RS-232 Tx / Rx monitoring LEDs?

Steve Moulding fti1983 at
Sun Aug 23 11:54:26 CDT 2015

-----Original Message-----
From: cctalk [mailto:cctalk-bounces at] 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> 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.

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.

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