Hornby decoder track voltage

[2foot6]

I have been using Hornby TTS decoders for about two years ,my DCC systems has a track voltage of 20 volts that Hornby decoders are able to handle.I have been looking at buying more decoders and I have noticed a number of dealers in the U.K are advertising the TTS decoders with a track voltage of 15 volts.Is this a mistake or has the voltage been changed?....Peter.

[Bulleidboy]

I stand to be corrected, but the Hornby Elite/Select put out 14/15v to the track - so I suppose any Hornby product (TTS) would say 15v.  I have not heard any comments about a wide variation in voltages when using different systems to operate a DCC layout.

[Yelrow]

15 volt AC is normal track voltage. Are you sure you measured it right..

[RDS]

Which controller are you using that gives an output of 20 volts? 

The standard DCC voltage for track is around 15 volts (measured on the AC scale).

The voltage has not changed.  All decoders operate at the same level.

[Chrissaf]

What DCC System are you using?

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Because DCC track power is a Bi-polar square wave and not sinusoidal, Then the accuracy of many multimeters cannot be relied upon to be accurate. The actual DCC track voltage is 28.8 volts 'peak to peak' using Hornby controllers, but usually measures somewhere between 12 volts and 15 volts AC with most meters.

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Nearly all multimeters are calibrated to read accurately on Sinusoidal waveforms. In order to measure DCC track voltage accurately, you need a special kind of meter. One such meter is called a 'RAMPmeter'. A more expensive multimeter that has a calibrated option to read 'TrueRMS' can also read fairly accurate results.

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The Sinewave (sinusoidal) waveform in the image above is a single pure frequency, thus your average multimeter can be made cheaply to measure this very specific waveform very accurately.

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Below is an extract from an unpublished document I wrote some time ago about measuring DCC track voltage.

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The DCC Bi-polar Squarewave is electronically a very complex wave form as it is created by mixing together a base Sinewave together with many of its odd harmonics, far more than 9 harmonics mentioned in the example. This means that you need a multimeter that takes multiple measurement samples over a very wide frequency range and performs complex mathematical computations on them. This is what makes the RAMPmeter and 'TrueRMS' meters so expensive. This measurement is further complicated because the DCC Bi-polar waveform is not a fixed ratio waveform. The 'marks' and 'spaces' for representing binary 1s and 0s are different, thus a DCC signal transporting ever changing DCC command data is even more complex for a meter to accurately read.

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Here is another extract from my unpublished document that compares the readings I get with a number of test meters I have in my possession. The bottom line in the table was taken with my 'TrueRMS' meter. Note that the AC frequency bandwidth range for this meter is 100Khz which is 100 times more than the next nearest meter. Note also that when reading the 240VAC Sinewave, the meters give reasonably consistent measurement results, apart from the Fluke. My Fluke is at least 40 years old, which probably accounts for the discrepancy.

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However, that all said. A measurement of 20 volts does seem to be on the high side (hence why I asked what controller you are using), If it is a Hornby controller then that 20 volt reading is false and due to your meter not being up to the standard required.

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The 20 volt reading may also be an indicator that your DCC track voltage is 'ringing'. This is not a good thing to have on your DCC power distribution. 'Ringing' can be reduced by fitting a 'snubber' to condition it.

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This scope display, shows a DCC waveform with 'ringing' on it. The wavy oscillation at the start of the pulse.

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Notice the VPP value (Voltage Peak to Peak) in the right hand side column, it is now reading 62 volts instead of 28.8 volts. This higher VPP value could easily account for a 20 volt AC reading on a cheap multimeter.

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TIP: As a newbie poster on the forum, just be aware that the 'Blue Button with the White Arrow' is not a 'Reply to this post' button. If you want to reply to any of the posts, scroll down and write your reply in the reply text box at the bottom of the page and click the Green 'Reply' button.

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Particularly as my reply includes an image, using the 'Blue Button' may result in your reply being held back for image approval, even though it is an existing image.

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See also – further TIPs on how to get the best user experience from this forum.

https://www.hornby.com/uk-en/forum/tips-on-using-the-forum/

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[Yelrow]

Do you wish you had not asked,.

[Chrissaf]

Ringing can, and if severe, will damage decoders. So if the 20 volt meter reading is indeed an indicator that the OP has ringing on his DCC track, he needs to know about it, so that he can resolve it.

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If I was in his shoes, I would be very glad that I had asked the question.

[96RAF]

TTS track voltage limit is 27V - it is so stated on every TTS user manual.

[2foot6]

Thank you for your replies.All of the eleven decoders I have purchased state max voltage is 27 volts and operate very well.As I stated in the original post ,some UK retailers are quoting 15volts for the decoders on their adverts,hence the question,is it now 15volts or the original 27 volts?I'm using a Ulenbrock Intellibox II system and love it,they state on their info  the system is a 20 volt system.I use Ramp meter to measure the voltage and it is 20 volts.A number of decoders will give problems when the voltage is over 16 volts. MRC is one in particular that fails early(personal experience),there is a number of decoders that operate well up to 22 volts as per their specs.(TCS,ESU) but getting the voltage limits can be occasionly hard to get from suppliers or web pages.Trial and error has been the learning curve for the past few years for me and Hornby decoders have been a proven decoder for my layout.The Aussie dollar is not good at the moment but Hornby's value for money still beats the other brands for me..................Peter.