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Multimeter discrepancy

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2 hours ago, WIMorrison said:

 

This would suggest to me that you have a current draw in each of the sections that you are switching on and that the command station isn't able to provide the power being drawn by the additional sections or this might also be caused by using to small a CSA wire for the bus - might even be a mixture of both.

 

17 hours ago, Grovenor said:

Maybe, maybe not,

Where is the meter connected when this is going on?

How many trains have you got sitting on the layout taking current?

Can you try each of your sections one at a time? That may show up that one section is taking excess load.

Can you measure the current instead? Most meters have a 10A setting which you can use on the DC input to the booster/command station where it is not affected by the DCC waveform.

At the moment  the electrics are disconnected but I will investigate later and come back with more info. 

Thanks so much for your answers

Cheers

Edward

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2 hours ago, DavidCBroad said:

Can someone re cap.  DCC.     Does the square wave go negative as well as positive?  I always thought it stayed the same polarity.

 

Yes it goes negative as well as positive, so there is no DC offset.

 

John P

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1 hour ago, TheQ said:

children. have no experience of real life.. try a Valve powered scope,

Does it give a warmer waveform :rolleyes:

 

John P

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1 hour ago, TheQ said:

children. have no experience of real life.. try a Valve powered scope,

lol :-) you sound like me at work ;-) 

 

I started work as a Microwave test technician (radar/missile parts, not ovens) in 1979 and we still had some valve equipment in use - noise figure meter for one, plus some sig. gens.  To stay very vaguely on topic the daily driver 'scope was the Tektronix 475 which had about 20 times as many knobs and buttons as the 'scopes I'd been using at college, which took some getting used to... and no autoscale of course.

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Posted (edited)
3 hours ago, TheQ said:

children. have no experience of real life.. try a Valve powered scope,

Even better a colour tv with valves need a fork lift truck to move them use to throw them about as easy as anything and they need two to move a flat screen

Edited by kev
spelling mistake

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2 hours ago, spamcan61 said:

lol :-) you sound like me at work ;-) 

 

I started work as a Microwave test technician (radar/missile parts, not ovens) in 1979 and we still had some valve equipment in use - noise figure meter for one, plus some sig. gens.  To stay very vaguely on topic the daily driver 'scope was the Tektronix 475 which had about 20 times as many knobs and buttons as the 'scopes I'd been using at college, which took some getting used to... and no autoscale of course.

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

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1 hour ago, kev said:

Even better a colour tv with valves need a fork lift truck to move them use to throw them about as easy as anything and they need two to move a flat screen

Try a " pool " radar display , a tube of at least 4 ft across,  facing upwards,  so several operators could stand around it, a more modern version of the horizontal maps that WW2 Wrafs used to push signs across representing aircraft.

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This off topic, my Dad's scope was bigger that your Dad's scope, is taking me back to my school days.  Anyone remember "Army Surplus
 stores?  Lots of wowee CRT's and stuff to play with and learn from back then !

 

My DCC has been turned off for while during some extended wirkshop relocations. However when rectifying the track voltage, for other projects, IIRC,  I got around 12 v DC with reference to ground. Not +/- 6v DC .

 

Andy

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On 16/06/2020 at 17:03, Graham Radish said:

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

 

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.

 

5 hours ago, DavidCBroad said:

Can someone re cap.  DCC.     Does the square wave go negative as well as positive?  I always thought it stayed the same polarity.

 

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.

 

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Blimey! With all this going on it's no wondr sometimes a loco seems to have a mind of it's own:locomotive:

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Simple cheap multimeters are designed to read 50Hz sine wave AC (like the mains) and can neither read true RMS of a square wave or have a rectifier that will run at 10KHz. Thus you cannot trust them to read your track voltage. For this you will need a RRampmeter which is the inexpensive solution.

 

Different multimeters will give different voltage readings. The ones with slower rectifiers will read a low voltage. If you read the voltage further away from the command station it will read different to near the command station because the waveform will have been rounded off and appear more like a sine wave, but not enough to give an accurate reading.

 

You could make yourself a bridge rectifier from some fast diodes and use your meter on DC and get a pretty accurate result, you just need to know what voltage the diodes are dropping in the bridge so you can make an allowance.

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Posted (edited)

With threads like this - it's no wonder that folks are put off DCC...….

Edited by newbryford
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Has anyone noticed that a DCC signal is also asymmetric?

 

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.

 

8 hours ago, newbryford said:

With threads like this - it's no wonder that folks are put off DCC...….

 

I'll raise you:

 

https://www.amazon.co.uk/Train-Tech-Multigauge-Track-Tester/dp/B01L35LLIA

 

Indicates presence of both DC and AC or DCC voltage. If they're there, they're there; if they ain't, they ain't! Seemples!

