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Pre-Charge Precautions
DS-1000 Operating Manual
5.0 Pre-Charge Precautions
In order to compensate for voltage drops across the test lead, the DS-Tracer pre-
charges the capacitive load to approximately -15 V. When an I-V curve is taken,
the PV module voltage starts at -15 V and increase to V
oc
. Most PV modules
(single crystal silicon for example) easily withstand this negative pre-charge
voltage. However, this pre-charge voltage can damage some thin-film modules—
particularly those with very low Voc values. Higher voltage modules have more
cells in series and therefore the pre-charge voltage is spread across more cells.
Only individual low-voltage, thin-film modules are affected. It is not a concern for
systems with many modules in series.
Avoid damage to a thin-film module by inserting a by-pass diode across the
module terminals as shown in Figure 9. This diode limits the reverse voltage to
approximately -1 V maximum. Locating the diode at the PV module terminals still
allows the pre-charge to offset voltage drops across the test leads.
Figure 9. Use of by-pass diode to protect thin-film PV modules.
The by-pass diode should be rated to handle the instantaneous peak current
created by shorting the pre-charge capacitor through the by-pass diode. The DS-
1000 design ensures this current will not exceed 150 A and lasts less than 2 ms.
The diode reverse voltage rating must be greater than module Voc. A Motorola
MR750 series rectifier or similar is recommend. This rectifier has a reverse
voltage rating of 50 V and can handle peak currents over 300 A.
DS-Tracer
PV Array
System
Disconnect
Switch
Test Lead
+
-
