RS-232 Tx / Rx monitoring LEDs?
drlegendre at gmail.com
Sun Aug 23 12:31:59 CDT 2015
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
On Sun, Aug 23, 2015 at 11:54 AM, Steve Moulding <fti1983 at xmission.com>
> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Brent
> 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 <cclist at sydex.com> 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
> power factor or reactive power.
> The impedance (capacitive reactance, Z=Xc=1/(2*pi*f*C) of the C does
> 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
> 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.
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