heat. In a perfect world we would have pure conductors with no voltage drop across them. But since we don't, we need heatsinks.

    It is interesting that your link said that Ohms law does not apply...that may be so, but it seems to work out well mathmatically.

    However, I am not an engineer, so there may be better answers from an engineering aspect, but my answers seem to match a number of sources also.



    (I can just imagine there are folks reading this, going "damn all I wanted to know was whether to use a 20gig hd or 40gig hd.")

  5. #25
    Raw Wave Rob Withey's Avatar
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    P=IE

    That's your basic diode's dissipation. There is no R in that equation. It is based entirely on I (current) and E (potential difference, usually Vf). The bulk resistance dissipation is usually insigificant when compared with this.

    Since the I is not linear with V, describing it as resistance doesn't make a whole lot of sense.

    I've been messing with electronics for about 20 years now, including a 3 year degree which included a fair wedge of electronics. When I'm designing a circuit, P=IE is what I use to determine the diodes dissipation. Vf is what I use to find the voltage drop. If you wanna do it a different way that's up to you. ;-)


    Rob
    Old Systems retired due to new car
    New system at design/prototype stage on BeagleBoard.

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