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Author Topic: Make A Centre Zero Meter  (Read 447 times)

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Offline MalcolmAL

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Make A Centre Zero Meter
« on: August 06, 2017, 03:22:46 pm »
1. Prolog & warning : The DC controller saga
I started out with two Farish sets which came with controllers. Some time later I found I had a few more locos and my good lady also had a couple. Although we only had one roundy and one shunty it was time to plan for the future when a local emporium was found selling an ex-set Bachmann controller.
Fast forward a few months and one of the controllers developed a bad case of 'noisy pot' syndrome causing the loco to take wild kangaroo leaps and bounds as its speed was varied :( No worries, we still had two controllers, a roundtuit plan was formed to make my own controller somewhen.

Fast forward a few more months and during an Inglenooking session my little Jinty came to an abrupt halt and sat there growling and gnashing its teeth. Argh! panic !! stripped gears, loose pinion, all sorts of thoughts flashed, what could have gone wrong.
So, without logically thinking, I took a J36 out of its box to continue the session, Horror of Horrors : it sat there growling and gnashing its teeth !! This could have been an expensive mistake.
The roundy meanwhile had been trundling a 4F with its three coaches ( all it can comfortably pull, very disappointed with those two sets) So, to confirm or other what damage I had done I put the Jinty on the roundy :
JOY, it still worked ! so did the J36 :)
I think you may, by now, be ahead of me :- I put a meter across the output of the controller and found it uncontrolled at about 18v Grrrr. I chucked the offending object into a box, what I should really have done was put a scope on it to see if (as I suspect from the growlings) it was outputting raw AC ? Thank goodness it seems not to have harmed either the Jinty or J36 !phew!
Later it was a simple job to swap the potentiometers so we still have two working ( xxfingers etc.) controllers.

The moral of the story :- always have a Voltmeter on your track feed to watch for things going wrong and to avoid making a bad situation worse.

2. Make a center-zero meter to monitor your controller.

Some time ago I wrote about how a bridge rectifier could be used to make a unidirectional indication of both V and A with a common economic meter movement. Apart from it being inelegant it also reduced the voltage by twice the diode forward voltage (typically 2 x 0.3 -> 0.6 V) No great deal though, just recalibrate the voltmeter !
But it is not nice in that it does not indicate in which direction your little loco is about to set off, or is currently moving. Yet more electronics involving more diodes and LEDs and/or opamps with bias, would do it but there must be a simpler way !
Yes,  use a center-zero meter, but how to do this without selling an arm and a leg ? :-
Easy, buy an economic ordinary meter from our friends in the East and set-to it with gay abandon and no regard to the financial risk (approx 1.64 ) LOL !
Looking at the pics online of these el-cheapo meters it looked like some are built in traditional style with screws, not the usual glue and plastic clips that thwart many modern re-purposing projects :)  raising optimism that they cold be taken apart and reassembled.
 this :-

297a

can easily be turned into one of these :) :-

434a

First, remove two TINY screws and carefully take off the face-plate to expose the ingenious mechanism beneath. Beware the screws are really tiny and well within the teleportation range of the average carpet monster.


It is interesting to see that the wee face-plate screws though appearing to be self-taps are seated in 'proper' bushes in the plastic stand-offs, very nice, how do they do all this for the price !

306a
Here you can see the slotted lever-arm (and below (1) ) which, when engaged by an embossed pin on the rear(inside) of the black adjuster thingy (bottom centre of the faceplate) effects zero-calibration.

306mA

However the available adjustment is not sufficient to bring the pointer to centre scale. I found by accident (whilst wiggling the arm back into its original position) that the nut (5) came loose, allowing (2)+(3) to be rotated clockwise. The nut could then be pinched back up to re-engage the arm in its customary micro-adjust role, with the pointer in a new position.
The lug (3) carries the end of a spring attached at its other end to the rotating mechanism comprised of the pointer and the frame (4) carrying the coil of wire that does the clever magnetic force stuff :)
Resist the temptation to twiddle the centre slotted screwhead (6), I think that carries the bearing cup at its other end !
I have no idea what size that nut is but a small screwdriver acting as a tangential drift was good enough to do the job, time will tell if that is good enough to hold well in the future !
Next time (if anyone is still awake!) I will describe how to tailor the voltage range and how to make a shunt ammeter, if anyone wants ??
these Chinese meters are well nice.
« Last Edit: August 06, 2017, 10:13:51 pm by MalcolmAL, Reason: To move all my pics into my gallery »

Offline Malc

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Re: Make A Centre Zero Meter
« Reply #1 on: August 06, 2017, 03:42:01 pm »
Did you post pictures on this thread?
I'm not sure if life is passing me by, or trying to run me over.

