Multimeter discrepancy

[Nigelcliffe]

1 hour ago, Crosland said:

2 hours 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!

 

 

This thread has moved to the railway to hell....

 

 

[Graham Radish]

I dont know how much simpler i can make it, both ac and dc are used, ac in dc out. (I/O) but that dc out depends on the ac in, its a very fast switching process going back to the controller to read the decoder its an ac/dc switching

Edited June 16, 2020 by Graham Radish

[jpendle]

3 hours ago, Graham Radish said:

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

DCC is not DC. It's a Pulse Width Modulated signal that oscillates around zero volts, so there's no DC component.

 

1 hour ago, Graham Radish said:

the AC track power is always on, the dcc data stream doesnt have to be.

On a DCC system you can't have track power on without the DCC signal being present. There may not be any actual commands being transmitted, but a stream of '0's will still result in a PWM signal oscillating around zero volts.

 

Regards,

 

John P

[TheQ]

1 hour 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, 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

You won't get one of those DMMs for £1000, they're closer to £ 20,000. A used 8508A, the old model, is on sale in the states for $12,250.

A decent scope say a Tektronix 1072B ( we've a lot of those round the factory) will set you back around £800, not a cheap Hantek . But even that is cheap compared to some we calibrate here, the highest listed price I could find of a Tektronix  scope  is £21,900 and there are models above that where they just say ...contact us. 

 

I used to drive around the country with more than £250,000 worth of scopes, spectrum analyzers etc in the boot and that was 30 years ago. 

 

But anyway a scope is extremely useful, especially on DCC, just keep an eye out and you can pick one up cheaply like I did, at a little village car boot sale.. 

[Graham Radish]

Yea but people in the real world cant drop 20 grand on a scope thats just ridiculous lol, hantek are pretty good for their price tbh or a good second hand techtronix. i use mine for cassette deck/vhs/cd player allignment etc

[spamcan61]

9 minutes ago, Graham Radish said:

Yea but people in the real world cant drop 20 grand on a scope thats just ridiculous lol, hantek are pretty good for their price tbh or a good second hand techtronix. i use mine for cassette deck/vhs/cd player allignment etc

It's nice to be able to get a 200MHz 'scope new for a few hundred quid (can't recall what I was paying for that level of performance back in the 80s, but it would've been over a grand).

[spamcan61]

30 minutes ago, TheQ said:

But even that is cheap compared to some we calibrate here, the highest listed price I could find of a Tektronix  scope  is £21,900 and there are models above that where they just say ...contact us. 

 

 

If we're doing 'scope top trumps then, as Crocodile Dundee nearly said, nah, this is an oscilloscope ;-)

 

https://www.keysight.com/en/pdx-x202219-pn-DSOV084A/infiniium-v-series-oscilloscope-8-ghz-4-analog-channels?nid=-32530.1150389&cc=US&lc=eng

 

Starting at 116K USD.

 

whoops what was I thinking, this one's 370K USD:-

 

https://www.keysight.com/en/pdx-x202220-pn-MSOV334A/mixed-signal-oscilloscope-33-ghz-4-analog-plus-16-digital-channels?nid=-32530.1150390&cc=US&lc=eng

Edited June 16, 2020 by spamcan61

[Graham Radish]

11 minutes ago, spamcan61 said:

If we're doing 'scope top trumps then, as Crocodile Dundee nearly said, nah, this is an oscilloscope ;-)

 

https://www.keysight.com/en/pdx-x202219-pn-DSOV084A/infiniium-v-series-oscilloscope-8-ghz-4-analog-channels?nid=-32530.1150389&cc=US&lc=eng

 

Starting at 116K USD.

 

whoops what was I thinking, this one's 370K USD:-

 

https://www.keysight.com/en/pdx-x202220-pn-MSOV334A/mixed-signal-oscilloscope-33-ghz-4-analog-plus-16-digital-channels?nid=-32530.1150390&cc=US&lc=eng

Nice, pick two of them up for me, ill drop the cash around later

[Edward]

I have 5 sections to my layout and I found that when they are switched on one after the other,  my meter reading drops from about 13.8 to about 11.5.  ( My meter set to AC volts)  I wonder in light of the discussions above, do the vary ing readings have any usefulness? 

[Grovenor]

26 minutes ago, Graham Radish said:

Yea but people in the real world cant drop 20 grand on a scope thats just ridiculous lol, hantek are pretty good for their price tbh or a good second hand techtronix. i use mine for cassette deck/vhs/cd player allignment etc

People in the real world don't buy a scope just to have a look at their DCC waveform, they might use one if they have it for other uses but a simple multimeter is adequate for faultfinding on the track wiring. Suggesting the OP should buy a scope is just unreal.

