In considered one of my current tasks, I needed to seize the dynamic present profile of a short-range wi-fi embedded platform for a lot of operation situations. As a result of dynamic nature of the present consumption concerned, I used to be unable to make use of a typical voltmeter for this job. On the time axis, I wanted a minimal decision of 1 millisecond and the accuracy of the present measurement was purported to be higher than 2 mA. This job could be a chunk of cake with a Digital Storage Oscilloscope, which I didn’t have on the time and subsequently I needed to give you a less expensive methodology to get the job accomplished.
The highest-level diagram of the answer I got here up with is proven beneath. I used 3.0 V button cell battery as my energy supply in all of my measurements, nevertheless you need to use an unusual DC energy provide in your measurements as required by the actual load you utilize. The load is the system whose present profile we’re concerned with. The load’s optimistic terminal is linked to the battery and the bottom terminal is in sequence with a 0.3 ohm shunt resistor (Rs) that’s terminated at floor. The shunt resistor must have a small worth so as to not trigger a big voltage drop throughout the Rs, which decreases the required operational voltage throughout the load (You may consider Rs as an unusual wire with a little bit of resistance). The ability of Rs is normally a excessive worth (e.g. 1 Watt) so as to not get broken by the present flowing by way of it. The voltage throughout the shunt resistor (denoted Vs) varies relying on the present drawn by the load. This present is just equal to Vs/0.3 amperes.
Observe that Vs is usually small because of the small worth of the shunt resistor. Due to this fact, digitisation and the measurement of Vs instantly is troublesome and an intermediate amplification stage is critical. In my software, an amplifier with a achieve of ~11 was used. When you want greater positive aspects in your measurements, merely modify the R2/R1 ratio as wanted. The general achieve for the amplifier topology offered is ( 1 + [R2/R1] ). I merely used a general-purpose LM324 opamp which labored rather well.
The output of the amplifier is fed to an analog-to-digital converter (ADC) that had a decision of 10 bits per pattern and ran at a sampling frequency of 20 kHz. This sampling fee equates to 50 microseconds between successive samples, which is best than what I wanted. Larger sampling charges can present significantly better decision in time however do not forget that your storage necessities will even enhance because of this. The digital output of the ADC is transformed to milliamps and for every pattern a time stamp in milliseconds can also be related. Observe that the blocks indicated contained in the dotted strains are an integral a part of the microcontroller platform (i.e. AtMega2560) I utilized in my software.
In my software, I dumped the SRAM content material after every measurement session because of the reminiscence limitations of the platform I used. Nonetheless, I additionally experimented with real-time serial dump of every ADC pattern and that methodology will even work in case your measurement decision necessities are extra relaxed. You may simply use a serial terminal program corresponding to TeraTerm to save lots of the ADC samples.
Utilizing the above arrange, it’s attainable to pattern present ranges as little as 1.15 mA. Determine beneath illustrates a captured present profile throughout the RF transmission of the load I utilized in my undertaking. The repetitive transmission duties can simply be recognized utilizing the dynamic present profile as proven within the determine.
I hope you will discover my tried and examined methodology for present measurement helpful in your tasks. The most effective a part of it’s it means that you can get correct measurements with out an costly digital storage oscilloscope!
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