Suppression and interference.
From: NCE-DCC@yahoogroups.com [mailto:NCE-DCC@yahoogroups.com]
On Behalf Of fbcupp Sent: Monday, 27 June 2005 4:48 AM To: NCE-DCC group
Subject: [NCE-DCC] FM RADIO INTERFERENCE.
I also have a larger layout and find little FM interference unless the radio is within about 10 feet of the rails. Twisting the wires is great for the feeders, but you still cannot physically twist the rails.
1. I did have more problems before I added "Snubbers"
to reduce the "Ringing" effect at the ends of each branch of the power districts. (I have 3.) The snubbers are a .01 to 0.1 MFD capacitor in series with a 150 ohm resistor. (Not critical values.) This is placed across the feeder at the end of the feed bus.
2. This may not be important, but I have a pair of #1156
tail light bulbs in parallel with each other, This is then placed in
series with one feeder, right at the output of the booster. This
is a cheap version of current limiter, good for about 3.5 amps. This added impedance at the feed point may also be a factor
in reducing the higher frequencies, ie. the FM radio band. Since this is a series circuit, it is only needed in one side of
------------ Original Message ----------------------
Date: Sun, 26 Jun 2005 05:06:39 -0000
From: "mec701" <email@example.com>
Subject: FM radio interference
wired the first one and had a little FM Radio interference on the radio in the same room, but none else where in the house. Today I wired the second district and have considerable interference with an FM radio in another room. This radio has an exterior antenna which is on the other side of the house. Can't wait to hook up the 3rd district to see what will happen (neighbors maybe). Anyway, is there anything I can do to the NCE unit or wiring to lessen the interference. I am winding the buss wires and my layout is a 12 x 10 around the room design. David Stickney
Michael Beckemeier wrote on 22 August 2005 Subject: [NCE-DCC] Power bus terminators
I recently scoped the DCC signal at the far end of my booster districts and saw considerable ringing and excessive overshoot on the rising edges of our DCC signal.
After reading previous post on this subject and checking out the data on the below links, I decided to see what effect they had. To see a learn more about blown decoders and long bus runs and a RC filter solution....
Well I must say I was stunned by the remarkable difference they made in cleaning up the overshoot and ringing. I used a 330 ohm 1/2 Watt resistor in series with a 0.1uf 50V capacitor. With this in place my Tony's DCC RR Volt ammeter no longer gave odd readings at the power district either, as it was now seeing a cleaner
DCC wave form.
I have found that the 130 ohm resistor is quite HOT. Not warm, Hot!
In looking closer at the un-terminated DCC wave on the scope, I see that the nominal voltage is 14.5 peak, but the overshoot voltage is 35-40 volts peak for a duration of about 200 nanoseconds. Lets see....
80v p-p / 130 ohm = 0.615 Amps
0.615 amps x 80v = 49 Watts !
However this should only be for the 200 nanosecond duration of the spike that the resistor would ever see this much power, so I never suspected that the temperature would rise so quickly for a small duty cycle. I was surprised that the resistor got so hot.
I did an experiment to see what wattage resistor would not be hot to the touch. Not having a 1 or 2 watt resistor handy, I put 4 of the 130 ohm 0.5 watt resistors in a series / parallel arrangement, which still measured 130 ohms.
With this I could feel some minor warmth from the individual four resistors, but nowhere near the heat off the 1/2 Watt resistor when it was the only one in series with the cap. I suspect that I will have to search for a 2 watt resistor. Has anyone else who has used this terminator noticed how much heat this circuit gives off?