Heh, all you had to say was "power factor" and I've have understood. I suspected that was the case, but it seemed too easy..
Guess I'm too used to thinking in simple ohmic terms, with Watt & Kirchhoff always looming large. Something told me that, in the end, there was no way around dealing with the E^2/R heat - anything else seemed like a thermodynamic "cheat". On Sun, Aug 23, 2015 at 11:54 AM, Steve Moulding <[email protected]> wrote: > > > -----Original Message----- > From: cctalk [mailto:[email protected]] On Behalf Of Brent > Hilpert > Sent: Sunday, August 23, 2015 3:07 AM > To: General Discussion: On-Topic and Off-Topic Posts > Subject: Re: RS-232 Tx / Rx monitoring LEDs? > > On 2015-Aug-22, at 11:55 PM, drlegendre . wrote: > > On Sun, Aug 23, 2015 at 1:17 AM, Chuck Guzis <[email protected]> wrote: > >> On 08/22/2015 10:23 PM, dwight wrote: > >> > >> I would think the reverse voltage sum of the diodes is enough. > >>> Different diodes also can handle different voltages. Since the sum > >>> of the forward voltages is enough to handle AC, I'd suspect the > >>> reverse voltages each would handle is quite small as well. > >>> The problem is when the current limiting is done with a resistor > >>> that in the forward direction drops a lot of voltage. > >>> The diode has to handle the voltage until breakdown when reversed. > >>> If the resistor was handling 1 Watts, with the right break down, the > >>> LED could be taking .5 Watts. This is more than most are designed > >>> for. > >> > >> ...and that's just the nub of it. The success of this depends > >> largely on the consistent characteristics of every LED in the string. > >> Since LEDs tend to fail short if submitted to overvoltage, I've often > >> wondered if a spike in the AC supply would precipitate a cascade > >> failure in the string. I've looked hard and there are no rectifier > >> diodes in the string--just the LEDs themselves. Probably saves about 5 > cents or so of manufacturing cost. > >> > >> I've also seen LED "night lights" from China that employ nothing more > >> than a safety capacitor (usually about 104) in series with a resistor > >> connected to two back-to-back LEDs, all across the AC line. > >> > >> I've wondered what the lifetime of such a setup is. > >> > >> --Chuck. > >> > > I've also seen C-R series voltage dropping circuits, here & there. > > > > Correct me if I'm wrong, but doesn't the series cap dissipate power > > just as it would, were it a series resistor? I mean, if the LED is > > passing 20mA, the cap is also doing 20mA - and at whatever the Vdrop is. > > > > Right? If not, why? > > I doubt if any brief explanation here is going to the topic justice. Look > up > power factor or reactive power. > FWIW: > > The impedance (capacitive reactance, Z=Xc=1/(2*pi*f*C) of the C does > produce > the desired voltage drop but the C also shifts the phase of the current > relative to that of the V. To apply the power equation P=VI properly, you > can't just multiply the RMS values together, you multiply the instantaneous > values of the V & I sine waves together through a cycle. You get a third > sine wave, that for power. If V & I are in phase, the power sine wave will > all be in the positive region and is real power consumption. When they are > out of phase, some portion of the power sine wave will be negative: a > portion of the energy the C sucked down the line is being returned during > each cycle. > > Yes, it does reduce energy consumption relative to a purely R solution. > > On a large scale, the power company doesn't like it because it > unnecessarily > adds to the currents circulating in the system, but then, this is from C > which shifts the current in one direction, so it's doing some compensation > for the inductive wall warts you have plugged in around the house, which as > L shift the current in the other direction. > > ----------------------- > > Brent, that is an excellent explanation in just a few sentences. One > quibble however. The power company does indeed like components that shift > the current in the capacitive direction. Taken as a whole for the power > grid, the power source sees the load as inductive because of all the > industrial motors it powers, including the ones that exist in almost every > home (washer, dryer, heating/air conditioning, mixer, disposal, etc.) > Capacitor banks are frequently installed in large industrial operations to > shift the inductive load more toward the capacitive power factor. This is > because the power company, as you have implied, charges more for power that > is current shifted away from zero %. I have even seen large motors > installed in industrial situations that run continuously without load, > because such motors appear as a capacitive load, and indeed are called > 'rotary capacitors'. I recall one time when the CFO ordered such a motor > turned off because "it is wasting power." It took a little plain and fancy > instruction by the engineer to let him know that it was actually saving the > company money. > > >
