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Hi all,can anyone explain this,both connected to track and on same settings,my system is a NCE power pro 5amps

20200615_134254.jpg

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I can' t see where you have connected the second meter. If you connect them both to the same piece of track in the same place separately do they show the same voltage?  If they do that suggests a problem with your track/soldering/connections where the second meter is connected. I'm assuming that there is nothing else on the track.

If there is still a discrepancy then look to the leads or the meters themselves.  Do you have any other power supplies that you could use to do in independent check of the meters?

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If the meters are measuring the same voltage (as you imply) then you would expect the same reading to appear on both - to within the tolerance of the meters.

 

However, you have the meters set to read AC - but DCC is not AC!

DCC is a square wave with the Mark/Space ratio varied to convey digital information.

 

Also, both meters are not "true RMS" meters (more expensive).

True RMS will measure a varying voltage and display the DC equivalent.

 

Both of your meters must be using different algorithms, to each other, which approximate (average) a sinusoidal (only) AC voltage to display the DC equivalent.

(One might be using the peakvoltage*0/7071, whilst the other could be using an averaging circuit based on 50Hz only, etc...)

 

 

Kev.

 

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Be aware that the DCC voltage is at a high frequency square wave format that causes some multi meters to read incorrectly. They are designed to work on 50Hz AC voltages and a different frequency Hz supply will cause a different result

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1 hour ago, BoD said:

I can' t see where you have connected the second meter. If you connect them both to the same piece of track in the same place separately do they show the same voltage?  If they do that suggests a problem with your track/soldering/connections where the second meter is connected. I'm assuming that there is nothing else on the track.

If there is still a discrepancy then look to the leads or the meters themselves.  Do you have any other power supplies that you could use to do in independent check of the meters?

Hi BoD,the second meter is connected to the same track as the first about an inch behind it with no joint inbetween,if I just connect the meters one at a time they still give the same readings as shown in the pics

Bob

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As others have said, it's due to the frequency and waveform of the DCC signal.  Multimeters which don't have a true RMS capability will give erroneous readings.

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Even Multimeters that have true RMS will give inaccurate reading on DCC.

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To measure DCC voltage and load correctly you need something like the RRAmpmeter from DCC Specialities. Quite a few retailers have it available. 

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Guest Half-full

Put a 9v battery on the track (disconnect your DCC power supply) and check the readings then - in DC

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I have the same, one meter reads 16, the other 7. But its of no consequence, all I use a meter for on the track is to look for dead sections, either meter will do this job.

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Posted (edited)

Grab yourself an oscilloscope on ebay, even a cheap one will be 150x more accurate than the best multimeter. dcc is a bipolar DC voltage

Edited by Graham Radish
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When measuring DCC, you might as well set your (digital) meter to measure Ohms instead of AC - that's how meaningful the result will be!

Zero, a non-zero, or a changing/fluctuating answer - is all you can expect.

 

That said, a cheap digital meter does give useful information if you know its' limitations.

It's just that DCC screws up any quantification you may have wanted/expected.

Cheap digital meters are good for measuring DC, Resistance, continuity, and Mains* though.

 

(*Only measure mains if you know what you are doing, you know the limitations of your equipment, and there is a second person there to help if things go horribly wrong.)

 

 

Kev.

 

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18 hours ago, SHMD said:

However, you have the meters set to read AC - but DCC is not AC!

DCC is a square wave with the Mark/Space ratio varied to convey digital information

 

DCC is most definitely AC, just not the same as what you get from the mains. It's a square wave, a bit less than 10kHz. Cheap multimeters are designed to measure 50 or 60 Hz sine waves.

 

Having said that, some DMMs will give a good reading as the OP has found. It very difficult to predict in advance which will be accurate, even by price.

 

16 hours ago, WIMorrison said:

Even Multimeters that have true RMS will give inaccurate reading on DCC.

 

If they have the frequency response to cover DCC they should be fine.

 

16 hours ago, RFS said:

To measure DCC voltage and load correctly you need something like the RRAmpmeter from DCC Specialities. Quite a few retailers have it available. 

 

Or just connect a bridge rectifier to the racks and measure the resulting DC. Add a volt or two for the diode drop and you have an accurate enough reading of the track voltage.

 

Another way is to measure the voltage between the blue wire and a function output of a decoder, when the function is on. Add another volt or two to compensate for the decoder.

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26 minutes ago, Crosland said:

 

DCC is most definitely AC, just not the same as what you get from the mains. It's a square wave, a bit less than 10kHz. Cheap multimeters are designed to measure 50 or 60 Hz sine waves.

 

Having said that, some DMMs will give a good reading as the OP has found. It very difficult to predict in advance which will be accurate, even by price.

 

 

 

Yes DCC alternates the direction of electron flow to convey information along with the transference of energy.

So you are very correct...

 

...but in the context of the OP you are wrong.

As the low-end digital multi-meters "ACV" and "V~" setting are only valid for oscillating sinusoidal waveforms approaching 50/60Hz.

DCC, as you point out, is well outside this range.

 

 

I liked you suggestions regarding adding a bridge rectifier and also the blue wire idea though.

 

 

Kev.

