Maximum Power Point Problem

[Sunking]

This is really a very complicated drawing to measure Voc and Isc. It takes years to understand the complexity . Note there is no resistor.

[davidegrima]

This is really a very complicated drawing to measure Voc and Isc. It takes years to understand the complexity . Note there is no resistor.

lol... that is exactly what im doing

[Sunking]

I did that and the voltage came out as 5.86V and the current 8.1mA

You did it wrong. Run 2000 Ohm's Law equations of Voltage = Current X Resistance: Use R = 0, If you come up with any answer other than 0, you did it wrong, do it again another 200 times until you understand 0 x anything equals 0.

[Sunking]

lol... that is exactly what im doing

No you are not. It is impossible to read voltage with 0 ohm's.

You do not know how to use your meter or interpret results.

[Sunking]

Look at your meter. Does it have 3 inputs called Com, Volt, and Amp?

If it does, the AMP INPUT means stick you finger in this hole to measure Amps.

[davidegrima]

You did it wrong. Run 2000 Ohm's Law equations of Voltage = Current X Resistance: Use R = 0, If you come up with any answer other than 0, you did it wrong, do it again another 200 times until you understand 0 x anything equals 0.

I did exactly what you told me to do and im sure i didnt do it wrong.

When I lower my potentiometer to give 0Ohms, no voltage and no current comes out since like you said V=IR is 0.

[davidegrima]

Look at your meter. Does it have 3 inputs called Com, Volt, and Amp?

If it does, the AMP INPUT means stick you finger in this hole to measure Amps.

I just told you that at 0Ohms (lower the pot to give 0Ohms) no voltage and current come out and yes i know how to use a multimetre stop kidding and try to understand what im telling you

[Sunking]

You are doing it wrong, there should be NO RESISTOR in the circuit, just your meter as I drawed out for you. Forget resistance until you understand Voc and Isc.

[Sunking]

stop kidding and try to understand what im telling you

I am not kidding now, you do not understand how to use your meter and interpret the results.

It is impossible for you to read voltage across a 0 ohm circuit. It is impossible to read current in an open circuits. It is also impossible for you to read voltage and current at the same time with the same meter unless you know exactly how to do that which right now is above your head.

[davidegrima]

You are doing it wrong, there should be NO RESISTOR in the circuit, just your meter as I drawed out for you. Forget resistance until you understand Voc and Isc.

With no resistance I get a reading for the voltage as well as a reading for the current.

I shouldnt get a reading for the current alone (voltage = 0V) since there is no resistance (0Ohms) right?

[Sunking]

I shouldnt get a reading for the current alone (voltage = 0V) since there is no resistance (0Ohms) right?

No sir wrong. You can have current with no resistance and voltage. There is something about a solar panel you do not understand. I suspect this is part of a test question that you have to figure out and has you stumped. I will give you a hint:

A solar panel is not a VOLTAGE SOURCE. If it is not a voltage source then what is it? Look at the curves I showed you the link too of IR curves. Ask yourself this, why is it a knee curve and not a straight line?

I am not trying to be mean, I am trying to give you the info to figure it out yourself.

A solar panel has 4 electrical characteristics:

Voc = Voltage Open Circuit (No current flows in an open circuit Voltage / infinity = 0)
Vmp = Voltage @ maximum power (which is slightly less than Voc)
Isc = Current Short Circuit. (no voltage in a short 0 ohm circuit)
Imp = Current @ maximum power (which is slightly less than Isc)

You can measure Voc and Isc with nothing other than a DMM connected across the output of a cell or panel. When you measure voltage, there is no current to speak of. When you measure current, there is no voltage to speak of. YOUR METER CANNOT READ BOTH AT THE SAME TIME.

[Sunking]

Build this circuit exactly as drawn. Note it will take 2 meters, one for voltage and 1 for current.



If you do not have 2 meters there is another way, but will require a pencil calculator to obtain current, but it will not be accurate at ZERO and low resistance values of R

[Sunking]

David it would help a lot if you understand how your DMM works in the Voltage and Current modes.

In the VOLTAGE mode your meter input impedance is very high around 20,000,000 ohms per volt. So if you measure say a 10 volt battery, the current through the meter = 10 Volts / [20M x 10] = .00000005 amps or .5 mico-amps. Close enough to ZERO to say it is ZERO amps. OK? So when you measure voltage with your meter, no current flows through the meter.

In the CURRENT mode or Amps a small precision resistor is switched in between the meter leads inside the meter. This resistor is called a SHUNT RESISTOR. The value of the Shunt Resistor depends upon the full scale reading the meter is set up to read. Full Scale voltage will be 50 milli-volts or .05 volts. So for example if the scale is 10 Amps, the Shunt Resistor has to equal .050 volts / 10 amps = .005 Ohms. So when you measure current the meter is actually measuring the voltage developed across the Shunt Resistor up to 50 mV full scale @ 10 amps. .05 volts is close enough to ZERO to call it ZERO. So when you read amps, there is no significant or meaningful voltage drop across the meter.

[davidegrima]

No sir wrong. You can have current with no resistance and voltage. There is something about a solar panel you do not understand. I suspect this is part of a test question that you have to figure out and has you stumped. I will give you a hint:

A solar panel is not a VOLTAGE SOURCE. If it is not a voltage source then what is it? Look at the curves I showed you the link too of IR curves. Ask yourself this, why is it a knee curve and not a straight line?

I am not trying to be mean, I am trying to give you the info to figure it out yourself.

A solar panel has 4 electrical characteristics:

Voc = Voltage Open Circuit (No current flows in an open circuit Voltage / infinity = 0)
Vmp = Voltage @ maximum power (which is slightly less than Voc)
Isc = Current Short Circuit. (no voltage in a short 0 ohm circuit)
Imp = Current @ maximum power (which is slightly less than Isc)

You can measure Voc and Isc with nothing other than a DMM connected across the output of a cell or panel. When you measure voltage, there is no current to speak of. When you measure current, there is no voltage to speak of. YOUR METER CANNOT READ BOTH AT THE SAME TIME.

If it is not a voltage source it should be a current source. And yes, if you are reading voltage, you have to switch the meter to read current because the meter cannot give you both readings at the same time.

The knee curve is produced due to the change in resistance because as resistance increase it is sort of approaching infinity and therefore it becomes like and open circuit and vice versa.

When you told me to read the solar panels without any resistance (+ve terminal of solar cell to +ve of probe and -ve terminal of solar cell to common of probe) I had a voltage reading. Than to read current I switched the mter and had a current reading. Is this correct?

[Sunking]

Correct. A panel is a current source, and that is what is throwing you off in your test measurements. A panel acts as a current source between Isc and Imp, and as a voltage source from Vmp to Voc. All your test you made were well above Vmp near Voc. YOu never even got close to the MPP starting at 500 ohms. Based on what you said about the 14 series 100 ma panels, MPP will be around 40 to 80 Ohms depending on the irraiance illumination.

So if you start at 0 Ohms you sill see Isc current and no voltage. As you start to increases resistance the current will not change, but the voltage will rise proportionally to the resistance until you hit the MPP. At MPP things switch as you increase resistance. The voltage will quit rising and the current will start dropping proportionally to the resistance.