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Multimeter discrepancy

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To help in describing conditions we could do worse than take a leaf out of the domestic wiring book.

 

"high impedance short" is an oxymoron. A too high current that does not cause a trip, is an overload. It's beyond the design parameters of the equipment but does not necessarily cause an immediate, catastrophic, failure. You can, for example, overload the fuse in a 13A plug for a long time, depending on the magnitude of the overload.

 

A true short circuit is a "fault" that needs to be detected quickly, e.g. by blowing a fuse or tripping a breaker.

 

DCC boosters have (should have) fast acting overload protection but there will always be a grey area between the stated limit and the actual trip current, due to component tolerances. Allowance for this should be built into the booster.

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On 17/06/2020 at 17:24, Crosland said:

 

Sorry but what you are describing is not DCC. If you want to know how DCC works then I suggest you read the NMRA standards. They are freely available on the NMRA website. I'm tiring of correcting the nonsense that gets written about DCC.

 

 

Depends how you measure it.

 

If you measured between ONE rail and the booster 0V connection then you would see a square wave switching between 0V and the positive track voltage. Measure the other rail in the same way and it will be 180 degrees out of phase.

 

On the layout all you have is the two rails. Half the time one rail is more "positive" than the other. The other half of the time it's more "negative". There are no absolute "positive" or "negative" voltages, just the voltage difference between the two rails.

 

Ac current doesn't have a polarity, it has a phase.

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59 minutes ago, Graham Radish said:

Ac current doesn't have a polarity, it has a phase.

I've seen it all now.

You've quoted a post & argued with something which was never actually stated or inferred in it. :search:

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1 hour ago, Graham Radish said:

Ac current doesn't have a polarity, it has a phase.

 

AC current/voltage does have a polarity - lots of them, continuously changing.

(Even power has a polarity - and just to stir it, current, voltage and power have a phase angle too - but is it relevant? Does it help? Is it out of context?)

AC does have a phase - but with reference to what? In a single phase situation, what analysis method are you using?

 

Picking out a fact - seemingly at randomly - and picking out another buzz word - out of context - to prove an un-said point! Does this help at all!

 

Some methods of AC analysis work better with AC (rotating/frequency domain theory) than others. In some analyses, the use of instantaneous (time based) analysis, instead of frequency based analyses, can be - but be very careful when crossing from one to the other.

Can I be the first to mention Fast-Fourier-transforms BUT WHAT WOULD BE THE POINT?

 

 

What, with all due respect, was the point of that post?

It's no wonder people are quickly turned off threads like this.

 

 

Kev.

(We can also throw commination's theory into the mix here too!   :)   )

 

 

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Now that provided a word that I had to google and find its meaning!

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That's TWO rabbit holes opened  :)

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On 17/06/2020 at 16:48, TheQ said:

Youngster,  I have a whole 4 years on you,  the valve powered scopes were hidden in odd buildings,  to service ancient bits of kit,  we mostly used the 7000 series of scopes back then. 

The pictures are of the RAF Neatishead radar museum

I started on little things like these.. I've worked on the radars,  with the equipment shown.. 

69ec048f59eed5f417b665c2be8dcd3b.jpg

20200617_164444168.jpeg

 

Before you you guys disappear down yer rabbit hole can you just point out to me in the pictures above where I plug me wires in to check me track voltage ? I've ordered one of them in the top picture from Ebay but I'm a bit confused where I plugs me leads in ! :jester:

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Posted (edited)

Top picture,

Well if you go through the doors you can see with that wood leaning across. , pass the two six ft high fire extinguishers on your right,  and the duty man's office on the left,  then on through the next pair doors into the big room behind. 

On your right is the 19inch wide rack 6 ft high digital take off equipment , on the left,  are several racks of SSR 750 equipment .

In front of you is a 7ft high equipment about 10ft wide  in semi-metalic grey with big chrome letters saying Marconi. 

There are several compartments,  I would recommend the right hand side, open the doors to find half a dozen 2ft high valves glowing.. Bright red orange.  Or the centre door,  then inner doors to the magnatron providing a 4 megawatts pulse, the Connect to them in the wrong place , there will be a big blue flash,  a cloud of smoke,  and you'll be past caring about DCC.. 

 

Should you be more cautious and pick an AVO8 from the lower picture.. 

 

Red lead to red + socket,  Black lead to black - negative  socket,  set switch the left hand switch  to AC, the right  hand switch to 25V ( or Higher) 

Touch rails with prods. Read meter. :D

Edited by TheQ

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3 hours ago, TheQ said:

Top picture,

Well if you go through the doors you can see with that wood leaning across. , pass the two six ft high fire extinguishers on your right,  and the duty man's office on the left,  then on through the next pair doors into the big room behind. 

