How to connect a Moving coil gauge

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Kaellis991
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Re: How to connect a Moving coil gauge

#21 Post by Kaellis991 »

Joe,
I just received a replacement meter but I am confused about how to test it.
In a video I saw, the test was a “shake and short” test.
Supposedly when the meter is shaken the needle should bounce a little and when the terminals are connected together it dampens the needle movement. I do not remember if the needle in the first one that I eventually damaged had a freely moving needle. Don’t think I learned about the test until after my screw up.
The needle in this meter does not move during the shake test. Is that normal with some types of moving coil meters?
Is it just a matter of hooking up the circuit you detailed to actually test the meter movement?

So I tried the test the way you described. Went down to a 22k resistor and no movement at all.
Decided to try Mike Powell’s procedure again using my multimeter and a 9 volt battery with the pot and a 10k resistor.
For that first meter I purchased Mikes procedure worked perfectly.
This time I get no movement from either test.
With the fact that the needle doesn’t pass the shake test I am beginning to think I got an inop meter this time.
Figures…

I think I have it connected correctly to the back of the meter. This meter has a third terminal that the first one didn’t.
What does UCS use only mean?
It also has two smaller screws for what I think is a light. Not sure..
Do I have it connected to the meter properly?
ADD085B4-6384-42CE-8B1C-26116123D493.jpeg
Kirk
Last edited by Kaellis991 on Fri May 20, 2022 12:35 am, edited 1 time in total.

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Keith Baxter
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Re: How to connect a Moving coil gauge

#22 Post by Keith Baxter »

Hi,

An approach i would venture into rather than messing with unknown resistor values, is to simply write a test instrument in code, using a dial function to control a opacity output via the hardware board.

You do understand that the three terminals are Ground, + and probe?
Normally the probe is a resistance of some sort, mostly bi-metal (dissimilar metal). This "PROBE" acts as a variable resistance between "GROUND" and the "PROBE" terminal.



You would have to check the current draw and the operating voltages, but I would assume it should be within the 500ma limits of a Arduino board. An external voltage supply for the instrument most probably will be necessary as I suspect it to be about 36v

Me not having the bits to check. But something considering...

Keith
Last edited by Keith Baxter on Fri May 20, 2022 1:47 am, edited 1 time in total.
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Kaellis991
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Re: How to connect a Moving coil gauge

#23 Post by Kaellis991 »

Keith,

Writing the instrument code was going to be my next challenge. I wanted to make sure I have a usable meter before I spend the effort and time learning that part.
The procedure below from Mike Powell worked perfectly on the first gauge.
I am convinced this gauge is inoperable. There is a 30 day return policy on this part.
So I am shipping it back for an exchange if they have another one that works.

This second gauge is wired slightly different than the previous one and I’ve tried connecting the positive wire to the different terminals.
Still neither testing method moves the needle.

So will try a third time….still beating myself up for breaking that first gauge with my ineptitude.

Kirk
44179D01-8221-4645-915D-D31A49D75EA4.jpeg

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Keith Baxter
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Re: How to connect a Moving coil gauge

#24 Post by Keith Baxter »

Hi,

The instrument you posted looks like it needs a +V and a -V to work.

Do you want the gauge to work original? or are you wanting to jimmy some other moving coil into a gauge?

Keith

EDIT: http://acversailles.free.fr/documentati ... _Alcor.pdf
Last edited by Keith Baxter on Fri May 20, 2022 4:44 am, edited 3 times in total.
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Kaellis991
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Re: How to connect a Moving coil gauge

#25 Post by Kaellis991 »

I want to use it as is with an Arduino as Joe has explained it can be done.
And as described in Mike Powell’s book.

Edit:
Now I am even more confused with that third terminal and the variable resistor.
From Joes comments I should not use a variable resistor alone without another resistor in parallel.
Just trying to reconcile Joe’s method and Mike Powell’s methods for testing the gauge with the addition of that third terminal.
The first gauge I had was so simple and worked great during the test. It only had the two terminals and was easy to connect.
Last edited by Kaellis991 on Fri May 20, 2022 2:45 am, edited 1 time in total.

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Keith Baxter
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Re: How to connect a Moving coil gauge

#26 Post by Keith Baxter »

Kaellis991 wrote: Fri May 20, 2022 2:06 am I want to use it as is with an Arduino as Joe has explained it can be done.
And as described in Mike Powell’s book.
Hi,

No problems. Thought I could provide another simpler view. Mike Powell probably does not consider Air Manager in his book. Not sure because I have never read it.

For that EGT instrument you say does not work. It most likely does work. You just need to understand how it works.
All you need to do is create a probe using a few lines of code. Connection to you arduino is a ground and a ADC output.

Keith
Last edited by Keith Baxter on Fri May 20, 2022 2:58 am, edited 1 time in total.
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Kaellis991
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Re: How to connect a Moving coil gauge

#27 Post by Kaellis991 »

Right now I am just trying to determine if the gauge is good. It didn’t even pass the shake test.
https://m.youtube.com/watch?v=wbRx5cQZ8Ts

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Keith Baxter
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Re: How to connect a Moving coil gauge

#28 Post by Keith Baxter »

..
Last edited by Keith Baxter on Fri May 20, 2022 4:20 am, edited 3 times in total.
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Sling
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Re: How to connect a Moving coil gauge

#29 Post by Sling »

Hi Kirk,

I’d suggest you have a read about how EGT is sensed IRL. You will find they use thermocouples which output a very small signal. Look for a K type thermocouple table and it will tell you what voltage gives what reading. To simulate this accurately you need to consider the cold junction. Again if you read up on thermocouples you’ll understand why that’s important. At EGT temps though a room temperature cold junction variation may not be that noticeable.

Read carefully as these gauges are only designed for the tens of mV low level thermocouple signals.

Tony

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jph
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Re: How to connect a Moving coil gauge

#30 Post by jph »

Firstly, NEVER EVER EVER do what Keith suggests here, you WILL almost certainly destroy the meter !!!!! PERIOD !

The native meter is designed to be used on VERY low currents. Often in the order of microamps. Connecting the arduino as Keith suggests would potentially blow the shit out of it. :shock: Be VERY careful what you read on forums !!! Always verify from multiple KNOWLEDGABLE sources and cross reference.

Tony is correct (sling) in that this unit will use extremely small levels, hence why we use the SAFE and controlled resistive 'halving' method from high level to determine the approximate range. Not some 'unknown' resistor method as the ill informed put it. ! :?

Also, just to clarify. I said never connect the meter with a variable resistor without a safety resistor in SERIES, not in parallel. Major difference.
The reason is simple. If you attach a supply with a variable resistor only - then at one limit of travel there will be NO resistance - ie, a short - so the meter can take as much current as it likes and destroy itself, which can happen at not very many mA. With a series safety resistor to limit the current to some knowledge based 'safe' value' then this cannot happen. I note from the picture you post from the guys book that he also uses a safety resistor ! .. So you can trust that book !! ;)

If you hold the meter face towards you and rotate it left and right with your wrist it should cause the meter to deflect. You can be quite rough... If it doesn't pass that test it is 'captain cooked' as they say in Cockney Rhyming slang.....


Joe.
Joe. CISSP, MSc.

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