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Don't see you point, it is actually quite a useful little tool.

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Posted (edited)
1 hour ago, WIMorrison said:

Don't see you point, it is actually quite a useful little tool.

 

Then you probably haven't read the pedants' posts (including one I quoted) about multimeter readings!

Edited by JohnDMJ

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I still fail to see the point you are trying to make - this will light orange on a DCC track and red or green (depending on track polarity) on a DC layout.

 

cant get much simpler than that.

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13 minutes ago, WIMorrison said:

I still fail to see the point you are trying to make - this will light orange on a DCC track and red or green (depending on track polarity) on a DC layout.

 

cant get much simpler than that.

 

MY point is that who cares what the numerical voltage displayed on whatever mutlimeter one cares to use is irrelevant, that there is a voltage there at all is what matters here. Regardless of whether it is 13V, 7V, 5V.

 

It ain't the size but how you use it what counts.

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2 hours ago, JohnDMJ said:

 

MY point is that who cares what the numerical voltage displayed on whatever mutlimeter one cares to use is irrelevant, that there is a voltage there at all is what matters here. Regardless of whether it is 13V, 7V, 5V.

 

It ain't the size but how you use it what counts.

What if the voltage is reduced for some reason - a partial short for instance? A go/no go may not help here.

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If you use a DMM for fault finding I would read the voltage at the Booster/command station with the layout disconnected

Use this reading as a reference reconnect the layout & measure the voltages around the layout 

If you get any readings significantly less than the reference reading then there is something to investigate 

 

 For anyone interested the drawing shows Voltage against Time for AC DCC & DC (Blue line is 0V)

I believe the times shown on the DCC graph are correct

The DCC wave form looks to be a square wave with the pulse width modulated to give the information for 1bit

Wave Width of 100µs then bit = 0

Wave Width of 58µs then bit = 1

 

 

305175362_DCCACDC.png.7ee51ad0982971aa7185182e8d9051ef.png

John

PS can you spot the hitchhikers Easter egg

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1 minute ago, John ks said:

If you use a DMM for fault finding I would read the voltage at the Booster/command station with the layout disconnected

Use this reading as a reference reconnect the layout & measure the voltages around the layout 

If you get any readings significantly less than the reference reading then there is something to investigate 

 

 

That would not work very well. I only use my multimeter as a voltmeter to confirm that a controller or booster is still alive.

 

The faults we usually need to locate are areas of slightly higher resistance. The resistance of a voltmeter is huge so it will not interfere with components it is measuring.

The voltage across devices in a circuit is proportional to the resistance of them. Since a bad connection will have a much lower resistance than the voltmeter, the reading may only change marginally at best.

 

Most of us have multimeters but it looks like many overlook their more useful settings. These can measure resistance, which is far more useful for troubleshooting:

 

1. Disconnect your booster or control system & replace it with a wire.

2. Starting close to the wire you have just placed where the booster was, measure the resistance across these 2 points. It should be very low, possibly even so low that the meter reads zero.

3. Move further away from where the booster should be. Every wire & rail has a little resistance but we want to keep this to a minimum. As you get further from the booster, the resistance will increase, but this may be so small that you will not see it. The current usually needs to flow from the booster to the decoder then back again. The resistance of this path is exactly what we are measuring.

4. Keep getting gradually further away. If you see a sudden jump in resistance, the fault will be between this point & the last place which gave you a good reading.

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Posted (edited)
28 minutes ago, stewartingram said:

No such thing as a "partial short". Impossible.

High impedance shorts are very possible. High enough that they limit the current to less than that required to trip the booster. They tend to get very hot!

 

Generally though the more difficult faults to find are the ones with high impedance in the wiring that, when measured on a DMM, show the full expected voltage, but when a loco is present drawing current the voltage falls significantly. Generally caused by too thin cables. I saw a great example of this on a friend's exhibition layout that was using printer data cables as cross-board jumpers. 

 

Andi

Edited by Dagworth
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It's also possible to have rectified shorts, which are a (partial) short circuit when power is connected one way but no short circuit when power is connected the other way.

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42 minutes ago, Dagworth said:

 

Generally though the more difficult faults to find are the ones with high impedance in the wiring that, when measured on a DMM, show the full expected voltage, but when a loco is present drawing current the voltage falls significantly. Generally caused by too thin cables. I saw a great example of this on a friend's exhibition layout that was using printer data cables as cross-board jumpers. 

 

Andi

I've seen this happen 'in the wild' as well, due to a high resistance solder joint , hence although I have a couple of old but functional  Fluke DMMs knocking around I debug track voltage with a 5W car sidelight bulb with croc clips attached.

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Posted (edited)

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