DCC volt meter

[GRUNFOS]

Can anyone recomend a DCC volt meter, I'm having trouble with ac class 37 and need to know voltage. Thanks in advance.

Paul.

[WIMorrison]

any multimeter will work, it may be a slightly wring in the absolute value displayed, but it will slightly wrong everywhere therefore good enough for testing and fault tracing.

[RAF96]

RRAMP meter but they are not as cheap as a Lidl or Screwfix multimeter. The accuracy depends upon how close the sampling rate is to DCC frequency. I have had readings on 3 different priced multimeters ranging from 9 volts to 15 volts when the RRAMP is saying 15.4 volts.

[spamcan61]

5 minutes ago, RAF96 said:

RRAMP meter but they are not as cheap as a Lidl or Screwfix multimeter. The accuracy depends upon how close the sampling rate is to DCC frequency. I have had readings on 3 different priced multimeters ranging from 9 volts to 15 volts when the RRAMP is saying 15.4 volts.

It would be interesting to know what a cheap 'scope reads. (not a hint !)

Edited November 23, 2023 by spamcan61

[RAF96]

47 minutes ago, spamcan61 said:

It would be interesting to know what a cheap 'scope reads. (not a hint !)

I have a USB Picoscope as it happens. Here is a partial screen grab from an article I wrote for them about using it with an Elite. The ringing is before I tuned the probe correctly. About 15 volts.

 

 

Edited November 23, 2023 by RAF96

[spamcan61]

13 minutes ago, RAF96 said:

I have a USB Picoscope as it happens. Here is a partial screen grab from an article I wrote for them about using it with an Elite. The ringing is before I tuned the probe correctly. About 15 volts.

 

 

Sorry me again ;-) - I can't read the x axis there, what frequency range are we looking at roughly for those square waves please? I'm wondering if my very cheap handheld 'scope has enough bandwidth to measure DCC voltage reasonably accurately.

 

One of these to be specific:-

 

https://www.ebay.co.uk/itm/204530362829?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=tk_m-8fksgm&sssrc=4429486&ssuid=CUw97j2zRRy&var=&widget_ver=artemis&media=COPY

Edited November 23, 2023 by spamcan61

[Crosland]

2 hours ago, WIMorrison said:

any multimeter will work, it may be a slightly wring in the absolute value displayed, but it will slightly wrong everywhere therefore good enough for testing and fault tracing.

 

It may be more than slightly wrong. It really depends on the multimeter. It's counter-intuitive in that cheaper ones can be better than dearer ones.

 

1 hour ago, RAF96 said:

RRAMP meter but they are not as cheap as a Lidl or Screwfix multimeter. The accuracy depends upon how close the sampling rate is to DCC frequency. 

 

Many do not sample the waveform. They have simple circuit that is only good for low frequency sine waves (e.g. 50/60 Hz derived form the mains). Even many true RMS meters do not have a frequency response adequate for DCC.

 

1 hour ago, spamcan61 said:

It would be interesting to know what a cheap 'scope reads. (not a hint !)

 

It should be at least as good as any cheap multimeter.

 

23 minutes ago, spamcan61 said:

Sorry me again ;-) - I can't read the x axis there, what frequency range are we looking at roughly for those square waves please?

 

DCC uses cycles of 112 us and 200us, so appx 9 kHz and 5 kHz.

 

23 minutes ago, spamcan61 said:

 

I'm wondering if my very cheap handheld 'scope has enough bandwidth to measure DCC voltage reasonably accurately.

 

One of these to be specific:-

 

https://www.ebay.co.uk/itm/204530362829?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=tk_m-8fksgm&sssrc=4429486&ssuid=CUw97j2zRRy&var=&widget_ver=artemis&media=COPY

 

It claims a sample rate of 1 Msps so it will be good for DCC.

[SHMD]

A cheap scope will definitely let you "see" a few cycles of waveform (so you can measure the voltage difference between the top and bottom of the waveform), but you will not come anywhere near to being able to view, or capture, a full message packet. (Or even tell you where you are in a message! eg if you want to work out the address, or data, being transmitted.)

 

Here's some further detail of how the DCC is made up...

The "red square" gives you an indication of how much (read little here) a scope will let you "see".

(Still very useful though...)

 

 

Another vote for the Pico scope, (with its very useful built in DCC decoding), from me.

(The other serial decoding formats supported, are also very useful.)

It's recently had a software update too.

 

 

Kev.

 

 

[spamcan61]

13 minutes ago, Crosland said:

 

Many do not sample the waveform. They have simple circuit that is only good for low frequency sine waves (e.g. 50/60 Hz derived form the mains). Even many true RMS meters do not have a frequency response adequate for DCC.

 

I would say for DCC, as essentially a bipolar square wave, RMS and peak AC voltages should be the same. I suspect this may also confuse DMMs measuring AC voltage as they may well assume a sine wave input.

[Crosland]

16 hours ago, spamcan61 said:

I would say for DCC, as essentially a bipolar square wave, RMS and peak AC voltages should be the same.

 

And the prize goes to...😀

 

A very easy way to measure the track voltage is to simply rectify it and add a bit for the diode losses. Maybe add a small smoothing capacitor. The DC voltage you measure will be the RMS, which is the same as the peak.

 

16 hours ago, spamcan61 said:

I suspect this may also confuse DMMs measuring AC voltage as they may well assume a sine wave input.

 

True RMS meters will work, but they need to have a high enough frequency response. Most seem to be 1 kHz or below.

 

[RAF96]

18 hours ago, spamcan61 said:

Sorry me again ;-) - I can't read the x axis there, what frequency range are we looking at roughly for those square waves please? I'm wondering if my very cheap handheld 'scope has enough bandwidth to measure DCC voltage reasonably accurately.

 

One of these to be specific:-

 

https://www.ebay.co.uk/itm/204530362829?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=tk_m-8fksgm&sssrc=4429486&ssuid=CUw97j2zRRy&var=&widget_ver=artemis&media=COPY

Rather than me chop images out of the Picotech articles have a look here…

Basic DCC decoding using a Select …

https://www.picotech.com/library/oscilloscopes/digital-command-control-dcc-protocol-decoding

Elite decoding in more detail…

https://www.picotech.com/library/application-note/dcc-demonstrating-the-Hornby-elite-using-picoscope

[spamcan61]

15 minutes ago, RAF96 said:

Rather than me chop images out of the Picotech articles have a look here…

Basic DCC decoding using a Select …

https://www.picotech.com/library/oscilloscopes/digital-command-control-dcc-protocol-decoding

Elite decoding in more detail…

https://www.picotech.com/library/application-note/dcc-demonstrating-the-Hornby-elite-using-picoscope

thanks for that, looks right up my street

 

[RAF96]

1 minute ago, spamcan61 said:

thanks for that, looks right up my street

 

The key to decoding in a form that makes sense to the likes of me is the link file that turns the binary into text. I never did finish doing that.

Nor did I get round to submitting the follow on articles.