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Topic: ATMEGA328p Standalone 8MHz @ 2.5v (Read 12484 times) previous topic - next topic

winner10920

Its probably like my meter, the regular (non shunted) terminal has a max 200ma current measue ability and it works just like that
You could alternatively put a precision 1 or .1ohm resistor in series with the supply and measure the mv across it to get the current

dhenry

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precision 1 or .1ohm resistor


They are tough to find.

Alternatively, you can use a small power resistor (0.1, 0.22, or 0.47ohm, or in parallel), to measure the current. You can calibrate / measure the resistor's value via a constant current source.

winner10920

Hard part is having a reliable precision device to start with to calibrate thr rest, unless its a precision current source it won't be accurate, unless its a precision resistor it won't be accurate, might as well just buy a better multimeter lol

john1993

unlikely you need one part per million accuracy. 10% is probably fine. and power resistor is definitely not required. regular tiny 1ohm, 10ohm, or 100ohm of any low wattage will do here. at really low reading even 1k may work. or maybe replace the fuse in that meter. #1 cause of bad meters is using any of the the current ranges to measure across a high current supply like a battery. guaranteed to blow the fuse or smoke the leads on no-fuse setting.

even cheapo $3 harbor frieght meters are accurate within fraction of a percent. not like the ol' day where mechanical meter could be 2% or even 3% off. still ok here though. accuracy and should not be confused with sensitivity either. however both are cheap these days.

Zapro

It doesn't seem like anyone in this thread is aware of "Burden Voltage" - You cannot measure the current the chip is using without adjusting the voltage of the power supply, so it's correct at the target.

Have a look here on what i'm talking about http://www.alternatezone.com/electronics/ucurrent/

// Per.

dhenry

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Have a look here on what i'm talking about http://www.alternatezone.com/electronics/ucurrent/


Way exaggerated.

If you truly care about it, put a current sensing amplifier there and call it a day.

Zapro


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Have a look here on what i'm talking about http://www.alternatezone.com/electronics/ucurrent/


Way exaggerated.


I wouldn't call it exaggerated when you set your power supply at 2,5V, put your meter in series with the chip and then the chip only gets maybe 1,9V and you wonder what happened!

// Per.

dhenry

I would argue that if a chip draws so much current that it causes that substantial of a voltage drop over the meter, the chip is no good; and/or the person doing the measurement is no good.

The issue he raised can be easily addressed.

Zapro


I would argue that if a chip draws so much current that it causes that substantial of a voltage drop over the meter, the chip is no good; and/or the person doing the measurement is no good.


The burden voltage of a multimeter is the same, regardless if you pull 1 mA or 200 mA trough it.... It does not change.

// Per.

dhenry

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The burden voltage of a multimeter is the same, regardless if you pull 1 mA or 200 mA trough it.... It does not change.


Take out your multimeter and measure it.

There is a reason that "burden voltage" is specified in mv/ma. And it is typically lower for high amperage settings - for good reason.

retrolefty

Well of course the 'burden' of the fixed shunt resistance of the amp meter will have an effect on the actual voltage applied to the load being measured. It forms a simple voltage divider with the load resistance and follows ohms law. A simple 'fix' is to simply measure the voltage actually being applied to the load with a second meter and adjust the source voltage for the value you wish the load to be operated at as you measure it's current consumption at that applied voltage.

Understanding ones basic test equipment is important if you want to account for all the possible sources of variation and error in taking precise measurements, if that is indeed the task at hand.

Lefty

dhenry

The particular meter that David Jones was talking about has a equivalent resistance of 1.8ohm (1.8mv/ma), in the 200ma current setting.

As the current we are talking about here in this threads are on the tune of 2ma, the "burden" induced by the meter is 4mv, or about 0.1% of the supply voltage.

I think the avrs can deal with that without any problem.

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