Experiment 5 - Capacitive Sensors I

Owned by Jana Ihrens
Last updated: 13 May 2022
3 min read

Tasks:

In this Lab the goal is to learn about capacitors. As capacitive sensors can be used for many applications, measuring capacitances is an important task. For different types of capacitors the capacitance will be measured. You will have to use the knowledge about oscilloscopes and function generators from experiment 2.

Be careful with the polarisation of the capacitor, usually the terminals are marked with +/- or one pin is longer than the other, in this case the longer pin is the positive terminal.


Fig. 1: Capacitors (ceramic, electrolytic, foil) and inductance (left to right)


  1. Start measuring the electrolytic capacitor by using the multimeter. Before you connect the capacitor to the multimeter, make sure that the capacitor is not loaded by connecting the 20kOhm resistance to the terminals.
    1. How does the multimeter measure the capacitance? What other options to measure a capacitance do you know?
  2. Measure the ceramic capacitor by using the setup shown in fig. 2. First connect the oscilloscope across the capacitor. Then connect the battery to the circuit. The oscilloscope should show the voltage across the capacitor rising until at full charge. 
    1. Calculate the value of 63.2% of the applied voltage and move one of the cursors to the point where the voltage reaches this value.
    2. Move the other cursor to the beginning of the loading process. Measure the time difference between the two cursors. Save the screen to USB.
    3. This time difference is called τ=RC.
    4. Calculate the value of the capacitance.  

 

 Fig 2: Time constant measurement

3. Now measure the ceramic capacitor by using the setup shown in fig. 3.

a. Use the breadboard to build the circuit.
b. Set the function generator to continuous sine wave with f=100Hz and an amplitude of 1.9V.
c. Connect CH1 of the oscilloscope to Point A and CH2 to point B. Also connect mass appropriately.
d. Take a look at fig. 4 and fig. 5 and use the following equations to calculate the impedance between A and B and the angle γ to find the value C. 


Fig. 3: measuring with sine wave 

  

Fig. 4: equivalent circuit           Fig. 5: vector diagram

4. Measure the foil-capacitor by using the circuit from fig. 6. Connect the inductance (220 µH) and the foil-capacitor to a resonator circuit on the breadboard and set the (internal) function generator of the oscilloscope to a sinewave with frequency 1kHz and peak-to-peak-Voltage to 4V.

a. Connect the oscilloscope in parallel to LC.
b. Now change the frequency slowly and watch the voltage carefully. You will be able to see a sharp voltage drop, once you reach the resonance frequency.
c. What condition is satisfied when reaching resonance frequency?
d. Calculate the capacitance by using the following equations:

Fig. 6: resonance measurement



Sources:

[1]     P. P. L. Regtien, Hg., Sensors for Mechatronics // Sensors for mechatronics. Amsterdam: Elsevier, 2012.

[2]     Prof. Dr.-Ing. G. Ackermann, „Grundlagen der Elektrotechnik“. Manuskript der Vorlesung, Institut für Elektrische Energiesysteme und Automation, TU Hamburg, Hamburg, 2017.

[3]     Freddy Alferink, Measuring capacitance & ESR. [Online]. Verfügbar unter: https://meettechniek.info/passive/capacitance.html

[4]     Russo, Thomas V., „Capacitance and Inductance Measurements Using an Oscilloscope and a Function Generator: Application note Tektronix“, 2013.









Institut für Mechatronik im Maschinenbau (iMEK), Eißendorfer Straße 38, 21073 Hamburg