How to connect a Moving coil gauge

[Kaellis991]

Joe,

I’ve understood the use of one side or the other for the pot connection from Mikes book. It changes the direction that the values on my multimeter increase or decrease as the knob is turned either clockwise or anti clockwise.

Here is the image of the back side with the terminals marked.
Yesterday when I was setting up your configuration I was doing it with the case off and the mechanism exposed.
It was then I realized I should be working on this gauge with the case on it or I might damage it.
So I grabbed the case that was off to the side and noticed the rating label. It notes a RTG of 3.7 ohms.
Does that rating information mean that this test isn’t required?

Anyway, I tried connecting it per your diagram, with the case off and the mechanism exposed using the 5v from the Arduino and the 470k ohm resistor yesterday….which was a big mistake. There was no movement in the needle. Thinking that I did something wrong I went back and connected it the same way with the 9v battery that I used per Mike Powell’s diagram like I had the day before when it was working…I.e. the needle was deflecting.
And guess what…. it didn’t move the needle as it did before using Mikes setup.

I finally realized that since I was working with the gauge with the protective case off while connecting the leads for your setup, I think I put either too much pressure on the needle mechanism or shocked it somehow while trying to screw the wires on the terminals.

Whatever I did, I destroyed it by accidentally touching the needle and mechanism.
I even tried the shaking test to see if the needle would bounce as it did the day previously.
No luck. The needle did not freely move with the shake test.
So I am pretty sure I damaged the instrument and will need to buy another one. So this test will have to be done when I get another EGT gauge.

Kirk

[jph]

Hi Kirk,
The 3.7 Ohms is not important. It is probably the coil resistance + temp comp. Not what we need to know here.
As far as damaging the meter. That could only happen if you were apply pressure to the needle area, especially the cage assembly.
No, it wans't good idea to try it with the case off I presumed you were just inspecting the inside for images.

Anyway, it is highly unlikely that you have damaged the unit irreparably. You have probably just dislocated the coil assembly from the pivot.
Have a careful look at the area. You can dismantle it but be very very careful of the return spring assembly. The (usually jewel) bearing is underneath the flat head screw with locknut at the centre of the rotating coil assembly. - DO NOT remove this unless absolutely necessary and mark exactly where it is for replacement. There is also a very fine hair spring coil that is easily damaged.
It should be possible to see what has happened by looking carefully. It may just pop back into place.

Post another image of the inside of the needle coil area as it is now to see if I can see what you did. The needle drum should move freely and return freely. As said, it is probably just out of location.
Worst case is that you replace it with an X27 or replace the coil assembly from a modern moving coil unit which are only a couple of dollars.
Joe

[Kaellis991]

Joe,

Can’t say I really know what to look for.
But to me it looks like I managed to deform both of the coil springs.

It’s a lost cause. Looking around in there I think I messed up the springs even more.
I’m buying another one…hard lessons learned.

[jph]

Hmm. Yes, not good. It looks like the whole drum assembly is tilted and not located in the top Jewel, or, the jewel is smashed - which happens. It would take some pressure to do that usually.

On closer examination It looks like the bottom bearing is located correctly but the top is not.

In the image below you can see the points on the end of the shaft. These usually locate into a small jewel bearing. The bearing is inside the threaded shaft with the slot head screw that has the locknut. There is also one at the base.
When the drum is not located in the bearings and it is turned then the spring tends to overlap a turn which is what it appears to have done here.
sometimes it can pop back into place. Sometimes you can very slightly release the slotted threaded bearing shaft that has the locknut just to allow the central alignment to be regained. Whatever you do though you simply cannot touch the hair springs as they can deform. At the moment they are not deformed, they just appear to be overlapped.

[Kaellis991]

Joe,

For me it’s easier to just throw more money at the problem. I will chalk it up as my boneheadedness in not following proper safety precautions and vow to never do it again.

I already purchased another EGT gauge which I hope will get shipped to me by end of the week.
This next time I will be extra careful in how I handle it.

When it arrives I will connect it per your suggestions.

Kirk

[jph]

No worries Kirk.
No need to remove the case this time though so that's a bonus.
Joe

just for your interest and to give you an idea how cheap the units / mechanisms are

https://www.aliexpress.com/item/1005003 ... 9653%21rec

[Kaellis991]

That is a lot cheaper but I’d have to install it in the EGT case to have use of the adjustable red pointer.

[jph]

Oh, absolutely yes, and graft the old white pointer onto the new moving coil unit. It was just as an example to say that in these cases it is possible to replace the moving coil units with a bit of cloak and dagger grafting
Sometimes the instruments simply are not replaceable so in that case it is worth knowing for someone who may look at this thread maybe in the future.
Also, some of these units for a few dollars can be made into pretty good replicas of existing instruments.
A nice thing about moving coil meters is that to all intents and purposes they are all the same principle and you dont have to buy a specific 'value' . All the gauges on that link in the last post are exactly the same moving coil unit, it is simply the shunt resistor inside the unit (as it is being used as an ammeter) that is different, and the markings on the scale of course. Otherwise they are identical.
People use servos and steppers etc for points, but where a moving coil can be used it is FAR more realistic and so easy to drive from AM.
I am sure version 2 will be amazing Kirk
Joe

[Kaellis991]

Joe,

I’ll get back to this thread in a week or so to continue with the discussion on getting my second attempt to the finish line.

Kirk

[jph]

No worries.
I am sure it will be excellent.
Joe