In reply to John Winterflood's message of Wed, 15 Nov 2006 01:54:51 +0800: Hi, [snip] >With regard to Johnson noise, if you short or open the resistor, then >the entire 4kT watts generated is simply dissipated back into the >sourcing resistor as heat and there is no net power flow. If you load >it with a matched resistance then you can draw off half of this power, >but if the resistor you load it with is at the same temperature, then it >also generates this same power back in the first resistor and again >there is no net power flow. > >Coupling to it via a transformer is no different to using a different >value of resistor as the source - the voltage to current ratio changes >but the power available remains constant. Similarly connecting many >such resistors in series or parallel simply changes the impedance (or >voltage to current ratio) without changing the available power.
It isn't the transformer that is meant to have an effect. It's the diode. > >A diode is not of course a very good switch and has a gently changing >V/I slope (ie impedance) near zero bias. Which is precisely why you put the transformer in between. That shifts the voltage up the curve of the diode away from the zero bias point. However you would need an incredible transformer ratio, and the resulting minute current on the diode side may be "lost" in the noise of the diode. This depends somewhat on whether or not these purported signals from the resistor can be "ganged" together. Since they would have random phase relative to one another, they would likely at least on occasion enhance one another leading to a "spike" that might be transformed and rectified. >Thus it must also generate >Johnson noise by the same mechanism (whenever there is a path for >electrical power to be dissipated as heat, then there is the reverse >path in which the heat bath can generate electrical power - this is >called the "fluctuation dissipation theorem" in physics). Presumably >this noise power source/sink will vary slightly in impedance with the >voltage/current fluctuations The transformer "transforms" the impedances, so that there is a deliberate mismatch between resistor and diode. >- but I am sure nature will have organised >it such that no configuration you can dream up will allow net power to >be generated from thermal energy! A solar cell already does this, it just operates at a higher "ambient" temperature. Its built in diode, acts like a 0 K heat sink. Regards, Robin van Spaandonk http://users.bigpond.net.au/rvanspaa/ Competition provides the motivation, Cooperation provides the means.
