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Author Topic: Fault Finding from first principles  (Read 1991 times)

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Offline Innovationgame

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Fault Finding from first principles
« on: April 03, 2018, 08:28:09 PM »
Fault Finding from First Principles

This tutorial is about fault finding (why doesn’t it work?) as opposed to finding fault (why doesn’t it look good?).

Using a multimeter

With model railways, the main reason for fault-finding is that there is an electrical problem.  That is where I will concentrate my thoughts.  Before starting to find an electrical fault, it pays to have a multimeter of some sort.  When I began in this field, we used AVO 7s, which were a bit crude compared with the AVO 8s, which were considered to be the Rolls Royce of multimeters.  They were analogue devices which employed what was known as a moving coil mechanism and the AVO 8 had a sensitivity of 50mA per volt.  Now, you don’t need to worry about all that because, nearly fifty years ago, the digital multimeter was born.  These days you can buy a reliable multimeter from as little as £7.29. 

This is a cheap digital multimeter that I purchased more than ten years ago.



The three areas that are of interest are DC volts (DCV), AC volts(ACV) and resistance (OHM Ω).  If you have a DC system, you will be interested in DC volts.  The most convenient range to select is 20V.  This indicates the voltage to within 5 mV (five thousandths of a volt).



The most useful range for fault finding is resistance.  To use this, you must switch off the power.  Modern multimeters are quite robust, but older ones could be damaged if you try to measure resistance with the power on.  Even with a modern one, the results will be meaningless unless the power is switched off.  The most useful range is 200Ω, which will give a measurement to within 0.05Ω.



In this case, I have connected the two probes together to create zero resistance and the meter reads 0.5 Ω so, if the battery deteriorates further, it will require replacing.  If the probes are separated, the indication is of open circuit.



Some meters read ‘-1’ for open circuit.  Finally, there is the AC volts range, which is useful for DCC systems.  The most useful range is 200V, which gives a reading to withing 0.05V. 



Most DCC systems use an AC voltage (actually it is a very fast switching DC voltage) which measures between 13V AC and 18V AC.  In the next part of this tutorial, I will concentrate on using the resistance range for fault finding.

« Last Edit: April 03, 2018, 08:30:30 PM by Innovationgame »
With kind regards
Laurence
My personal website is a bit of a mish mash
www.innovationgame.com

Offline weave

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Re: Fault Finding from first principles
« Reply #1 on: April 03, 2018, 09:20:14 PM »
Hi Laurence,

Thank you for starting this thread. I know you said there are no idiots here, on your layout thread, but please be gentle with some of us (me) when it comes to electrics. I need to learn at some point in my life so will be following with great interest.

Cheers weave  :beers:

Online Philip.

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Re: Fault Finding from first principles
« Reply #2 on: April 03, 2018, 09:34:36 PM »
Thanks for doing this Laurence, will be following with great interest :thumbsup:

Offline GreyWolf

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Re: Fault Finding from first principles
« Reply #3 on: April 04, 2018, 12:56:32 PM »
A most excellent beginning to the tutorial Laurence, and many thanks for doing this! I'm one idiot "lek" (limited electrical knowledge) who will be reading it avidly.

Cheers  :beers:

(PS have clicked the "add bookmark" button so I can find it again!)
« Last Edit: April 04, 2018, 01:58:55 PM by GreyWolf »

Offline austinbob

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Re: Fault Finding from first principles
« Reply #4 on: April 04, 2018, 01:01:50 PM »
A lot of people with limited electrical knowledge will thank you for this Laurence.
 :beers:
Size matters - especially if you don't have a lot of space - and N gauge is the answer!

Bob Austin

Offline The Q

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Re: Fault Finding from first principles
« Reply #5 on: April 04, 2018, 01:47:55 PM »
A Very  good Idea this thread,

Ah Avo 8's,
I still have access to one if I need it, but now I normally have a bank of 3 or 4 of these on my workbench.
 


The picture is faked up, as they are mains powered, it's shown in scan mode, which uses rear and front terminals.
The front displays the left hand display is in 20V DC range, the right hand display is in DC 1kV range, which is impossible, they should be the same.

Time  I carried on measuring the resistance of a 2A current shunt,
With the 8508A set to
2V DC range,
Scan mode,
Referenced against a standard resistor..
Both the 2A current shunt and standard resistor fed by the same 2 A supply.


« Last Edit: April 04, 2018, 02:11:20 PM by The Q »

Offline daffy

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Re: Fault Finding from first principles
« Reply #6 on: April 04, 2018, 01:50:16 PM »
I'm looking forward to moving myself out of the electrical darkness when the only words in my head are "What's gone wrong now?! :doh:"

Thanks Laurence. :thumbsup:
Mike

Sufferin' succotash!