On your right is the 19inch wide rack 6 ft high digital take off equipment , on the left,  are several racks of SSR 750 equipment .

In front of you is a 7ft high equipment about 10ft wide  in semi-metalic grey with big chrome letters saying Marconi. 

There are several compartments,  I would recommend the right hand side, open the doors to find half a dozen 2ft high valves glowing.. Bright red orange.  Or the centre door,  then inner doors to the magnatron providing a 4 megawatts pulse, the Connect to them in the wrong place , there will be a big blue flash,  a cloud of smoke,  and you'll be past caring about DCC.. 

 

Should you be more cautious and pick an AVO8 from the lower picture.. 

 

Red lead to red + socket,  Black lead to black - negative  socket,  set switch the left hand switch  to AC, the right  hand switch to 25V ( or Higher) 

Touch rails with prods. Read meter. :D

 

 

Ahhh ok sorry, was that through the doors and turn left, no right ummmm    :unsure_mini:

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Posted (edited)
On 23/06/2020 at 00:14, SHMD said:

 

AC current/voltage does have a polarity - lots of them, continuously changing.

(Even power has a polarity - and just to stir it, current, voltage and power have a phase angle too - but is it relevant? Does it help? Is it out of context?)

AC does have a phase - but with reference to what? In a single phase situation, what analysis method are you using?

 

Picking out a fact - seemingly at randomly - and picking out another buzz word - out of context - to prove an un-said point! Does this help at all!

 

Some methods of AC analysis work better with AC (rotating/frequency domain theory) than others. In some analyses, the use of instantaneous (time based) analysis, instead of frequency based analyses, can be - but be very careful when crossing from one to the other.

Can I be the first to mention Fast-Fourier-transforms BUT WHAT WOULD BE THE POINT?

 

 

What, with all due respect, was the point of that post?

It's no wonder people are quickly turned off threads like this.

 

 

Kev.

(We can also throw commination's theory into the mix here too!   :)   )

 

 

Oh im not trying to be a dick, and im sorry if i come across that way, i never explain myself very well due to learning difficulties, but what i was trying to say is mains ac power for example, it doesnt matter which way live and neutral are wired for a lot of things, ie: my digikeijs dr5000 is powered by a mains figure8 plug that can work in both directions, in the case of dcc modified ac track power, it carries sync pulses to and from the decoder. its like a hybrid thing. another example is speakers, they can be out of phase not polarity as audio is ac. its because the ac on dcc track power is modified so much (about 7-11Khz) standard multimeters are not accurate you need to have a visual on a screen of whats going on, a multimeter is only really showing half the story and even dmm's from the same manufacturer can give different readings, simply because theyre not equipped for such high frequencies, you will get a "ball park" but it will never be accurate without the proper tool for the job. to measure absolute perfect output from the track standard household dmm's are no good. either a proper track tester or a handheld scope is the ideal thing

Edited by Graham Radish
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On 16/06/2020 at 15:44, kevinlms said:

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.

 

This ^^. Check what voltage your box SAYS it should be putting out to the track, then using the black multimeter  check the track voltage close to the box, if it's reasonably close to what it should be start moving round the track checking each section. If you get a drop in voltage on any section switch off the power and check the joints/ connections in that area for problems. Fix and check again. I made up a couple of leads with crocodile clips at each end. If I found a section with voltage drop I would clip a lead to one rail from an adjacent section of track and to the same rail in  the problem section and see if that cured the drop. If not check the other rail. Its a quick way of identifying which rail/ connections might be giving you the problem.

 

When you have finished get a cuppa and come back and read the information overload posted in your thread then go lay down in a dark room  :jester:

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1 hour ago, Mattc6911 said:

 

This ^^. Check what voltage your box SAYS it should be putting out to the track, then using the black multimeter  check the track voltage close to the box, if it's reasonably close to what it should be start moving round the track checking each section. If you get a drop in voltage on any section switch off the power and check the joints/ connections in that area for problems. Fix and check again. I made up a couple of leads with crocodile clips at each end. If I found a section with voltage drop I would clip a lead to one rail from an adjacent section of track and to the same rail in  the problem section and see if that cured the drop. If not check the other rail. Its a quick way of identifying which rail/ connections might be giving you the problem.

