First off I am somewhat new to the Arduino and completely new to DC current sensing. So if I ask something stupid, post in the wrong area, or something that I should have easily found while searching I apologize in advance.
Summary:
I need a sensor to measure 12VDC current in the neighborhood of 50mA but be tolerant of going off scale high 60A to 100A I've never used DC current sensors so I don't even know if what I want to do is possible. If you can help I'd greatly appreciate it. Please explain how I might accomplish this as if I know nothing about DC current sensing, because I only know slightly more than that.
Details:
I have a system with a 12V battery that intermittently has a phantom drain that kills the 12V battery when it's supposed to be off. I want to use an Arduino to monitor current draw from a 12V battery to alarm when current draw exceeds 50mA. I will only be monitoring when the 12V load is supposed to be off. However when the load is active on the 12V battery it could in theory be as high as 100A. I don't care what it actually is as I only want to monitor current draw when the load is not active. The highest load would be when the dead/low 12V battery is charged by a 100A DC-DC converter. The most I have measured is just under 60A when the 12V battery was flat dead so maybe the sensor only has to be tolerant of slightly more than 60A. TIA for any links, info, or pointing me in the right direction.
As an alternative, is there a less expensive/better way to measure 12VDC current in the 50mA neighborhood that is tolerant of going off scale high up to 5A? If it's advantageous to go about it with this much lower cap on maximum current, I might be able accomplish my goal this way.
I suppose if all else fails I can monitor voltage and trigger when it drops below some preset value but that just seems crude.
pylon:
I'm not aware of a current sensor in that range.
I would try to switch the current meter in only if no load is applied and circumvent it the load is switched on.
Let me clarify. I only care to measure up to 50mA. Actually more specifically I need to measure 50mA with an accuracy of about +- 5mA. I don't care how high the range of measurement is above 50mA. Since it is impractical to switch the sensor in and out of battery circuit it needs to be able to handle the rare but worst possible max load of 60A without the magic smoke coming out of the sensor. Typically the max load is only 4-6A but I have to consider the rare worst case scenario. Obviously I won't be measuring current when this main load is on.
When the load is on I will have in input that tells the Arduino to stop monitoring the current as I'd imagine the "on" load would be off scale high of anything that can accurately measure 50mA. When the load is off that input to the arduino will go off and I'll have the Arduino start measuring battery current with a 50mA trigger point.
hammy:
I think I would just do the conventional fault finding instead .
Supposing you measure a 70A signal , then what do you do ?
I would actually be using this as a fault finding tool. The problem is one or more of many components intermittently doesn't shut down when it's supposed to resulting in a load on the battery that kills it. With the problem being intermittent I have to catch it in the act of malfunctioning to determine what isn't shutting down. The idea is to use the arduino as an alarm to let me know that something hasn't shut down when it should have so I can then fault find and identify the culprit. Later if the problem can't be fixed or is cost prohibitive to fix I might want to use the arduino as a cheap battery saver to disconnect a misbehaving component from the battery.
As far as measuring a 70A signal, See my response to pylon above, I won't be measuring more than 50mA. I am just looking for a drain on the battery when the load is supposed to be off that is greater than about 50mA. Essentally, I want to have an alarm if the "off" current drain on the battery exceeds 50mA.
Other than using a meter I have very little knowledge about the types of DC ammeter sensors available or how they work. I just need a way to measure about 50mA and that won't go up in smoke with a multiple amp load. I do not expect to be able to measure high A loads and mA loads accurately from the same sensor. I know that is not possible. I think I may be chasing a unicorn here but I guess it was worth reaching out to those more knowledgeable on the off chance there exists something that would work for my situation.
The only way I can think of is to use a very beefy schottky diode as a shunt. At those max current levels
it will need a heatsink of course, but like all diodes the voltage across is a logarithmic function
of the current through, so you'll be able to get rough measurements across many orders of magnitude.
Let see, 100A : 50 mA = 2000 : 1 dynamic range, that is 11 bits, plus 1% accuracy at the 50 mA end = 7 bits. Overall 18-bits, not much, there are plenty sigma-delta ADC on the market, DIP package IC (NAU7802) or ADC module (ADS1232) and written libraries.
So, buy a shunt current sensor for 100A and wire it up to adc module. Usually, SD adc have internal amplifier with very high input impedance, 1-100 MOhm or so, and to keep an adc safe, inserting single resistor 10-100k into connection line should not be an issue.
