I have a project to convert analog meter installed on the machine to digital and monitor it online.
From my understanding about analog meter, it requires current to move the needle and the maximum current I measured for the needle to move until the end is about 2mA.
But surprisingly when I measure the current directly from the wire connected to the analog meter while the machine is running, I get around 40mA for the needle to move until its end.
Besides, since Arduino only read Voltage signal. Is there anyway to convert current output to voltage output?
I’ve tried this approach of using resistors to convert current to voltage, but ended up burning my resistors and Arduino board.
My approach is as follows:-
Arduino only takes 0-5V signal and current output of the meter is about 40mA. According to Ohm’s Law, V=IR, the resistor required is 125 Ohm.
+-----------------------
|----------------------|
|----[125Ohm]------| *Ignore the white lines
|----------------------|
|_____(40mA)____|
I connected the + to A0 and - to GND of Arduino, but this approach ended up frying by board.
Any suggestion guys?
You will have to post the specifics of the meter you are trying to interface with. Current control isn’t the only option since there are voltage controlled analog meters as well. Most current controlled industrial meters use a 4-20mA loop so your measurements of 2mA and 40mA are suspect. Can you detail exactly how you are measuring the current going to the meter?
4-20mA systems typically have a relatively high excitation voltage in the 24V range available to drive multiple displays over long distances. To read the current in a loop, you place a shunt resistor in series with the loop and measure the voltage drop across it. The resistor can be put on the high-side (before the meter) or the low-side (after the meter) but there are precautions that need to be taken. With a high-side resistor, the voltage drop will be the same as a low-side resistor but you have to deal with up to a 24V common mode voltage which will release the smoke from your Arduino. High-side measurements need special interface ICs that can handle the common mode voltage or you need to isolate the measurement of the resistor using analog isolators. With a low-side measurement, you need to make sure the shunt resistor is the last item in the loop and then you can make measurements with non-isolated equipment.
Get us some more details of your system and we can help steer you in the right direction.
-Bill
After writing my post, I found this link which should help you better understand 4-20mA systems: http://www.dataq.com/blog/data-acquisit … surements/
If your meter turns out to be voltage controlled, there are different considerations.
-Bill
phalanx:
You will have to post the specifics of the meter you are trying to interface with. Current control isn’t the only option since there are voltage controlled analog meters as well. Most current controlled industrial meters use a 4-20mA loop so your measurements of 2mA and 40mA are suspect. Can you detail exactly how you are measuring the current going to the meter?
4-20mA systems typically have a relatively high excitation voltage in the 24V range available to drive multiple displays over long distances. To read the current in a loop, you place a shunt resistor in series with the loop and measure the voltage drop across it. The resistor can be put on the high-side (before the meter) or the low-side (after the meter) but there are precautions that need to be taken. With a high-side resistor, the voltage drop will be the same as a low-side resistor but you have to deal with up to a 24V common mode voltage which will release the smoke from your Arduino. High-side measurements need special interface ICs that can handle the common mode voltage or you need to isolate the measurement of the resistor using analog isolators. With a low-side measurement, you need to make sure the shunt resistor is the last item in the loop and then you can make measurements with non-isolated equipment.
Get us some more details of your system and we can help steer you in the right direction.
-Bill
I dont know there is a voltage controlled analog meter, thanks for the info. By the way, how to distinguish between these 2 meters? Is there any test to show it is controlled by current or voltage?
I input a 2mA of current to the meter externally by using a 9V battery and 4.5k Ohm resistor.
Do you have a part number for the meter? Do you have any details on how it’s currently integrated into your system (pinouts, what’s providing the signal, etc.)?
The easiest way to tell what kind of meter it is, is to check the resistance between the input terminals. DC voltage controlled meters will have values in the range of thousands to millions of Ohms where DC current controlled meters will be in the hundreds. There may be some overlap as you move towards meters that measure small values like DC millivolts and DC microamps.
-Bill