# How to allow DCC only, but block DC?



## John Gaarde (Oct 11, 2008)

I have some older MRC decoders which will not block DC current from reaching the motor, irregardless of how CV 29 is set. Is there some other way or something I could add that would prevent current flowing to the decoder if it was DC but still allow the DCC through? Thanks. John


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## Greg Elmassian (Jan 3, 2008)

You could try a big capacitor on one track pickup, it should allow AC but not DC.

BUT it needs to be a 75 volt or higher non polarized and while I have not calculated the MFD value needed, I'm pretty sure this is a big expensive capacitor, since it needs to pass a lot of current.

Better to buy a $35 4 amp decoder and replace the MRC.

Greg


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## John Gaarde (Oct 11, 2008)

Thanks, Greg. Just out of curiosity, is that how a decoder works? When set to "no analog" it routes current through a capacitor like you described, and otherwise does not? John


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## Dennis Cherry (Feb 16, 2008)

The command station should not have any effect on the decoder CV values. It just sends and receives the commands and power to all the decoders.

What is CV 29 set too on that decoder?


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## John Gaarde (Oct 11, 2008)

Hello, Dennis. The decoder's CV29 is set to "002" and I have confirmed this by reading it back. I haven't made any reference to the command station. John


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## John Gaarde (Oct 11, 2008)

Greg - Might any of these work? John

http://www.allelectronics.com/category/140900/capacitors/non-polar/1.html


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## Greg Elmassian (Jan 3, 2008)

OK, the capacitor will work in THEORY, but to pass that much current it needs to be physically large... the real requirement is passing the current, so, let me be clear:

1. Thoretically it could be done.

2. My BELIEF is that the capacitor to do this will be very large, expensive.

3. I am in NO WAY recommending you try it.

4. I am in NO WAY going to be able to specify the exact values that are needed.

I am confident that the best solution is to spend $35 for a new decoder.

Sorry if I implied that I know the exact specifications and the part to pick.

DCC varies approximately from 6 to 10 Kilohertz.

You need the filter capacitor to pass several amps, at 6 to 10 kilohertz with minimal effect on the square wave that IS DCC.

Now, different decoders have different sensitivity to distorted DCC signals.

This filter WILL DEFINITELY DISTORT the DCC signal, so it might take a more complex filter.

Designing this high pass filter to also handle several amps is a problem.

So, if you already have a poor quality decoder, it will make the filter design more difficult (meaning more expensive components)

The big problem is that you need several amps to flow through this.

If you could find a home speaker crossover and find one with a crossover point of perhaps 2 khz and could handle 5 amps it might work.... realize 5 amps on a 8 ohm speaker is 200 watts, then this would block the DC and POSSIBLY allow enough DCC through to properly operate the decoder, but I believe you will still get erratic operation, the decoder has already established itself as not good quality.

So, I hope you have an explanation you can live with. You could experiment if you wish, but capacitors can explode if run outside their limits, so I don't want anyone hurt.

Greg


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## John Gaarde (Oct 11, 2008)

Thanks for your answer and time on this, Greg. 

I am still curious how the ordinary decoder accomplishes this when CV 29 is appropriately set; does anyone know? John


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## Greg Elmassian (Jan 3, 2008)

Easy, the system senses if there is DCC or DC on the rails, and then the microprocessor either lets the track voltage decide the speed, or the DCC signal.

In either case, the microprocessor drives the motor, via FET transistors. (modern decoders, old ones used ordinary transistors and even a relay sometimes for reversing)

Greg


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## John Gaarde (Oct 11, 2008)

Thanks again, Greg. John


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## Greg Elmassian (Jan 3, 2008)

No problem John, and we need more questions and activity on DCC here, so if I can help, just ask.

You can also email me privately, my email is in my signature.

Regards, Gerg


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