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# Power monitor

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#### 0mega

##### New Member
Watup,

I am trying to construct a circuit to measure the output of a solar panel. I need some way of measuring the power the solar panel is recieving... this can be a relative value, ie. 1 =full power, 0 = no power.

Cheerz
JB

0mega said:
I am trying to construct a circuit to measure the output of a solar panel. I need some way of measuring the power the solar panel is recieving... this can be a relative value, ie. 1 =full power, 0 = no power.

If you're wanting a simple yes/no response, you first need to decide where you want the switching point - in between full power and no power there are an infinate number of intermediate points, you would need to chose one of these and decide above that point is full power, and below it is none.

For a start I would advise feeding it into a dummy load (resistor), and monitoring the current through the resistor and the voltage across it. Take the readings under different conditions, and plot a graph of the output of the solar panel. