I have designed a printed circuit board that contains an MCU, and some buttons to get external interrupts working with 2 x AAA batteries which provide around 3.1V. Current consumption is critical for me so when I measure the current drawn by the circuit connecting series with a multimeter I have seen 30uA in low-power mode while it is supplied by an external power supply with 3.1V. However, when I connect two AAA batteries series and measured with the same method I have seen values like 200uA. Why there is a huge difference in this case? The problem might be related to the difference between internal resistance and power supply so which value is correct? I am a bit confused to see such a difference
Is there a processor on the board?
Yes, there is a processor on the board. It is tested with the same operating conditions.
Presumably a 3.3v nominal one? Maybe it's not getting enough volts to work properly. On the psu measure its current with 3v3. Try fresh AA batteries, they are usually 1.65v or so.
The operating voltage of the MCU is between 1.8V-5.5V. So it must be in the range.
Which mcu are you using? Please could you provide a schematic and your code?
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Are you powering the MCU directly from the battery, or power supply, without a voltage regulator. If so, does the Vcc voltage measure about the same either way?
Maybe a schematic, code and a photo of actual circuit show what is going on?
Maybe a step up converter is involved? Or a few floating pins?
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I don't know what's causing the difference but it's not the internal resistance of the power source. If the voltage is the same and nothing in the circuit changes, the current should be the same (Ohm's Law). Resistance in the power supply (or in series) causes a voltage drop, and a proportional current drop.
The source resistance is insignificant at low current.
The internal source resistance along with the load resistance makes a voltage divider... If the load resistance is equal to the source resistance, the voltage would drop in half (and the current would be half of what it is with full-voltage). ...But buy the time the voltage drops in-half the power supply has probably fried (or shut-down, or blown a fuse, etc.). Regular AA batteries will survive that, but modern higher-power rechargeable batteries can catch fire or blow an internal fuse.
Thank you for your comments guys. It is so strange that I fix this problem by making some gpio adjustments in low-power mode. I call gpio config function every time when MCU got into low-power mode. Now it consumes nearly the same power as it is supplied by a battery. Still, it seems pretty odd to me because it hasn't drawn that current when it is supplied by the power supply.
A common mistake with beginners projects is no power supply decoupling.
The MCU could/will misbehave without that.
Leo..
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