page C24
DS-100C Operating Manual
5.0 Pre-Charge Precautions
In order to compensate for voltage drops across the test leads, the DS-Tracer
pre-charges the capacitive load to -25 V. When an I-V curve is taken, the PV
module voltage will start at -25 V and increase to 0 V (I
sc
) and continue to V
oc
.
Most PV modules (single crystal silicon for example) can 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.
You can avoid damage to a thin-film module by inserting a by-pass diode across
the module terminals as shown in Figure 10. This diode limits the reverse voltage
to approximately -1 V maximum. Locating the diode at the PV module terminals
allows the pre-charge to still offset voltage drops across the test leads.
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. Due to
circuit resistances, this current will not exceed 250 A and can last less than 2 ms.
The diode reverse voltage rating must be greater than module Voc. A Motorola
MR750 series rectifier is recommend. This rectifier has a reverse voltage rating
of 50 V and can handle peak currents over 300 A.
Figure 10. Use of by-pass diode to protect thin-film PV modules.
DS-Tracer
PV Array
System
Disconnect
Switch
Test Lead
+
-
Summary of Contents for DS-100c
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