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Digitrax Zephyr Voltage Question

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Digitrax Zephyr Voltage Question
Posted by gcri on Friday, August 8, 2008 11:32 PM

I just got an Athearn N scale Challenger and I noticed that it was not starting out too smooth.  I have a Digitrax Zephyr system and  I decided to check the track voltage with a multimeter (yes I know this is not the best method) but it showed 15.68 volts AC.  So I got a couple of full wave bridge rectifiers and wired them up (see diagram).  I then checked the voltage after I wired them up and it showed 14.1 volts.  The Challenger runs much better now, but that leads me to my questions.  How much power is a Zephyr supposed to put out and how accurate is a standard multimeter when it comes to measuring DCC voltage?

 

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Posted by mfm37 on Friday, August 8, 2008 11:58 PM

Unless you have a True RMS meter the readings wil be off. One way to get a more accurate reading is to set the meter to DC. First make sure that address 00 is at speed 00. Place the positive probe on Track A terminal and negative probe on the ground terminal. record the reading. Make the same measurement between Track B and ground. Add the two measurements together and you have the total voltage.

I did this with a non true RMS meter my own Zephyr and got a total of 13.66 volts.

I then measured the AC voltage between Track A and Track B with a True RMS meter and got a reading of 13.68 volts. As you can see, the other way gets pretty close. Both measurements were made with no load on the Zephyr.

My PS315 power supply measured a little over 17 volts with no load.

13.5 to 14 volts seems to be the popular voltage reading for the Zephyr when using a PS315 power supply. Higher voltage supplies may increase the Zephyr's output.

 

Don't know why the Challenger's decoder would cause the loco to jump with a higher track voltage. NMRA compliant decoders should be able to handle 22 volts safely but not all dceoders comply to the limits. If you are getting good results with the diode's, keep them. You can take the measurements after the diodes using the DC meter method to see the actual track voltage.

 Martin Myers

 

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Posted by locoworks on Saturday, August 9, 2008 2:31 AM
surely with the diagram shown the decoder in the loco is actually running on DC as the AC  DCC signal has been rectified to DC?? i can't get my head round AC being there with both sides going through a rectifier each?? the diodes would still stop back/reverse voltage yes???  did the loco even have a decoder and was it being run on 00 in the first place?  depending on the power supply ( and DCC system ), anything from 15 - 20 volts AC on the track is 'normal'.
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Posted by modelmaker51 on Saturday, August 9, 2008 5:26 AM
This was discussed ad nauseum a year with many different opinions, here's a link: http://cs.trains.com/forums/856152/ShowPost.aspx

Jay 

C-415 Build: https://imageshack.com/a/tShC/1 

Other builds: https://imageshack.com/my/albums 

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Posted by cacole on Saturday, August 9, 2008 8:46 AM

The bridge rectifiers are defeating the DCC signal and the decoder is running as if it is DC powered.  Remove the bridges and check CV 2, the Start Voltage.  If the engine starts with a sudden lurch on DCC, CV 2 may be set too high.  Start with a value of 0 and see how the loco performs.  If you have to turn the throttle too high to get it moving, put a value of 1 into CV 2, and keep this up until you get the desired setting.  Also check CVs 3 and 4, the momentum settings.  CV 3 is the start (delay) momentum, and CV 4 is the stop (coast) momentum.  Turn off DC operation of the decoder by programming a value of 34 into CV 29.  Some decoders perform poorly if DC operation is turned on.

There's nothing wrong with the Zephyr's track voltage -- a DC or DCC multimeter cannot accurately measure a DCC waveform.  The RRAmpmeter sold by Tony's Train Exchange can measure DCC voltages accurately because it is designed specifically for that purpose, but digital or analog VOMs cannot because they don't respond fast enough to the DCC waveform frequency.

If your Athearn N Scale Challenger has the MRC decoder, that's a problem, too.  MRC decoders have a very poor reputation.

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Posted by Phoebe Vet on Saturday, August 9, 2008 9:04 AM

 locoworks wrote:
surely with the diagram shown the decoder in the loco is actually running on DC as the AC  DCC signal has been rectified to DC?? i can't get my head round AC being there with both sides going through a rectifier each?? the diodes would still stop back/reverse voltage yes???  did the loco even have a decoder and was it being run on 00 in the first place?  depending on the power supply ( and DCC system ), anything from 15 - 20 volts AC on the track is 'normal'.

Actually, the way those two bridges are wired, they aren't doing anything at all except adding a little load.  They are in series with the circuit with the outputs shorted together, therefore allowing AC straight through.

Draw them out and trace the signal path through them.

