Hi All,
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....
and
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.
However......
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?
Mike Beckemeier