Offline Malc

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Re: Make A Centre Zero Meter
« Reply #2 on: August 06, 2017, 03:50:05 pm »
I have just uploaded a picture to my Misc gallery without any problem.
I'm not sure if life is passing me by, or trying to run me over.

Offline Dorsetmike

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Re: Make A Centre Zero Meter
« Reply #3 on: August 06, 2017, 03:54:46 pm »
Why make one when you can get one of these, I have one built in to my control panel, works a treat, gives a + or - indication. You can also get them for different max current, (mine is 0-2A reads down to 0.01mA) also similar volt meters and ones that have dual display of volts and current.

http://www.ebay.co.uk/itm/DC-Blue-0-199-9mA-200mA-Digital-Panel-Amp-Current-Meter-Ammeter-Milliammeter-LCD-/142048775415?hash=item2112c44cf7:g:cYcAAOSwuzRXf3ru
« Last Edit: August 06, 2017, 03:59:11 pm by Dorsetmike »
Cheers MIKE


How many roads must a man walk down ... ... ... ... ... before he knows he's lost!

Offline MalcolmAL

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Re: Make A Centre Zero Meter
« Reply #4 on: August 06, 2017, 03:59:49 pm »
Why make one when you can get one of these, I have one built in to my control panel, works a treat, gives a + or - indication. You can also get similar volt meters.

http://www.ebay.co.uk/itm/DC-Blue-0-199-9mA-200mA-Digital-Panel-Amp-Current-Meter-Ammeter-Milliammeter-LCD-/142048775415?hash=item2112c44cf7:g:cYcAAOSwuzRXf3ru

Yes I have those, but I pref. an analog needle for this application.
« Last Edit: August 07, 2017, 04:30:15 am by MalcolmAL »

Offline Railwaygun

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Re: Make A Centre Zero Meter
« Reply #5 on: August 06, 2017, 04:05:39 pm »
Did you post pictures on this thread?
No, still trying to, the add to gallery is giving me an error
 "something wrong" but not telling me what is wrong !


instructions under "help & FAQ" button on top of screen

i always make sure that i put a cursor in the text box - if you don't do this it seems to upset the system ( it loads the pics to Gallery/add/my images though).
This has been a public service announcement
It may contain alternative facts

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Offline MalcolmAL

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Re: Make A Centre Zero Meter
« Reply #6 on: August 06, 2017, 04:57:56 pm »
Yea! we got there in the end !! Everyone can go back to the start now :)

Suggest if a mod. has the time to spare, tidy the thread up by removing all the "Help Malcolm in his blond moments" posts ! thus getting back to business at hand Thanks.   :D

Offline MalcolmAL

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Re: Make A Centre Zero Meter
« Reply #7 on: August 06, 2017, 04:59:38 pm »
Why make one when you can get one of these, I have one built in to my control panel, works a treat, gives a + or - indication. You can also get them for different max current, (mine is 0-2A reads down to 0.01mA) also similar volt meters and ones that have dual display of volts and current.

http://www.ebay.co.uk/itm/DC-Blue-0-199-9mA-200mA-Digital-Panel-Amp-Current-Meter-Ammeter-Milliammeter-LCD-/142048775415?hash=item2112c44cf7:g:cYcAAOSwuzRXf3ru

6.99 vs. 1.64
« Last Edit: August 07, 2017, 04:28:36 am by MalcolmAL »

Offline Bob G

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Re: Make A Centre Zero Meter
« Reply #8 on: August 06, 2017, 07:40:49 pm »
Or a pint and a half in your local.
I'm with Dorset Mike.
There are only so many hours in the day...and some of them are allocated to working, eating, sleeping and modelling.

Bob

Offline MalcolmAL

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Re: Make A Centre Zero Meter
« Reply #9 on: August 09, 2017, 04:53:24 pm »
Well, that went down like a lead balloon !
I plead insanity from seeing this first time round in the cinema
(and if the Krell didnt use Digital Panel Meters that is good enough for me  :bounce: )

so I guess I neednt go on about the other end ! :-



Offline Bob G

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Re: Make A Centre Zero Meter
« Reply #10 on: August 09, 2017, 05:02:05 pm »
There is something immensely satisfying about a bank of analogue meters that you just don't get with digital displays, unless you go to very dynamic virtual displays.

The use of an alarm sound is also very important to heighten the anxiety level.

Certainly if the control panel noises drown out the DCC sound from a small loco, you know you have achieved something!

Bob

Offline MalcolmAL

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Re: Make A Centre Zero Meter
« Reply #11 on: August 09, 2017, 05:14:15 pm »
DCC ! something else that the Krell and I dont have.

agreed, no soul to the dpm, how could one not be full of admiration for Mr Faraday, Mr Lenz and his right&left hands, and the other boys and girls of yesteryear when peering into that charming mechanism ?

 

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