Measuring PV cell current with Transimpedance Op Amp

Hello.
I am trying to measure the output current from a couple of PV cells I have. However, I am not getting the expected output.

I have constructed the following circuit:



My understanding is that Vout = -IR. I then constructed the circuit as follows:

Overview:



Detail:



I measured the output current to be -4mA, so with the 1k resistor, I would expect the output voltage to be around 4V. However, it comes to only 0.8V. Moving the light closer increases the output as expected, and moving the light source away decreases the output too. However, it does not seem to reflect what I would expect. I tried using a larger resistor value, expecting the output to increase, but if anything I found the output to be _inversely_ proportional to R, which is not right at all...

If anyone has an idea as to why the circuit is behaving as it is, I would be very grateful to hear.

The op amp is a LM358N (All I have to hand right now, but I can purchase something else if need be). Datasheet here: https://pdf1.alldatasheet.com/datasheet-pdf/view/22762/STMICROELECTRONICS/LM358.html

Thanks.

Your measurement setup is attempting to measure the "short-circuit" current of the PV cell (into zero V imposed by the virtual ground of the opamp) . You can do that by just connecting a millampmeter directly across the PV. This is not a useful measurement, because a PV delivers zero power into a dead short.

Characterizing an unknown PV is all about finding at what combination of load Voltage and load Current the maximum Power out of the PV is achieved (called MPPT). Connect the PV to a variable resistor (with sufficient power rating). Vary the load Resistance. Measure I and V, and find the R at which I*V is maximum. That is where you want to operate the PV.

Here is a simulation of the process: Note that for this particular panel, the optimum load resistance is 152 Ohms, where the panel delivers ~3.5V @ 22mA. You can also see that the open-circuit voltage is ~6.5V, and the short-circuit current is ~45mA.

You could use a current meter to measure the current. I know the voltage will not be zero but close.

You are using a solar panel, not a photo-diode and you show it connected backwards on your schematic. The cathode of a solar device needs to feed the (-) input of the opamp for the output of the opamp to go positive.
When the output voltage from the opamp goes as high as its power supply voltage allows and the light intensity increases then the input voltage of the opamp might exceed its -0.3V maximum allowed input voltage with no negative supply.

A solar panel produces a voltage or a power but you are trying to measure only its short-circuit current that is not useful. A photo-diode is used as a light sensor when it feeds its short-circuit current with the correct polarity into an opamp (-) input.

Thank you for all of your replies. I see now where I was making the mistake of measuring the wrong parameter.

Could I ask for your thoughts on how to achieve what I am after? I am looking to use the PV cells to provide power to a DC/DC converter, the LTC3105. I would like to monitor the power output form the PV cells that would be available to the LTC3105 under varying conditions. As you have mentioned, this depends on the load on the PV cells. However, I cannot find such information in the datasheet for this.

Is there a relatively simple way that I could log the output power from the cells, that would be similar to the available power for the DC/DC converter? I plan to log with an arduino, which I have got working as a 0-5V logger.

If I've explained this in a convoluted way, please say and I'll try my best to explain again.

Andrew.

Most PV's have an MPPT voltage from 75 to 83% of Voc depending on solar input.

Therefore demand load must be regulated to 75% minimum PV voltage.
There must also be a regulator for over voltage to the battery to cut back voltage to float level based on coulomb counting or battery temp.

50mV to 75mV current shunts are normally used to sense max current.
If the PV voltage is held constant in this range, I is a good indicator of power supplied. If not all the power is needed, one. Must shunt the. Excess current to a dummy load such as a light, to compare input and out current to see excess power available.
Since Vpv and Vbat will be different, one could use scale faactors instead of a multiplier function. To compare.. Pin vs Pout....
http://cds.linear.com/docs/en/design-note/dn491.pdf

MPPT is Max Power Point. There are a number of methods to track this. One is called perturb and observe where th converter tries to change the operating point.

Other issues with your I-V converter are:
The transfer function is -I*Rf
Usually a capacitor is placed across the feedback resistor for a number of reasons.

The real transfer function is Vb-I*RF where Vb is the voltage at the + input of the OP amp.

The OP amp HAS to be able to supply the same current as the solar cell so the currents can sum to zero.

A single supply circuit is tricky because, in practice, you can't achieve zero volts. The offset voltage of the OP amp gets you every time.

I did solar cell measurements professionally and designed a rather nice I-V converter that was used in QE (Quantum Efficiency) measurements.

What is the Voc, Isc max ? ( open circuit and short circuit under bright light ( e.g. 300W Halogen)

and do you have any LEDs"?

From looking at your photo, my estimate is, you have two (2) x 5-cell silicon PV with each cell being approx 1 sq in. with a hardwired array of 2P5S.


my assumptions are:
Silicon PV being approx Voc=0.6 per cell and 12 sq.in. per Watt
...and your array being 2x5=10 sq.in.

The potential of 5 cells is Voc=3.0V
and Vmppt=75% to 83% of 3V = 2.25V to 2.5V ( Good for RED or YELLOW LEDs) (see my previous comment about MPPT voltage)

The max power = 10 /12 [sq.in.]/[sq.in/W] = 830 mW.
The Imppt= 830mW/2.25V ~370 mA (@2.25V)

This means you could power up twenty RED LEDs in parallel each rated for 20mA.
If you had a 1W White LED ( or Blue) LED which has a Vth or threshold voltage of 2.7V, and rated voltage of 2.85 on the best high efficacy LEDs to 3V max you could also use this although suffer loss in MPPT. 5mm White LED's are not high efficacy or low thermal conductance so their Vf is typically 3.1V due to ESR. give or take a wide margin.

Let me know how my assumptions compare from my experience

What type of battery cells did you have in mind?

With Solar power being standardized to 1000W/sq.m max or 1.5 W/sq.in and your PV of 12 sq.in/W ...I am assuming a PV efficacy of 1/12 /1.5 = 6% ... naturally recent PV's are better than my experience with 15~22% efficacy or 4.4 ~3.3 sq.in/W

Your mileage may vary... (YMMV)