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Solar cell
Control unit
Reference
voltage
Final stage
amplifier
Calibration
unit
Fig. 2. Block diagram of the read-out circuit
7-9 October 2009, Leuven, Belgium
which is registered as the changes in the irradiation sensor’s
output signal according to temperature. We’ve conducted
separate analysis to find out the thermal behaviour of our
sensor cell and the circuit, these test were also applied to an
industrial solar cell.
While investigating the circuit the solar cell was replaced
with a current source. This way the changes caused by the
solar cell could be ruled out and the circuit’s effective
temperature dependence could be determined. This
parameter can be defined if we measure the circuit’s output
voltage as a function of temperature Fig.4.
The most important active component is the control unit.
This unit consists of two parts, a voltage generator and a
regulator circuit Fig. 3. Compared to industrial solutions,
where small shunt resistors are used to transform the short
circuit current into a voltage signal, an active regulation is
used, which always ensures proper short circuit conditions.
Precise short circuit is adjusted by a reference voltage value.
Climate chamber
Temp.
Read-out
circuit
current
source
0.0V
W
V A
V A
OFF
V W
A
CO M
U ref
U ref
I PV cell
= I sc
U PV cell
= 0V
Fig. 4. Experimental setup for determination of the temperature
dependence of the circuit
Investigation of the solar cell is performed in a similar
manner. The photovoltaic cell is connected to the electronics
and if illuminated a measurement voltage appears on the
output of the circuit Fig. 5. As light source a Tungsram
EXN-WFL 12V 50W halogen incandescent lamp with a
colour temperature of 3000K was used.
Climate chamber
U ref
U m
→ Final stage amplifier
Temp.
PV
cell
V
W
0.0V
A
Fig. 3. Electrical diagram of the control unit
The voltage created over a measurement resistor (R 5 ) by
the short circuit current is amplified by the final stage
amplifier, because higher voltage levels can be more
precisely digitalized by commonly available devices.
Verification of the measuring device can be carried out with
the calibration unit, which sets the voltage of the final stage
amplifier.
C. Thermal testing
Both the sensor cell and the read-out circuit can be
affected by changing temperature. Similarly to the industrial
sensors the operation will be studied using different
methods. Thermal tests were performed in a climate chamber
in the temperature range -20 to 80 °C in steps of 10°C.
In order to determine the reliability of the sensor device
accelerated life tests were carried out. Thermal testing begins
with the determination of the temperature dependence,
Read-out
circuit
Fig. 5. Experimental setup for determination of the temperature
dependence of the solar cell
The second test that was carried out determined the sensor
cell’s spectral response as a function of temperature. In this
test the cell while fixed to a thermostated copper bulk was
illuminated through 8 narrow bandpass filters. The bulk’s
temperature (and solar cell’s temperature) was varied in the
range of -10°C to 80°C with a Cole-Palmer Polystat 12100-
25 type thermostat. At given temperature values the short
circuit current of the sensor cell is measured for each filter.
This way the spectral response can be determined at different
temperatures.
Third HTS (High Temperature Store) according to Mil-
Std-883 Method 1008 is performed to determine the effect
V
A
OFF
V W
CO M
A
©EDA Publishing/THERMINIC 2009 63
ISBN: 978-2-35500-010-2