To change the AC to DC you would put one bridge across the Zephyr output and the output of the bridge to the track.

Dave

Lackawanna Route of the Phoebe Snow

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Posted by locoworks on Saturday, August 9, 2008 9:36 AM
 Phoebe Vet wrote:

 locoworks wrote:
surely with the diagram shown the decoder in the loco is actually running on DC as the AC  DCC signal has been rectified to DC?? i can't get my head round AC being there with both sides going through a rectifier each?? the diodes would still stop back/reverse voltage yes???  did the loco even have a decoder and was it being run on 00 in the first place?  depending on the power supply ( and DCC system ), anything from 15 - 20 volts AC on the track is 'normal'.

Actually, the way those two bridges are wired, they aren't doing anything at all except adding a little load.  They are in series with the circuit with the outputs shorted together, therefore allowing AC straight through.

Draw them out and trace the signal path through them.

To change the AC to DC you would put one bridge across the Zephyr output and the output of the bridge to the track.

i thought a bride rectifier was basically 4 diodes wired together to convert AC to DC? the diode also does put a minor load/voltage drop in the system. but as all the contacts on a bridge rectifier are either one end or the other of a diode, the electric can surely only flow one way through one connection to another regardless of which way??  infact i would guess with 2 wired a certain way you could just stop electrical flow altogether??   perhaps you could post a drawing of how they are wired so i can work out how electricity can flow both ways through the diodes?? 

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Posted by gcri on Saturday, August 9, 2008 9:48 AM

Locoworks and Cacole,

I got the idea from the Atlas website.  This is what they suggest doing on the Atlas Commander (except they suggest to use 4 bridge rectifiers).  I can assure you DCC is working correctly, I can individually control locomotives, lights, sounds on the Challenger, etc.

http://www.atlasrr.com/voltage.htm

As for the decoder, yes I know MRC is not good.  I remember reading the Model Railroder review said they tried it with a 16 volt power supply and the control was not as good.  I have read on a couple other website to make sure to lower the voltage on some DCC systems so that it runs correctly at low speeds.  So that is why I was trying to lower the voltage.

I also knew that the multimeter was not the best method, but that was all I had and that is why I just wanted to know how off it really was.

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Posted by mfm37 on Saturday, August 9, 2008 9:55 AM

He's just using the diodes in the bridge to provide a voltage drop. According to his measurements, it's working. While a standard multimeter can't get an accurate reading of DCC, it can be used for comparisons.

Only one bridge rectifier would be needed to rectify DCC to  DC. with both DCC track leads connected to the AC inputs on the rectifier.  

BTW, my voltage figures listed previously are accurate. My True RMS DVM matches the readings of my Rrampmeter. Wouldn't use it if it didn't.

 

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Posted by gcri on Saturday, August 9, 2008 10:14 AM

After reading the thread modelmaker51 sent me I see that I am trying to open Pandora's box.  Fortunately I don't think this thread will be anywhere near as long as the other one.  I appreciate the information Martin as that was what I was looking for.  I will probably purchase one of the rrampmeters from Tony's as it will be useful to have when I expand my layout in the future.

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Posted by Phoebe Vet on Saturday, August 9, 2008 10:22 AM

Locoworks:

If your diagram is acurate then this is the way you have it wired. 

Electricity can only flow in the directon of the arrows in the diode symbols.

Trace the path in each direction with your finger.

As you can see, the bridges are doing nothing, you are passing the AC straight through them.

Dave

Lackawanna Route of the Phoebe Snow

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Posted by selector on Saturday, August 9, 2008 12:34 PM
You guys keeping levering that lid on the can o' worms and it's gonna break off!  Big Smile [:D]
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Posted by Randall_Roberts on Saturday, August 9, 2008 6:16 PM

The DCC signal is an AC square wave with varying pulse widths.  The RMS reading on a multimeter is calibrated for sine waves.  Tony's Trains makes a meter designed to measure DCC, DC, and AC accurately.

http://www.tonystrains.com/technews/rrampmeter.htm

No, I don't work for Tony's.

Best!

 

Randall Roberts Visit http://modeltrains.about.com Subscribe to the FREE weekly Model Trains newsletter.
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Posted by Vail and Southwestern RR on Sunday, August 10, 2008 8:23 PM
The rectifiers are just being used to add some diode drops and cut the coltage to the track.  The MRC decoders supposedly don't like DCC voltages well within the DCC specs (in fact they don't like the voltages that the MRC systems put out).  I had only heard of them blowing up, but poor operation may be another symptom, I suppose.

Jeff But it's a dry heat!

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