On 03/18/2010 05:58 PM, Harry Veeder wrote: > > > > > ----- Original Message ---- >> From: Jed Rothwell <[email protected]> To: [email protected]; >> [email protected] Sent: Thu, March 18, 2010 5:22:20 PM Subject: >> Re: [Vo]:circuit diagram >> >> Stephen A. Lawrence wrote: > >> By the way, I should say "Thanks!" for taking the time to post all >> these here. It's interesting, even if I don't believe for a minute >> that it's OU. > > Someone should communicate the >> gist of the comments here to the > author of the video. Tell him to invest in >> an ammeter, for crying out loud. > > - Jed > > I am ignorant about electronics but I don't see what the fuss is > about since it is all DC current. If you know the resistance and the > voltage can't you safely infer that as the voltage rises and falls so > does the current?
This isn't DC (and the load is mostly inductive, not resistive). Check out the scope shots, and look at that circuit diagram again. He's feeding the AC output of the signal generator to the gate of the FET which in turn chops the DC from the battery to produce AC. It's asymmetric AC (goes up from zero then back down to zero) but it's AC none the less; you can view this sort of chopped signal as a symmetric AC signal *plus* a DC signal, where the DC signal is the average, or "offset", voltage. It's the AC component of the signal which is going to "go through" the transformer and out the other side. The power consumed will be the integral of the product of the input voltage and the input current. Power is coming from the battery, where the voltage is (nearly) fixed but the current is varying wildly (in a rough square wave with, I suspect, some pretty substantial peaks), and power is coming from the signal generator, which is producing a neat square wave of *voltage* but, depending on the capacitance of the FET gate and the frequency, may be providing current in a rather squirrely waveform. The actual voltage wave forms are square waves, rather than sine waves. I said (in some earlier post) that those are harder to understand. They can be viewed as being a sum of an infinite progression of ever higher frequency sine waves, and it's the extremely high frequency "components" which are buried in the square wave which make its behavior peculiar. In particular, those high frequencies are likely to "leak" through the FET gate, which can be viewed as a tiny capacitor. The current "leaking through" the FET will be visible as a current drain from the signal generator -- but that hasn't been measured (or, at any rate, that measurement hasn't been shown). Without knowing the current drain from the batteries and the signal generator, we know exactly *nothing* about how much power is being provided to the circuit. > > I still think that in certain "simple" circuits voltage measurements > can serve as a pretty good indicator of current and power. Yes, but this circuit is anything but simple. With a partly inductive load and AC voltage, you don't necessarily know, a priori, whether the current and voltage are even going to be in phase with each other. An aside: When people talk about RMS voltage, they're talking about taking the "square root of the mean squared voltage". That's a useful measurement in exactly one case, which is the one you're thinking of: A purely resistive load. As long as the load is (or acts like it is) purely resistive, the current will be linear in the voltage, and the instantaneous power will consequently be proportional to the square of the voltage. The average power will, then, be proportional to the average of the squared voltage. And the square root of the average power will be the "equivalent" DC voltage which would put the same power as your AC waveform into a given resistive load. But that "equivalent" DC voltage -- the RMS voltage -- might behave very differently from the original AC wave form if the load is not purely resistive. This circuit is an example where knowing the RMS voltage applied to the circuit doesn't tell you much of anything at all. > > Harry > > __________________________________________________________________ > Yahoo! Canada Toolbar: Search from anywhere on the web, and bookmark > your favourite sites. Download it now http://ca.toolbar.yahoo.com. >