Offline themadhippy

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Re: Fault Finding from first principles
« Reply #7 on: April 04, 2018, 02:02:20 PM »
My goto fault finding tool kit,depending on the perceived fault level dictates which size to goto first


Quote
The most useful range for fault finding is resistance.  To use this, you must switch off the power.  Modern multimeters are quite robust, but older ones could be damaged if you try to measure resistance with the power on
But you do get  rather spectacular firework display if you try to measure the impedance of the national grid  with a tandy elcheapo  multimeter :-[

Offline The Q

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Re: Fault Finding from first principles
« Reply #8 on: April 04, 2018, 02:15:55 PM »


Quote
The most useful range for fault finding is resistance.  To use this, you must switch off the power.  Modern multimeters are quite robust, but older ones could be damaged if you try to measure resistance with the power on
But you do get  rather spectacular firework display if you try to measure the impedance of the national grid  with a tandy elcheapo  multimeter :-[

I saw someone try to measure current with an AVO 8 across the the input terminals (instead of in series), BIG blue flash!!! He welded the probes to the terminals and not suprisingly blew the fuse in the Avo 8.  This attracted the attention of the instructor somewhat..

Offline Newportnobby

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Re: Fault Finding from first principles
« Reply #9 on: April 04, 2018, 03:42:27 PM »
My multimeter is very similar to Laurence's but yellow. I set it up as per Laurence's notes i.e. the resistance mode at 200 and when I touch the probes together the figures on the display just dance all over the place and don't show 0.05 as Laurence's does (I hope they are getting better weather outside the caravan than we are in Lankyshire :rain:)


The most useful range for fault finding is resistance.  To use this, you must switch off the power.  Modern multimeters are quite robust, but older ones could be damaged if you try to measure resistance with the power on.  Even with a modern one, the results will be meaningless unless the power is switched off.  The most useful range is 200Ω, which will give a measurement to within 0.05Ω.



In this case, I have connected the two probes together to create zero resistance and the meter reads 0.5 Ω so, if the battery deteriorates further, it will require replacing.  If the probes are separated, the indication is of open circuit.


Offline austinbob

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Re: Fault Finding from first principles
« Reply #10 on: April 04, 2018, 03:59:53 PM »
My multimeter is very similar to Laurence's but yellow. I set it up as per Laurence's notes i.e. the resistance mode at 200 and when I touch the probes together the figures on the display just dance all over the place and don't show 0.05 as Laurence's does (I hope they are getting better weather outside the caravan than we are in Lankyshire :rain:)


The most useful range for fault finding is resistance.  To use this, you must switch off the power.  Modern multimeters are quite robust, but older ones could be damaged if you try to measure resistance with the power on.  Even with a modern one, the results will be meaningless unless the power is switched off.  The most useful range is 200Ω, which will give a measurement to within 0.05Ω.



In this case, I have connected the two probes together to create zero resistance and the meter reads 0.5 Ω so, if the battery deteriorates further, it will require replacing.  If the probes are separated, the indication is of open circuit.



Check the probes are fully plugged in and make sure the probes are clean NN. :beers:
Size matters - especially if you don't have a lot of space - and N gauge is the answer!

Bob Austin

Offline Newportnobby

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Re: Fault Finding from first principles
« Reply #11 on: April 04, 2018, 04:05:33 PM »
Unplugged, replugged, probes cleaned.
Same result, Bob :'(

Offline austinbob

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Re: Fault Finding from first principles
« Reply #12 on: April 04, 2018, 04:08:59 PM »
Unplugged, replugged, probes cleaned.
Same result, Bob :'(
Check the battery NN. Otherwise sound like you need to treat yourself to a new meter.
 :beers:
Size matters - especially if you don't have a lot of space - and N gauge is the answer!

Bob Austin

Offline ntpntpntp

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Re: Fault Finding from first principles
« Reply #13 on: April 04, 2018, 04:13:44 PM »
Ye Olde Maplin Precision Gold multimeter that I've had for decades.

Given that Maplin are selling off and closing down there may be some bargains to be had?


Nick.   2016 celebrating the 20th anniversary of "Königshafen" exhibition layout!

Online mickd247

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Re: Fault Finding from first principles
« Reply #14 on: April 04, 2018, 04:24:42 PM »
Brilliant Lawrence, for future instalments could you paint the selection switch position marker white, as I am sure it will make the correct setting super clear for dummies like me.

 :beers:

Mick

 

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