[Graham Radish]

3 minutes ago, Grovenor said:

People in the real world don't buy a scope just to have a look at their DCC waveform, they might use one if they have it for other uses but a simple multimeter is adequate for faultfinding on the track wiring. Suggesting the OP should buy a scope is just unreal.

A simple paperclip is enough to hold paper together, as others have mentioned here they are really useful tools for digital stuff

Edited June 16, 2020 by Graham Radish

[Grovenor]

2 minutes ago, Edward said:

I have 5 sections to my layout and I found that when they are switched on one after the other,  my meter reading drops from about 13.8 to about 11.5.  ( My meter set to AC volts)  I wonder in light of the discussions above, do the vary ing readings have any usefulness? 

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.

[Grovenor]

3 minutes ago, Graham Radish said:

A simple paperclip is enough to hold paper together, as others have mentioned here they are really useful tools for digital stuff

They are really useful tools for an electronic designer but not for a DCC user.

[Graham Radish]

6 minutes ago, Grovenor said:

They are really useful tools for an electronic designer but not for a DCC user.

 They are if you want accurate power readings

 

Even this will do the job: RRampMeter https://www.digitrains.co.uk/rrampmeter-v2.html

 

"The RRampMeter automatically detects and switches to the type of power it is measuring. Two LEDs indicate DCC or ac , no LED on indicates dc. RMS is an abbreviation for Root Mean Squared. It is a mathematical way of analyzing a distorted wave form"

Edited June 16, 2020 by Graham Radish

[TheQ]

12 hours ago, Graham Radish said:

 They are if you want accurate power readings

 

Even this will do the job: RRampMeter https://www.digitrains.co.uk/rrampmeter-v2.html

 

"The RRampMeter automatically detects and switches to the type of power it is measuring. Two LEDs indicate DCC or ac , no LED on indicates dc. RMS is an abbreviation for Root Mean Squared. It is a mathematical way of analyzing a distorted wave form"

For only a little more a cheap Chinese Scopemeter can be had from the house of strong ladies for about £100. Expensive if you are not going to use it much, but it's much less than the price of a loco these days..

 

Groveners questions are right as to us helping with a problem,

One odd thing are the sections wired in Parallel or series?

 

Edited June 17, 2020 by TheQ

[WIMorrison]

14 hours ago, Edward said:

I have 5 sections to my layout and I found that when they are switched on one after the other,  my meter reading drops from about 13.8 to about 11.5.  ( My meter set to AC volts)  I wonder in light of the discussions above, do the vary ing readings have any usefulness? 

 

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.

[kev]

My old avo 8 gives a good reading on DCC set to AC but the digital meter is all over the place 

[Free At Last]

Dug this out of the garage, lit a fire in it and it still lights up. How do I measure dcc voltage with it?

[DCB]

If you have one multimeter then that's fine. If you have two multimeters you don't know which to believe if they give different readings. You really need three then you can go with the majority verdict.   I have an old Voltmeter, it was old 45 years ago when I started work and I bought it when the place closed down.  It works great. I also have several dead chinese multimeters which started giving duff readings.

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

[John Clitsome]

33 minutes ago, kev said:

My old avo 8 gives a good reading on DCC set to AC but the digital meter is all over the place 

 Ditto here. A mate of mine who worked in industry explained these things to me when I couldn't understand the readings from a newly purchased multimeter.

He still had an old Avo and that gave a good reading. I now just use the DMM for taking comparative readings - usually shows around 2.33 and thats enough generally to know that all is well- at least as well as I need it to be.

I think these old Avo's still come up on Ebay, but I was warned that unless they've been calibrated you might not be getting good results and no doubt getting one calibrated would cost a bit.

[Dagworth]

20 hours ago, Graham Radish said:

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

Pardon? 
 

Andi

[spamcan61]

39 minutes 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.

I was interested to know that as well; having  done a Google image search then it is indeed swinging both ways, so to speak.

[TheQ]

54 minutes ago, Free At Last said:

Dug this out of the garage, lit a fire in it and it still lights up. How do I measure dcc voltage with it?

Connect red and black terminals, one to each side of the track

Switch to AC,

I assume Y shift gain is in Volts per division, set it to 5 V adjust the variable knob  on Y shift so it's centered  on the screen.

Play with the Time base switch till you see a pattern on screen not a blur.

Adjust the Y shift variable till the base of the pattern on screen is on a black line, measure on screen the size.

You may be able to switch down to 1V per division, move bottom of the signal down to the bottom of the screen and then read off the top level to get a more accurate reading..

[spamcan61]

Where's the 'autoscale' button? ;-) 

 

 

[TheQ]

2 minutes ago, spamcan61 said:

Where's the 'autoscale' button? ;-) 

 

 

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