Jun 19, 2011

New Multimeter

Why invest your money when you buy multimeters! My new BK Precision 2709B multimeter showed up about two weeks ago and I have just gotten around to playing with it. This meter actually won Dave Jones’ $100 multimeter shootout on the EEVBlog (check out the two part video if you are interested in getting an above average meter). Before this meter I had a cheap $20 Chinese meter that I used for college projects and labs so I figured it was time to upgrade. It’s truly amazing the difference quality equipment makes. A little while back I got my first temperature controlled soldering iron (Hakko FX-888) and my work with it has been so much better than anything I did with my cheap Radio Shack iron.

Now for some specs that I really liked about this meter:

True RMS: This meter can measure the true root-mean-squared (RMS) value of any input waveform. Some meters can measure RMS, but they assume the input waveform will be a sine wave so measuring a signal with any other shape will give you inaccurate results. This feature is especially helpful in switching converters because you need RMS voltages for calculating efficiency and selecting components with the right tolerances.

Micro-amps Range: Having a micro-amps range meter is probably going to be a luxury with most designers who don’t work on embedded systems, but one my interests is digital power so I think it may come in handy in the future. For general purpose applications, a micro-amps scale is useful to determine the leakage current of components or the quiescent current of integrated circuits (the current need to make sure the ICs are on even if they aren’t doing anything relative to the system).

3 ¾ Digit Display: Many people get this concept confused when it comes to LCD displays in electronics meters. The first piece is pretty simple: the integer number used for the number of digits represents the number of LCD digits that can display 0-9. So a three digit display can display 999 at the maximum.

Now for the confusing part. The fractional piece usually refers to the most significant digit. Typically you will see a rating like 3.5 digits, which means the maximum the meter can display is 1999 (not 4999 or 5999 like you might think). This meter has a 3 and ¾ digit display so you can get readings up to 5999. As far as I know there are no real defined parameters for these digits outside of using 0.5 to indicate a maximum of 1 as the most significant digit.

0.5% Accuracy: Another term that often gets misused when it comes to multimeters is accuracy. There are actually several accuracy ratings that a multimeter gets because of all the different measurements that it is capable of making (DC volts and amps, AC volts and amps, temperature, etc.). My understanding is that the DC accuracy measurements are the ones most often expressed as the accuracy of the meter because most devices will use DC power.

Accuracy is given as a percentage +/- a few counts, which means you are getting a combination of the measurement accuracy plus the resolution of the LCD display. This meter has a rating of 0.5% + 2 counts. So, for example, if you are measuring a 5V signal, the measurement could be anywhere between 4.975V and 5.025V, but what would actually be displayed on the screen is 4.973V to 5.027V.

Like everything in electronics, the best choices fit the needs of the application. In my case, power conversion requires accurate DC voltages and currents as well as RMS values so DC accuracy and true RMS were important when I chose this meter. I will admit  the fact that Dave Jones gave it his thumbs up helped a lot too, but it ultimately came down to what I needed and the fact that I had seen an in depth review of it somewhere else.

I didn’t really have any specific goal in mind for this post other than to show off my new meter, but my hope is that some of this will be useful in the future. If you want to learn more about multimeters – how they work, what to buy, what to look for – check out the EEVblog and look for any episode with “multimeter” in the heading. Dave Jones is the king of meters as far as I am concerned. He does some really great tutorial blogs on how they work and what to look for when you are buying.


Joel Ryan said...

Thats pretty interesting that the resolution of the screen has an effect on the accuracy of the measurement. It makes sense, but it reminds me of how everything is relative and depends on the viewing medium and all that. I'd never heard of the 3 3/4 digit display thing either.

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