Basic Test Theory And Specifictaions - Saluki STB6684 Operation Manual

STB6684/A Operation Manual

4.5.1 Basic test theory and specifictaions

STB6684/A can be programmed to run a sequence of measurements after being triggered
from the keyboard or remotely.
The measurements can be compared with limits and set the bin out siganals to appprove or
reject the device under test. In order to avoid some of the pitfalls associated with this,
especially when testing capacitors, a basic understanding of the functions of the
megohmmeter is required.
Uhv
HV
GND
Figure 4.5.1 Test Schematic Diagram
Figure 4.5.1 shows the HV supply and the input stage of the current amplifier. The test object
is a capacitance.
First, the capacitance is charged, the start current limited by the current limit setting of the
HV supply. In order to speed up charging and to protect the current amplifier, the charge relay
short circuits the input during charging.The charge relay is controlled by an internal signal
ILIM which is high when the output current of the HV suppply is larger than 2mA.In
sequence mode, the charge relay remains activated if the preset charge time exceeds the time
it takes for the current to fall below 2mA. (the charge relay is active until both ILIM is reset
and charge time has expired.) The charge relay is also activated during discharge.
When measuring capacitance the effect of "dielectric absorption" must be considered. This
effect is also referred to as "soaking". It can be described with the equivalent shown in Fig.
4.5.1. Cds will typically have a value of 0.1-1% of C and the time constant of Rda and Cda
will be app. 1-10sec. This means that after C has been fully charged a current larger than the
leakage current will flow into the capacitor. However, just after the charge relay is deactivated
the absorption current is delivered by the main capacitance and the current amplifier measures
C
Cda
Rda
◇ 42
Chapter 4 [SETUP]
Input Resister
Ic
( 10k or 1M )
1R
Change Relay
GND
GND