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16 hours ago, Graham Radish said:

Grab yourself an oscilloscope on ebay, even a cheap one will be 150x more accurate than the best multimeter. dcc is a bipolar DC voltage

Would you really buy an oscilloscope just to get an accurate voltage reading on DCC?

Is an accurate reading of any real value to the user?

All you need is a comparison to see if you are losing voltage in any section and so long as the meter gives a reading it can be used for comparison.

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17 hours ago, Graham Radish said:

Grab yourself an oscilloscope on ebay, even a cheap one will be 150x more accurate than the best multimeter. dcc is a bipolar DC voltage

 

"Bipolar DC" is a horrible contradiction in terms which is very misleading & simply incorrect.

DCC is in no way whatsoever DC because the voltage (& therefore the current) varies positive & negative.

Whoever chose this appalling term should be banned from working with anything electrical.

 

AC is generally assumed to mean SINUSOIDAL AC in the same way as DCC is generally assumed to be NMRA compliant DCC. (Zero 1 is DCC but it shares no standards with NMRA).

I can therefore understand why some prefer to not call DCC "AC" but it "Bipolar DC" is about the worst term I could imagine.

 

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Posted (edited)

Track power is AC dcc is DC thats what bipolar DC means, it uses both fella.

Edited by Graham Radish

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Posted (edited)
2 hours ago, Grovenor said:

Would you really buy an oscilloscope just to get an accurate voltage reading on DCC?

Is an accurate reading of any real value to the user?

All you need is a comparison to see if you are losing voltage in any section and so long as the meter gives a reading it can be used for comparison.

 

Because they dont just measure basic voltages like a multimeter, you can measure dcc pulses, the dcc part is embedded in the ac waveform.

Edited by Graham Radish

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20 hours ago, Grovenor said:

I have the same, one meter reads 16, the other 7. But its of no consequence, all I use a meter for on the track is to look for dead sections, either meter will do this job.

But in the case of the 2 multimeters in the photo, the black one is almost certainly closest to the actual voltage and should therefore be the one used for checking DCC voltage.

 

The orange/black one is reading far too low, assuming the actual track voltage is adequately running trains.

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39 minutes ago, Graham Radish said:

Track power is AC dcc is DC thats what bipolar DC means, it uses both fella.

 

I'm not sure what your statement means. The track power is DCC. You can't say one is AC and one is DC!

 

Now consider what you measure between the two rails of a DCC layout. The current flows one way, then it flows the other way. The current alternates. Do you see where I am going with this?

 

The only time a DCC signal is DC is if you measure ONE track output with respect to the 0V supply to the booster. Even then, being pedantic, it has a DC component and an AC component. On the layout there is no fixed reference, only the two rails. What you see is AC.

 

The NMRA use the term "bipolar", but not "bipolar DC" as far as I can see, to describe DCC in the standards. I suspect it's an American thing where they seem to assume AC is always low frequency that you get from the mains.

 

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Posted (edited)

What i mean is the AC track power is the carrier of the dcc signal, the AC track power is always on, the dcc data stream doesnt have to be. thats why multimeters cant read it properly, the ac power is usually around 16v then the dcc power is deducted from the track power, basically a couple of volts. both ac and dc voltages are present in a dcc setup multimeters cannot do this job,its extremely high frequency, you need a scope. in my instance i never go above 15volts on my n gauge setup, theres absolutely no need to all it does is reduce the lifespan of the decoders and heats up the motors to 65c, 1 way around the motor heat up problem is to add a thermal pad between the motor and the metal chassis, this will drop it down to 25-35c

 

multimeters only read upto 60hz, a mid priced scope measures to 100Mhz the switching speeds on dcc are unbelievably fast.

Edited by Graham Radish

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Some multimeter are more capable than others, you get what you pay for,  I have 3 of these,  and 3 of its predecessors  available for my use at work... .  Oh and it does display wave forms as well.  Mind you I couldn't afford to buy one.. 

https://eu.flukecal.com/products/electrical-calibration/bench-multimeters/8588a-reference-multimeter

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Posted (edited)
5 minutes ago, TheQ said:

Some multimeter are more capable than others, you get what you pay for,  I have 3 of these,  and 3 of its predecessors  available for my use at work... .  Oh and it does display wave forms as well.  Mind you I couldn't afford to buy one.. 

https://eu.flukecal.com/products/electrical-calibration/bench-multimeters/8588a-reference-multimeter

 

This is true of the very expensive flukes, but mainstreame DMM's are pretty useless for this kind of thing, but for the price of one of these expensive meters you may as well just buy an oscillosocpe, those dmms are anywhere upto £1,000 a decent scope is about 350 quid if you look around

 

https://www.ebay.co.uk/itm/Hantek-DSO5202P-Digital-Oscilloscope-200MHz-Bandwidth-2-Channels-1GSa-s-7inch/114257235641?epid=1544096641&hash=item1a9a430eb9:g:phAAAOSwozpe46c7

Edited by Graham Radish

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1 minute ago, Graham Radish said:

 

This is true of the very expensive flukes, but mainstreame DMM's are pretty useless for this kind of thing, but for the price of one of these expensive meters you may as well just buy an oscillosocpe

I did,  an old portable Farnell.. It cost me the huge sum of.... 

 

 

£5, in full working order. 

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