Tech Note 502 Detail

Charge Sensor Operation

Affected Products:

CI-6555 Charge Sensor
PS-2132 PASPORT Charge Sensor

Problem/Symptom:
Charge Sensor Operation

PASCO Solution:

The PS-2132 and CI-6555 charge sensors measure the charge across a 10 nF internal capacitor that is placed in series with the external capacitor. Capacitors in series must have the same charge, so by measuring the voltage across the known internal capacitor, the charge on the external capacitor can be calculated by Qexternal = Qinternal = Cinternal*V. (The charge sensors can measure the charge on capacitors much smaller than 10 nF, provided that the voltage used to charge the capacitances is proportionately larger; however, higher voltages at significant capacitance are not recommended due to safety considerations.)

Proper Measurement Procedure with the PS-2132 or CI-6555 Charge Sensors

Insulators with Trapped Surface Charges (Proof Planes, Fur, Acetate Films etc.)

Required:

Procedure:

  1. Connect the red lead of the charge sensor cable to the inside screen of the Faraday Ice Pail.
  2. Connect the ground lead of the charge sensor cable to the outside screen of the Faraday Ice Pail.
  3. Begin recording.
  4. Press and hold the zero button on the charge sensor until the value of the charge has stabilized at zero.
  5. Release the zero button.
  6. Place the charged object completely inside the inner pail.
  7. Stop recording when you get a stable value.

Low-Leakage Capacitors with Capacitances of 0.1-1000 nF

Recommended:

Procedure:

  1. Select a low-leakage (polypropylene (~ 83 hours at 25 C), polyester, polystyrene, or Teflon) capacitor having capacitances in the range of 1-100 nC. (Avoid using electrolytic, ceramic, or tantalum capacitors as the RC discharge time is only a few minutes.)
  2. Place the capacitor under test in a test fixture with a ground plane underneath.
  3. Connect the ground lead of the charge sensor to a the ground plane underneath the capacitor, Earth ground, and a groundable lead of the power supply to one side of the capacitor.
  4. Connect the other side of the power supply to the capacitor to charge it to a voltage between +/- 10 V.
  5. Disconnect the power supply from the component.
  6. Begin recording.
  7. Press the zero button on the charge sensor.
  8. Connect the leads of the charge sensor to the capacitor.
  9. Stop recording after the charge stabilizes.

If the charge did not change significantly before the leads were connected to the capacitor, then the stabilized value of the charge should be close to the actual charge on the capacitor; otherwise, a large amount of charge was transferred by the operator to the capacitor. Avoid getting wool or synthetic fabrics close to the leads of the charge sensor, discharge your hands to the ground plane, and repeat the measurements.

Objects with pico-Coulomb (pC) Charges (ES-9043 parallel plate capacitor, etc)

The PASPort and ScienceWorkshop Charge Sensors cannot give accurate results for these devices because the value of the voltage on the internal capacitor is too low and because the leakage time is too short (<< 100 s); these measurements require an electrometer-Faraday cage system, such as the ES-9078 Basic Electrometer and ES-9042A Faraday Ice Pail system. The charge Q would be determined as Csystem * Velectrometer, where Csystem ~ 50 pF.

The exact value of Csystem is determined calibrating the system with a low-leakage capacitor (examples: Teflon, polypropylene 10 pF) of known capacitance as described in the ES-9078A manual.

Objects with femto-Coulomb Charges

Charges in the range of less than 1 fC can be measured with the electrometer connected to a ScienceWorkshop analog input port ; The voltage output by the electrometer is 1/20th of the input voltage and the capacitance of the system is ~50 pF, so the minimum charge Q that can be resolved is given by Q [nC] = (Vres [V]), where Vres is the voltage resolution of the analog input channel of the interface.

Refer to the chart below for the resolution of our signal interfaces:

PS-2002

PS-2008/2009/2011

CI-6400

CI-7599

UI-5000

Bits on A/D converter

12

12

12

12

14

Voltage Gain, G

1

1

1
10

1
10
100

1
10
100
1000

Maximum Sampling rate (kHz), Smax

50

1

20

250

10000

Voltage Range [V]

+/- 10

+/- 10

+/- 10/G

+/- 10/G

+/- 20/G

Vres

Voltage Resolution (mV) @ 1x Gain

4.88

4.88

4.88/G

4.88/G

2.44*S/G/107

G ≡ User-Selected Gain Setting
S ≡ User-Selected Sampling Rate

When performing sensitive measurements with the electrometer, one will need to do the following:
  1. Zero the electrometer using both the button on the electrometer and software.
  2. Use at least one static dissipative mat to discharge the user and other materials that have stray charges. (The shielding on the electrometer and the ice pail is inadequate to prevent charges on the operator from interfering with such sensitive measurements.)
  3. If using the 850 interface, use a sampling rate of 10 Hz or lower. If using another interface oversample (10x or more) and smooth the data.

Creation Date: 07/16/2004
Last Modified: 01/29/2015
Mod Summary: