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Educational BoosterPack LC + ESR Meter Template


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Another quick experiment w/ the CircuitCo Educational BoosterPack.

 

 

  • Measures capacitance (< 1nF) and inductance using LC tank frequency measuring method.
  • Measures capacitance (> 1nF) with time-to-charge method.
  • Measures (or rather attempts to measure) capacitor ESR values by pulse injecting currents through capacitors.
  • Bare minimal component count, experimental hookup.

I called this a "Meter Template" instead of a "Meter" as I consider this hookup as a base to develop a "Full" meter. There are quite a few things "missing" when compare w/ other common designs you can find on the internet, that affects accuracy and usability.

 

  • Breadboard is no good for capacitance measuring, and LC tanks.
  • Instead of using transistors to source and sink currents through test subjects, I drive them direct (close to 30mA on one pin).
  • No relays to switch inputs, have to use your hand.
  • No relays / switches to map multiple ranges components. One range for all (well, we have high and low capacitance).

And it's fully functional with not-too-bad accuracy.

 

Schematic and additional write-ups will follow.

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Incremental progress update w/ photos and ascii schematic.

 

Breadboard circuit connects to LaunchPad via 5 IO pins, plus draws power from LP.

The breadboard circuit is make up of 6 resistors, 4 capacitors, 1 inductor and 2 switches, there are no active components.

 

IMAG0619.jpg

 

With the BoosterPack attached, there is limit choice on the use of IO pins. And I had to access P1.1 and P1.2 via probe hooks.

 

IMAG0612.jpg

 

 

 

Schematic:

             MSP430G2553
           ---------------  
       /|\|            XIN|-
        | |               | 
        --|RST        XOUT|-
          |               |                    /|\
     (black)  P1.3  .-----|-------o-----o---o   |
          |         ^     |       |     |   |   |
          |        / \    |       |     |  | | | |100k x 2
          |       /__+\   |       |     |  | | | |
          |        | |    |       |     |   |   |     10uF
     (blue)   P1.1 | o----|-------|-----|---o---o--+||--o-----o-----o-------o
          |        |      |       |     |       |       |     |     |       |
     (yellow)      | P1.2-|-o     |     |       |       |     |     |_      |
          |        |      | |    | |47k| |47k  | |     --1nF --1nF    )82uH |
          |        |      || |   | |   | |     | |100k ---   ---      )     |
          |        |      || |100 |     |       |       |     |      _)     |
          |        |      | |     |     |       |       |     |      |      |  
     (yellow) P1.6 o------|-|-----|-----o---||--o-------o     o    o o----o o
          |               | |     |        0.1uF|            \      \ .....\
     (white)  P1.4 o------|-o-----o  o          |             o      o      o-----(A)
          |               |          |          |             |      |       
          |               |          |          |             |      o------------(
          |               |         _|_        _|_           _|_    _|_    L=C 
          |               |         ///        ///           ///    /// 
          |               |                  
  

The LC (Cx up to 1nF) meter part utilizes P1.1, P1.3, P1.6. Comparator A is used along w/ other passive components to form a LC tank circuit. This oscillates at a frequency based on the inductor and capacitor values. Where Freq = 1 / (2 * Pi * Sqrt(L * C))

 

A DPDT switch is used to select capacitance or inductance measure mode. With the subject capacitor / inductor attached, we measure the frequency of the oscillation, and use the above formula to derive the unknown L or C value.

 

 

The high range Capacitor measurement utilizes P1.4 and P1.3. P1.4 is configured as Comparator A inverting input (-) while we use internal 0.55V as the non-inverting (+) input. We setup the comparator interrupt to trip TimerA and measures the time it takes to charge a subject capacitor to 0.55V. Since the time to charge is linear to the capacitance, we used a stored calibrated value as a reference to work out subject capacitance. P1.3 is used to start "charge" the target capacitor.

 

The ESR measurement is based on the high range capacitance measurement (P1.4, P1.3) with an additional P1.2 pin. Here we use P1.4 as ADC input. A 100ohm resistor is used between P1.2 and P1.4, with a target capacitor in place between P1.4 and ground, this three nodes (P1.2, P1.4, Gnd) forms a voltage divider. By measuring the voltage at P1.4 we can derived the resistance (or ESR) of the target capacitor.

 

 

The above is a high level overview on the set-up. You can find out more by googling the subject. I studied various designs from the internet and come up w/ this. And at this stage I want to have the most bare-bone set-up to play with.

 

I am not ready to release the source code yet as the ESR part is off a lot and still to be worked on. The challenge is that I need to have a sub 100 ohm resistor to get to the 1/10 or 1/100 resolution needed for ESRs, and w/ a small value resistor, P1.2 is drawing a lot of current and upon load attached, the voltage drop is affecting the calculation quite a bit. (and I still do not want to add transistors, just trying).

 

 

 

 

 

 

 

 

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  • 3 weeks later...

I am releasing the 1st version of the source code for whoever interested to play w/ it. Don't want to hold off too long as I can't work on it in the next week.

 

https://github.com/simpleavr/lc_meter

 

The ESR measurement is getting better but still not accurate (I think, I can't tell as I had not reference).

 

I had used 1, 2.2 and 7.5 ohm 1% resistors to try and calibrate it, but I think it's still off when measuring capacitor ESRs, it looks high. It's good enough to spot a bad cap though.

 

I had since add an additional range in the high-range capacitor measuring to speed up reading large (100uF+) caps.

 

The capacitor and inductor measurements are quite good for hobby use. I initially want to build this when I was trying to build a one ic radio (TA7642) and in need to measure hand-wind coils. (I build one radio w/ mk484 30+ years ago and want to build one again). So far the meter is serving this purpose well.

 

I will come back to this project later, intend to turn it standalone w/ a 4x7 segment LED (for retro look) on a 2x2 inch board.

 

Thanks for looking.

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