capacitor in sound only installation

[cliffsrr]

I have two units with MRC 1633 sound decoders that try as I might they still seem to stutter as they hit spots in the track that may cause a little hiccup in the DCC signal. I am wanting to try a capacitor in the hook up but don't know the value of the capacitor or where to wire it to. I am thinking across the two wires that feed the signal to the decoder. Any suggestions appreciated.

Cliff

[rrinker]

 Definitely NOT across the track power connection. A keep-alive capacitor has to go after the voltage regulator on the decoder. Where this is on an MRC decoder I couldn't tell you without careful examination.

                                           --Randy

 

[richg1998]

 Here is diagram that is generally used in decoders. Click on the below link. The are four diodes used to convert the DCC signal to DC voltage. There is a capacitor on the decoder which filters this DC voltage. What you want to do is put, say a 1000ufd to maybe 3000ufd capacitor in parallel with the capacitor in the decoder. Depends on the area available. Usually the capacitor on the board is the largest one. I have not seen a decoder with a voltage regulator but they may exist. Sometimes there are five diodes together as you will see in the diagrams in the link. The fifth diode will have another use. I have some Micro-Tsunami decoders that use the fifth diode in conjunction with the blue common lead. Only four are used to form a full wave bridge rectifier.

SoundTraxx includes a 220ufd capacitor with the Tsunami and Micro-Tsunami which they say is good for sound interruptions. Some people add much more to help with interruptions to the motor. With larger capacitors, the diode and 100 ohm resistor are suggested so you booster does not see a heavy current draw. With only 220ufd, those two items may not be needed. You will probably have to experiment.

 I recommend getting a multimeter, Optivisor, soldering iron with fine point, clip leads, good light source and ohm out the connections. Make a drawing. You will find out the red pickup wire connects to two diodes. The black with connects to the other two diodes. The output of two diodes is positive DC and the output of two other diodes is negative DC. The negative and positive will go to the largest square capacitor on the board to filter the DC.

I have no idea what your level of electronic experience is so good luck.

http://www.members.optusnet.com.au/mainnorth/alive.htm

The below link is what the pulses to the motor look like from the decoder. You do not want to put a capacitor accros the motor leads. Capacitors will not store pulses.

Even decoders run on DC power packs send pulses to the motor.

Rich

[jbinkley60]

richg1998

The below link is what the pulses to the motor look like from the decoder. You do not want to put a capacitor accros the motor leads. Capacitors will not store pulses

Actually the capacitor will "store" the pulses but placing an polarized electrolytic capacitor across the motor leads is a bad idea because the polarity will flip when the locomotive goes in reverse.   Eventually the capacitor will blow the seal and make a nice mess inside your locomotive.  Placing the capacitor on the motor will not help the sound either, since the power for the sound is pulled further back in the decoder circuit.  The URL above shows he proper way to do what you want.  I personally have never had a problem with dropouts on any of my 20+ sound units but I don't have any Soundtraxx decoders.  I did have a slight problem early on but found that dirty track was the cause.

 

 

[richg1998]

 You missed the 'Scope in the message. When the DCC signal is interrupted, the pulses immediately disappear and some DC voltage remains for a fraction of a second. I did the 'Scope readings. I do agree, capacitor innards will end up in the tender with reverse is selected.

One person I corresponded with in a different group had that happen when he connected the capacitor to the stay alive leads the wrong way. Reverse polarity.

Rich

[cmarchan]

The hold-up capacitor is actually placed in the circuit after the bridge rectifier. The role of the bridge is to convert the DCC signal current (which looks like AC current to solid state devices) into a DC power supply for the decoder's logic and control circuits. Remember you must have DC power to operate solid state components. The bridge also maintains proper polarity to the aforementioned decoder components regardless of the input polarity from the track, making operation on DC possible. For the curious, the reference signal to the decoder's brain to determine  DC forward or DC reverse is picked up AHEAD of the bridge rectiffier.

The hold-up capacitor "extends" the DC power level for a moment; works to offset issues from dirty track, dead frogs, dirty wheels. It will not hold-up the power for long periods of time.

If you are proficient in electronic related projects, you can go to the website link mentioned earlier to learn how to apply the hold-up cap to various decoders. I've done this in several installations, and it helps to minimize the sound "restart" that you get from Soundtraxx LC decoders. The DSX decoder capacitor upgrade is based on these principles.

For non-sound decoders, the result depending on the size of the cap is reduced hesitation over dirty track, dead frogs, etc.

 

[CSX Robert]

rrinker

A keep-alive capacitor has to go after the voltage regulator on the decoder.

richg1998

I have not seen a decoder with a voltage regulator but they may exist.

From what I have seen, most if not all decoders have a voltage regulator because the processors they use can not run directly off the DCC voltage levels. It is not, however, neccessary to tie in after the voltage regulator. With a sound only decoder, tying in after the regulator will give you more keep alive time because if you tie in before the regulator some of the capacitor's energy will be wasted as heat through the regulator. Tying in before the regulator is usually easier to do though, especially if the decoder has function outputs, because the unregulated positive voltage can be found at the blue wire. Also, if the decoder is a sound and motor control decoder, and you want the keep-alive capacitor to power the motor as well as the sound circuit, then you have to tie in before the regulator.