DS-1000 Operating Manual
Theory of Operation
page 21
Curve Sweep
The DS-Tracer sweeps an I-V curve by switching the PV system to a bank of
capacitors. As these capacitors charge, the PV system voltage increases until
open circuit voltage is reached. While the voltage is increasing, and the current
to the capacitors is decreasing from its maximum, short-circuit value, the voltage
and current are sampled by high-speed analog-to-digital converters. These data
points define the I-V curve.
Pre-Charge
Before acquiring data, the DS-1000 reverse charges its capacitive load to
approximately -15 V. This negative pre-charge is used to offset any voltage
drops created by the test leads and to allow any switching transients to settle
before I
sc
is reached. This method allows a true short circuit current reading to
be made. After the capacitive load charges to Voc, the curve sweep is complete.
At this point the circuitry discharges the capacitors through a resistive load.
Sweep Speed
Curve sweep time varies with PV system current and voltage. The sweep is
typically less than one second, but can be as long as 10 seconds.
The complete cycle actually begins with a discharge of any residual voltage on
the capacitors and a pre-charge to -15 V, curve sweep, and finally another
discharge/pre-charge period to ready for the next I-V curve sweep.
Some PV systems are sensitive to how quickly an I-V curve is taken. The speed
of a curve is most easily expressed in volts per second by taking the PV short
circuit current in Amps (I
sc
) and dividing it by the DS-Tracer load capacitance in
Farads. The DS-1000 load capacitance varies depending on PV system voltage
and current range selection. There are four possible load capacitance values
from approximately 2,500 uF to 11,600 uF.
