QuadTech 1865, Инструкция по эксплуатации, Страница: 76 / 87

Page 76 of 87 Applications
3.3.2 Charge Time Constant
The time constant for charging a capacitor in the charge phase is determined by the value
of the capacitor times the effective source impedance of the supply. The supply
resistance is approximately,
E E E
Ro = ----- ohms = ----- ohms = ------ kohms
I max 0.002 A 2
where E is the user specified test voltage in volts and I max is the current limit of the
1865 instrument, which is approximately 2mA. Therefore, the time constant is
E Cx
T = Ro Cx = --------- seconds
2000
where Cx is in
µ
F. As an example, for a 500V test voltage Ro is approximately 250k
Ω
so that the time constant for charging of a 1
µ
F capacitor is 0.25 seconds. The charge time
should be set for a minimum of 10 time constants.
When the resistance adapters (discussed in paragraph 3.3.1 above) are used the time
constant will be increased. In the example above if the 100k
Ω
or 1M
Ω
adapter is placed
in series with the effective source impedance of 250k
Ω
the charge time will be increased
by a factor of 0.4 and 4 respectively.
3.3.3 Discharge Time
The time constant for discharging a capacitor in the discharge phase is determined by the
value of the capacitor times the resistance of the 1865 discharge circuit. The discharge
resistance is approximately 66k
Ω
. The CAUTION HIGH VOLTAGE lamp is turned off
after the user specified discharge time. If the discharge time is set to zero or a very short
time a capacitor could remain with a charge after the indicator is extinguished. However,
the discharge time is so short this is not a practical consideration except for capacitors
greater than 10
µ
F or so.
Capacitors with high dielectric absorption can have a residual charge even after they are
shunted and must be repeatedly shunted to completely discharge. Usually a "voltage
recovery" of this type would be a very small percentage of the original applied voltage
and generally not dangerous to the operator. This risk is further minimized by the fact
that the 1865 circuitry remains in a discharge mode until the operator initiates another
charge and measure cycle.