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photovoltaic panel load problem

mshh

Nov 29, 2013
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I am doing maximum power tracking (MPPT)for a photovoltaic cell (max current 4.4 A ,max voltage 34 v) i did the attached circuit to control current consequently get MPPT but the circuit go mad when i turned the potentiometer the current go up and down and didn't stabilize .

I used this power supplyhttp://www.amazon.com/Mastech-HY3005.../dp/B000E14F56 to simulate the pv panel and adjust it to source 7 volt and it did well . i could change the current from 0-2 A when i tried to increase it upper than 7 v it went crazy . the led indicator called C.C changed to C.V every thing current and voltage go upside down ,and that what was happening with the pv panel. i don't know why . may be i should increase the number of mosfets.
but i used four resistors 3.3Ω 7 watt instead of those in picture
 

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CDRIVE

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Your circuit appears to be a shunt regulator of sorts. The control element seems to be an OpAmp but you didn't list what the OpAmp PN on your print.

This circuit, unless I'm misinterpreting it, seems to be using MOSFETs in a linear mode. I don't think they're the best choice for this because they have an extremely narrow (nearly non existent) linear region. Because of this MOSFETs are usually relegated to switch mode applications.

I'm not a solar cell expert but do they have to be shunt regulated? It's a very wasteful method of regulation except when current is very low.

Chris
 

mshh

Nov 29, 2013
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Your circuit appears to be a shunt regulator of sorts. The control element seems to be an OpAmp but you didn't list what the OpAmp PN on your print.

This circuit, unless I'm misinterpreting it, seems to be using MOSFETs in a linear mode. I don't think they're the best choice for this because they have an extremely narrow (nearly non existent) linear region. Because of this MOSFETs are usually relegated to switch mode applications.

I'm not a solar cell expert but do they have to be shunt regulated? It's a very wasteful method of regulation except when current is very low.

Chris
it is shunt regulated because the panel power is 150 watt . what do you suggest?
 

CDRIVE

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it is shunt regulated because the panel power is 150 watt . what do you suggest?

I don't think that's relevant unless there's something about solar voltaic cells I'm not aware of. Is there a voltage break-over if they're not loaded when receiving max Sun input?

Chris
 

mshh

Nov 29, 2013
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what do you mean by voltage break-over here ?
photovoltaic open circuit voltage is 42 v it can source current if loaded but you have to find the suitable resistance to get the maximum power.
 

CDRIVE

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what do you mean by voltage break-over here ?
photovoltaic open circuit voltage is 42 v it can source current if loaded but you have to find the suitable resistance to get the maximum power.

Voltage breakover or breakdown is a 'Maximum Voltage' device spec. When it's exceeded the device fails and typically does not recover. Since it was a mystery to you I'll assume that a solar cell will not fail if unloaded when receiving maximum light and producing max voltage.

You still haven't told us what OpAmp you're using in your simulation. I do know this.. That circuit is linear. The inverted input (-) of the OpAmp derives its voltage from the current developed across the source resistor (R1) which folds back the output voltage of the OpAmp. The more Source current M1 draws the higher the foldback voltage will be.

As I stated before MOSFETS are not the most linear devices on the block. They work great as switches but linear?... Not so much. I think NPN power transistors would be more appropriate in this application. Where did you find that schematic?

One last note. I still don't like circuits that dump excess power into a dummy load just for the sake of dissipating it. If you're not heating a room or water tank with that energy it's not GREEN thinking. It's WASTE!

Chris
 

(*steve*)

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I am doing maximum power tracking (MPPT)for a photovoltaic cell

I see no inductor.

I can't imagine how you can do MPPT without an inductor.

I imagine that you could do it without a microcontroller, but I don't know why you would.

Do you understand what MPPT does or how it achieves that goal?

Oh, and you'll also need (in most cases) a way to determine when you have changed the battery.
 

mshh

Nov 29, 2013
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Voltage breakover or breakdown is a 'Maximum Voltage' device spec. When it's exceeded the device fails and typically does not recover. Since it was a mystery to you I'll assume that a solar cell will not fail if unloaded when receiving maximum light and producing max voltage.

You still haven't told us what OpAmp you're using in your simulation. I do know this.. That circuit is linear. The inverted input (-) of the OpAmp derives its voltage from the current developed across the source resistor (R1) which folds back the output voltage of the OpAmp. The more Source current M1 draws the higher the foldback voltage will be.

As I stated before MOSFETS are not the most linear devices on the block. They work great as switches but linear?... Not so much. I think NPN power transistors would be more appropriate in this application. Where did you find that schematic?

One last note. I still don't like circuits that dump excess power into a dummy load just for the sake of dissipating it. If you're not heating a room or water tank with that energy it's not GREEN thinking. It's WASTE!

Chris
i used lm358.
i take this schematic from all about circuits forum.
i don't understand your last note , what should i do with 150 watt if i didn't loss it in a load .
 

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i am just doing load determine to find current and voltage.

Unless you live in a place with constant weather and a sun that doesn't move, the optimum load will change. This is actually the whole point of MPPT -- it's the T part actually.
 

mshh

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Unless you live in a place with constant weather and a sun that doesn't move, the optimum load will change. This is actually the whole point of MPPT -- it's the T part actually.
off course , weather isn't constant but at a time it can be handled . i can assure you that irradiation won't change every second so i can find optimal load value .
 

CDRIVE

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i used lm358.
i take this schematic from all about circuits forum.

Thanks for answering those questions.

i used lm358
i don't understand your last note , what should i do with 150 watt if i didn't loss it in a load .

My comment was based (solely) on a more basic concept than MPPT, which by the way, I read up on just prior to posting this. In fact, I've spent most of this morning reading and browsing over MPPT theory of operation as well as conceptual diagrams. Did I mention that I wasn't familiar with it until now?

Well, it didn't take a lot of reading before it became apparent that the relationship between MPPT ..

http://en.wikipedia.org/wiki/Maximum_power_point_tracking
http://bama.ua.edu/~bwbuckley/projects/mppt.html

and Maximum Power Transfer Theorem ("Jacobi's Law") were closely related.

http://en.wikipedia.org/wiki/Maximum_power_transfer_theorem
http://faraday.ee.emu.edu.tr/eeng224/Papers/Maximum Power Transfer.pdf

Initially, I was a bit annoyed that I was reading about MPPT or any alternative energy topic but once I realized its close relationship to MPTT I feel my time wasn't wasted at all.

The best part of the two MPTT links is the arguments that define the difference between MPTT and efficiency. They are definably not the same thing. The MPTT Wiki link even makes mention of the relation between MPTT & MPPT, stating that they should be merged into the same topic or at least linked.

Chris
 
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(*steve*)

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off course , weather isn't constant but at a time it can be handled . i can assure you that irradiation won't change every second so i can find optimal load value .

OK, so you find the best load for Tuesday 12th December 2013 at midday in your back yard with the solar panel on the ground in full sun.

What does that tell you and how does it help?

Many MPPT devices track the output current. If they are used to charge a battery, the output voltage can be considered almost fixed over a short period of time. The task is then to maximise the current.

Assuming your panels have a voltage significantly greater than that of the battery, you essentially have a buck regulator and you adjust the mark/space ratio to achieve the maximum output current.

And yes, you do change that second by second.
 
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