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MicroBlocks

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Joined: 12/05/2012
Location: Thailand
Posts: 2209

Posted: 04:15pm 22 Sep 2015

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I am currently designing a small current measuring device that sends data over usb.
If small enopugh i would like to add it to my current USB-Serial/Programmer solution.

The 'problem' is choosing the right range that will be most useful.
Chris Roper already mentioned it would be nice to be able to measure very low current so that you can see how much is used when a mcu is in sleep mode.

Another use would be to know how much you are using running the mcu.
The difference between these two is quite big.

I will be using the internal vref of the chip that gives either 1.024v or 2.048 volts. So i need to choose the right shunt resistor.
The ADC has 10 bits resolution.

I want this to be a low side measurements, meaning the shunt resistor will be between the GND pin and the real ground that is connected to the power supply.

I might be able to use two shunt resistors and 2 ADC channels on the mcu to have two ranges.

Another possibility is adding a INA225 with programmable gain. It offers 25,50,100 and 200 times gain. Which in combination with a good shunt resistor value might give a very wide range.

An MCP6001 is also a good candidate.

But first and foremost. Which range of current would you like to be able to measure?
(Keep in mind i only want to use it for measuring a absolute maximum of 500ma)

Edited by TZAdvantage 2015-09-24

Microblocks. Build with logic.

paceman
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Joined: 07/10/2011
Location: Australia
Posts: 1329

Posted: 01:06am 23 Sep 2015

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Hi TZ,

I'm very much a hobbyist user so with that caveat these are some of the uses I've needed and would be looking for:

1. checking various 'indicator' LED's - say in the 3 to 20mA range, and maybe up to 100mA. Obviously there's also a huge range upwards from this for various displays: multi-digit 7-segment, and small TFT's up to 7" TFT's, but no one would assume a small current meter would suffice to check a big display.
2. checking current sinking/sourcing from micro-controller pins - say 0.2mA to 30mA. The low end might be feeding one or multiple transistor base(s), the high end might be feeding a relay.
3. looking at current draw from a small motor or servo, 20 to 100mA range.
4. checking the current draw from an ESP8266 module when transmitting - say 40 to 80mA.
5. checking the draw from any module or small boxed PCB/project to make sure it's power supply is well within range, say 20mA to 150mA. This one is pretty much open-ended but the range is pretty typical for me.

They're some of my thoughts but obviously this is a pretty open-ended request. Another point of course is that pretty much anyone fiddling with electronics is going to have at least a standard DVM available - it's just a matter of convenience of use and a little in-line meter can be very useful.

I can see that the very low end would be useful for development of battery projects but so far I haven't done much of that.

Greg

MicroBlocks

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Joined: 12/05/2012
Location: Thailand
Posts: 2209

Posted: 07:15pm 23 Sep 2015

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Thanks Greg. Those are some things i did not think about.
I would like to only add a shunt register on the gnd path.
This would make the pcb a lot easier as i have limited myself to 18x18mm.
Calculating the right values though is more difficult then i expected. I will need an opamp to get a good resolution.

Anyone have suggestions for what values will work with the above mentioned current ranges?

Microblocks. Build with logic.

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