 

When you have finished get a cuppa and come back and read the information overload posted in your thread then go lay down in a dark room  :jester:

It wasn't my thread, but I agree with you 100%. If any multimeter shows anything like what the voltage SHOULD read (surely anyone with the slightest bit of electronic knowledge, ought to be able to work out a ball park figure - but perhaps not, as common sense, isn't that common!), then it is a pretty good tool to use.

 

It could be 10% out, up or down, but it's totally irrelevant as long as it's consistent. For the intended purpose, a guide is good enough.

 

This thread has to be one of the most 'realms of fantasy' threads on RMWeb and let's not go to the hinted at insults - whoops just did!

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1 hour ago, kevinlms said:

It wasn't my thread, but I agree with you 100%. If any multimeter shows anything like what the voltage SHOULD read (surely anyone with the slightest bit of electronic knowledge, ought to be able to work out a ball park figure - but perhaps not, as common sense, isn't that common!), then it is a pretty good tool to use.

 

It could be 10% out, up or down, but it's totally irrelevant as long as it's consistent. For the intended purpose, a guide is good enough.

 

This thread has to be one of the most 'realms of fantasy' threads on RMWeb and let's not go to the hinted at insults - whoops just did!

 

 

Picked yours as being one of the few that probably gives the OP as much information as he needs to know, to figure out which one to use.  Gotta love the knowledgeable RM webbers who cant resist jumping in with ever more complex information that each is SURE is right, convinced of course everyone else is absolutely wrong, posting ever more complex technobabble, that comletely loses those of us with lesser knowledge, and probably scares the bejeesus out of anyone dipping their toe into DCC Thinking its Waaaayyyy too complicated  !

 

:jester:

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19 minutes ago, melmerby said:

My scope does this: ......

 


My bar-code reader says that’s a packet of Corn Flakes and it costs £3.29

 

 

 

 

 

 

.

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9 hours ago, Ron Ron Ron said:


My bar-code reader says that’s a packet of Corn Flakes and it costs £3.29

 

 

 

 

 

 

.

Ah! But are they bipolar DC cornflakes or regular AC cornflakes?:D

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10 hours ago, melmerby said:

My scope does this:

Capture1.JPG.128ea2f069da18910eb102c85407a116.JPG

 

@melmerby

 

Keith, was this taken from the Lenz or Z21 DCC signal? I would really like to see an expanded image with the waveform produced by the Z21 - for no reason other than curiosity and seeing how close to a square wave they have managed to make the signal.

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1 hour ago, WIMorrison said:

 

@melmerby

 

Keith, was this taken from the Lenz or Z21 DCC signal? I would really like to see an expanded image with the waveform produced by the Z21 - for no reason other than curiosity and seeing how close to a square wave they have managed to make the signal.

That's from a DR5000 but after going through an opto isolator* and the scope probe hadn't been trimmed for response either. (I'm working on an Arduino DCC project)

The items were in the cupboard and on the bench whilst I was reading through the post and I just lashed them together to get something to capture.

 

I'll try again later direct from the , Lenz and the Z21

 

* I couldn't connect directly to track out else there would have been problem with the common ground & the DCC signal as all were connected to the Desktop PC.

It's best to use the scope module with a laptop, so problems with the grounds doesn't occur.

 

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12 hours ago, melmerby said:

My scope does this:

Capture1.JPG.128ea2f069da18910eb102c85407a116.JPG

 

Which Scope and software is that?

(How much, where from?)

It's easy enough to count the 1s and 0s and build the words (important to be able to too) but having a quick breakdown of the DCC packets looks really useful.

 

 

Kev.

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Posted (edited)
3 hours ago, SHMD said:

 

Which Scope and software is that?

(How much, where from?)

It's easy enough to count the 1s and 0s and build the words (important to be able to too) but having a quick breakdown of the DCC packets looks really useful.

 

 

Kev.

It's a Picoscope (UK designed and made) which is powered from a USB port on a computer.

https://www.picotech.com/products/oscilloscope

 

The software is as supplied and comes with a lot of serial decoding options

It has been updated several times which has added functions.

All scope units also include a useful function generator.

Prices start at around £100 inc VAT for the cheapest single channel 10Mhz device

The one I have is a 2205A which is a two channel 25MHz device.

 

You can buy direct from PIco or from Farnell, Amazon etc.

 

Edited by melmerby
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4 hours ago, melmerby said:

That's from a DR5000 but after going through an opto isolator ...

 

I'll try again later direct from the , Lenz and the Z21

 

 The comparison between the different waveforms will be very interesting and may be reflective of the different makes. :)

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If you want to retain your sanity, don't look at a DCC waveform with a 4GHz scope :o

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