RS-232 Tx / Rx monitoring LEDs?

Chuck Guzis cclist at
Sun Aug 23 11:06:48 CDT 2015

On 08/23/2015 07:10 AM, dwight wrote:
> I've used the capacitor method to provide most of the drop in the
> past. I don't usually max out the LEDs at 20ma. I find there is
> little difference between 10 and 20ma. Yes, the 10 ( or 20ma ) is
> current flow through the capacitor. It is necessary to have some
> resistor in series as well to suppress line spikes.

Another similar dirty trick back in the day was to run a 6SL7 
dual-triode form the line using a 1.0 uF nonpolar capacitor in series 
with the line to provide a supply for the 500 ma 6.3 v heater and then 
use one of the triode sections as a half-wave rectifier.  You thus had 
the other triode section for whatever stupid purpose.  Of course, this 
was horrible abuse of the tube, particularly in the area of 
heater-cathode voltage ratings.  It probably wouldn't work as well in 
220VAC countries, but it worked well enough in the 120VAC ones.


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