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Blog Keysight Technologies U1282A Digital Multimeter - Component Applications - Sorting
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  • Author Author: dougw
  • Date Created: 9 May 2016 4:03 AM Date Created
  • Views 1109 views
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  • road_test

Keysight Technologies U1282A Digital Multimeter - Component Applications - Sorting

dougw
dougw
9 May 2016

This segment of the road test will start exploring some of the applications I want to use this meter for - the real reasons I applied for the road test.

 

I design and assemble a lot of PCBs with surface mount components. Inevitably I end up with a variable number of random unused parts of unknown value. Capacitors in particular or small footprint resistors have no markings to indicate their value, so these parts become essentially useless. I have collected a small container with quite a number of such components with the vague intention to figure out a use for them. I went as far as to purchase a set of “tweezer” probes to measure surface mount components and it strikes me these probes might allow unknown components to be quickly sorted into standard value bins.

image

The probes did not come with a meter so this Keysight  U1282AU1282A should be ideal for this application The large display can be read quickly and the capacitance function will allow those unmarked capacitors as well as resistors to be measured efficiently I think it will be interesting to demonstrate whether this application is quick easy and useful

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Measuring and sorting SMT components is always going to be fiddly but with a tweezer probe and this meter and a little practice, it is feasible.

 

As you may know multilayer ceramic chip (MLCC) capacitors, like the ones I am measuring in this test, change capacitance significantly based on voltage. The following graph shows they can lose over 80% of their rated capacitance near their rated voltage. It is worse for dense capacitors so a 1uf 0603 will be affected more than a 1uf 1206. This will have a significant effect on filters and other circuits requiring stable capacitance. It may be worth placing such filters before your amplifiers.

image

The reason capacitance degrades with voltage is that the permittivity of Barium Titanate (ceramic dielectric) is a function of voltage – spontaneous local reversals of polarization at the crystal lattice level are increasingly inhibited by stronger electric fields. Polarization reversals increase permittivity and capacitance is directly proportional to permittivity, so the higher the voltage, the lower the capacitance.

To see where this meter is measuring on the degradation curve, I will monitor it with an oscilloscope. This may alter the measurement signal, but it should give some feel for the voltage level.

image

The scope trace tells us a number of things:

  • The wave form is a triangle wave, which means the Keysight is using a constant current source and constant current sink to generate this frequency.
  • The frequency is 99.1 Hz (implying the charge period is 5 ms), the capacitance is 21.46 nF and the voltage is about 850 mV; so the constant current is about 3.65 uA (because i=CV/t)
  • The peak-to-peak voltage is 856 mV but the peak voltage is 1.18 V because the waveform is offset from ground.

The signal looks to be around 1 volt which is good because it is below where 16V MLCC capacitance falls off the cliff - as shown above. Most of my my capacitors are 16V or more, except maybe a couple of 10uf caps.

But here is what a 10 uF, 6.3 V 0805 MLCCapacitor looks like on the scope.

image

Note even at this low voltage the slopes of the triangle waveform are not exactly straight lines. This shows the capacitance is changing with voltage. The frequency is about 2.5 Hz.

 

Here is a high voltage 4.7 uF plastic film capacitor for comparison – its capacitance should be pretty stable with voltage:

image

Note the better triangle waveform. The frequency is about double the frequency of the 10 uF measurement, as you would expect with a constant current driver.

 

Capacitors are often required to be reasonably accurate for desired circuit performance - this exercise shows the designer needs to be aware of MLCC performance limitations and even capacitance meter characteristics in order to obtain predictable results.

 

Links to other installments of this road test:

unboxing

component and DC voltage measurements exploration

waveform measurements exploration

teardown research

applications exploration - component sorting

applications exploration - water level measurement

applications exploration - sensor datalogging

formal road test link

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Top Comments

  • jw0752
    jw0752 over 9 years ago +1
    Hi Doug, This is a great idea. I wasn't aware of tweezer probes before. I can see where this would be a great way to sort the little devils. Combine the measuring with the ability to pickup and move them…
  • jw0752
    jw0752 over 9 years ago in reply to dougw +1
    Thanks, I will check it out and get some to play with. Let me know how the carving goes. John
  • shabaz
    shabaz over 9 years ago +1
    Hi Doug, Fantastic post as usual! I too have similar tweezers, and also a slightly more expensive TESTEC pair, but they both have different benefits. Your ones are nicely spaced apart for reduced capacitance…
  • shabaz
    shabaz over 9 years ago

    Hi Doug,

     

    Fantastic post as usual!

    I too have similar tweezers, and also a slightly more expensive TESTEC pair, but they both have different benefits.

    Your ones are nicely spaced apart for reduced capacitance, whereas the TESTEC ones are not. The

    TESTEC ones have a longer cable, but it is not very flexible, whereas your ones have flex on the cable and are more comfortable

    for extended use and short length means less inductance.

    The reason I got them was because a particular project had a different supplier BoM and I've been burned in the past

    with that supplier providing mixed-up components. So I wanted to hand-check each SMD component since it was an expensive

    project (a vector network analyzer test tool) and since it wasn't my design I would have a hard time knowing it was working correctly

    or giving inaccuracies due to wrong component values, so I needed to be confident all parts on the board were correct.

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  • jw0752
    jw0752 over 9 years ago in reply to dougw

    Thanks, I will check it out and get some to play with. Let me know how the carving goes.

    John

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  • dougw
    dougw over 9 years ago in reply to jw0752

    They cost about $2.51 on ebay. They are a bit stiff - I may try to reduce the stiffness with some judicious carving.

    Doug

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  • jw0752
    jw0752 over 9 years ago

    Hi Doug,

     

    This is a great idea. I wasn't aware of tweezer probes before. I can see where this would be a great way to sort the little devils. Combine the measuring with the ability to pickup and move them. If I may ask where did you get the tweezer probes?

     

    John

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