5-2 Coulombs Measurements
Measurement overview
Coulombs measurements — Model 6514 can make coulombs measurements from 10fC to
2.1µC using four measurement ranges; 20nC, 200nC, 2µC, and 20µC.
In the coulombs function, an accurately known capacitor is placed in the feedback loop of the
amplifier so that the voltage developed is proportional to the integral of the input current in
accordance with the following formula:
Where; V is the voltage
C is the known capacitance
Q is the charge
The voltage is scaled and displayed as charge.
External feedback — The external feedback mode of Model 6514 can be used to measure
non-standard charge ranges. Measurements using the external feedback mode are covered in
Section 11.
NOTE Accuracy specifications for all measurement functions are provided in Appendix A.
Auto discharge
Model 6514 has an auto discharge feature for the coulombs function. When enabled, auto dis-
charge resets the charge reading to zero when the specified charge level is reached. After the inte-
grator resets, the charge measurement process simply restarts at zero. The charge reading resets
every time the specified charge level is reached.
When auto discharge is disabled, you can use zero check to reset the integrator.
Perform the following steps to set an auto discharge level and enable it:
1. Press SHIFT and then AUTO-DIS to display the present auto discharge level.
2. Use the cursor keys (
and ) and ( and ) to enter a discharge level. To change polar-
ity, place the cursor on the “+” or “-” sign and press
or . To change range, place the
cursor on the range indicator and use the
and keys.
3. With the desired auto discharge level displayed, press ENTER.
NOTE Pressing SHIFT and then AUTO-DIS a second time disables auto discharge
(“DISCHRG OFF” displayed briefly).
V
1
C
----
itd
∫
Q
C
----==
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