Thursday, October 7, 2010

Low-Cost Photovoltaic I-V Curve Measurement System

I have noticed that my posts on photovoltaic test are getting lots of hits so here is another one. Here I am going to present a $3k photovoltaic I-V curve measurement system. The measurement system can be seen in the figure below (click on it to enlarge it).

Here is how it works:

  •  The op amp, FET, and Rsense act like a poor mans programmable electronic load with the PV panel connected across it. The op amp will drive the FET (lower its resistance) until the voltage at the op amps negative input equals the voltage at its positive input. If we can control the voltage at the positive input then we can control the current flowing out of the PV panel into the FET and Rsense just like an eload in consant current mode
  • Setting and stepping the voltage at the positive input of the op amp is done by the 34972A DAQ switch unit. It provides a DAC output from 0 to 15 V. When the DAC is set to 0V the FET is in an open condition and the PV panel is at Voc.
  • Rsense is 100 mOhm precision shunt. If we set the 34972A's DAC output to .5 V the op amp will drive the FET until the voltage drop across Rsense is .5 V. Since Rsense is 100 mOhm we know that 5 A of current is flowing out of the PV panel through Rsense (Ohm's law: .5 V/.1 Ohm = 5 A).
  • The 34972A's built-in 61/2 digit DMM combined with plugin MUX switch cards allow us to measure voltage, current, temperature, and more on a large number of channels. In the figure we use one channel to measure the PV panel's voltage, another channel to measure the panel's current (voltage measurement across Rsense), and two other channels to measure temperature. 
  • Putting it all together the way we get the I-V curve is by stepping the DAC's voltage up from 0 V (panel at Voc) until we reach Isc. At each step we measure the panel's output voltage and current to get our I-V curve. 
  • The way we know we have reached Isc is when further voltage steps from the DAC do not result in the voltage across Rsense increasing. At this point the FET is essentially a short.
This photovoltaic I-V curve measurement system will not work well with low voltage low power PV cells since Rsense and the FET cannot actually become a true short. It is better suited for PV modules and panels. Although the hardware is low cost you do need to wrap some type of software around it to control the DAC and gather the measurements (unless you want to do it manually). The value of Rsense and its power handling capability should be chosen based on your PV device's power range and your measurement accuracy needs. Below is a list of parts I used and their approximate cost. This was just a brief overview of the solution if you need more info just comment or email me.

Part description
Model/part number
Approximate price
Product Web site
DAQ switch measure unit
34972A
$1,850
Agilent.com/find/34972A
20-channel MUX module
34901A
$500
Agilent.com/find/34972A
Multifunction module
34907A
$400
Agilent.com/find/34972A
10-V power supply
B10G50
2 x $120
Acopian.com
Operational amplifier
NE5534AN
$2
Newark.com
Power MOSFET
IRFP150N
$3
Newark.com
0.1-ohm shunt resistor
15FR100E
$1
Newark.com
PCB prototyping board
8015-1
$8
Newark.com
Total
$3,004
Click here for more information on the 34972A DAQ switch unit

12 comments:

  1. One safety concern that I forgot to mention is heat dissipation. The FET is going to have a lot of current running through it so you want to be sure to attach a heat sink to it. Depending on how much current your PV device is putting out you may also want to add some active cooling.

    ReplyDelete
  2. What would you recommend for testing low power PV cells of 1/2 Watt or less? Does it make a difference if the MOSFET is working in the linear region? Would amplifying Vsense with an opamp fix the low power problem?

    ReplyDelete
  3. This solution was intended more for high power test. It would be hard for me to make a recommendation on how to alter the setup without testing. You may want to look at Agilent B2900A family of SMUs (check out blog post on May 18th 2011). They are not too expensive and would offer an excellent solution for 1/2 watt cell testing. Email me if you have any further questions.

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