Ampere meter for small current?

Matt Shira

Hi,

Could someone recommend an ampere meter to measure the small current of MSP430 board? I do have a multi-meter which can measure current for like through a small light bulb; however, for small sized Lithium batteries or cell batteries, the multi-meter seems to have limited the current to much lower than in normal operation due its internal resistance. For example, my whole board consumes maximum 50mA current, but when the multi-meter (as ampere meter) is connected in series, only less than 20mA current could pass through. I looked up in the multi-meter’s manual but didn’t find any specification of ampere meter’s internal resistance.

So is it appropriate to use multi-meter’s serial connection for measuring small current like for MSP430 boards? TI measures LPM modes' currents at uA precision, and what is the instrument used for such measurement? Is it very expensive?

Matt

over 13 years ago

0 Jens-Michael Gross over 13 years ago

Guru 227245 points

If your multimeter supports low voltages, you may put a 10 Ohm resistor in series with the MSP and measure the voltage on it. On 10 mA current, this will result in a voltage drop of 100mV. However, on 10µA, you'll have 100µV to detect. It realyl depends on your multimeter.

Alternatively, you may use a scope for checking such low voltages. You'll also see the voltage (=current) spikes then instead of the heavily averaged readings of a multimeter.

0 Matt Shira over 13 years ago in reply to Jens-Michael Gross

Expert 2180 points

Jens,

Thanks for the suggestions.

On the other hand, does there exist anything like what might been called "precision small current meter" or alike?

Matt

0 Peter Bigot over 13 years ago

Expert 2505 points

I like the uCurrent: http://www.adafruit.com/products/882. Very useful for the LPMx.5 modes.

0 Andy Neil over 13 years ago in reply to Matt Shira

Guru 35565 points

As already mentioned, meters give very heavily averaged readings - so not very useful for understanding the detail of a microcontroller's currect consumption.

Search for "current sense amplifiers";

eg, on this site: http://www.ti.com/sitesearch/docs/universalsearch.tsp?searchTerm=precision%20current%20sense&linkId=1

Other vendors, of course, are available...

0 Matt Shira over 13 years ago in reply to Peter Bigot

Expert 2180 points

Peter,

Nice stuff! I will check that.

Matt

0 Matt Shira over 13 years ago in reply to Andy Neil

Expert 2180 points

Andy,

I am aware of the sense amplifier solution. Just looking for some easy stuff off the shelf :)

Matt

0 Jens-Michael Gross over 13 years ago in reply to Matt Shira

Guru 227245 points

Matt Shira said:
I am aware of the sense amplifier solution. Just looking for some easy stuff off the shelf :)

A simplified version of the amplifier is a simple series resistor in tha current path (10Ohm for µA range, 10k for nA range or 10mOhm for mA range) and an OpAmp with a gain of 100 that amplifies the voltage across the resistor. Then you can measure 1mV/µA (nA/mA) with minimal voltage drop. If you don't need too much precision (noise/offset), it can be done with cheap parts you already have.

0 Matt Shira over 13 years ago in reply to Jens-Michael Gross

Expert 2180 points

Jens,

Thanks. But still prefer the lazier method...

Matt

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Ampere meter for small current?