RS-232 Tx / Rx monitoring LEDs?

Brent Hilpert hilpert at
Sun Aug 23 13:17:48 CDT 2015

On 2015-Aug-23, at 9:54 AM, Steve Moulding wrote:
> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at] 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 <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.

Sure, L shift predominates in the system, I was just speaking in general terms at that point.

The power companies may be playing it both ways though:
I was once told a story of a small mill or manufacturing co. and an electro-plating company situated next door to each other. But the mill was inductive (motors) and the electro-plating was effectively capacitive (not sure how that worked), so they pretty much cancelled each other out and weren't contributing to line losses for the power company. The power company was none-the-less charging both of them for poor power factor (industrial users having special power meters that track power factor).

As you mention rotary capacitors, a few miles away from me is a large gas-fired generating station that doesn't get used much for generation as we are primarily hydro in BC. But the hydro generation is distant from here - here being the primary load centre (Vancouver area). They unhook some of the generators at the gas-fired plant from the turbines and run them as rotary capacitors.

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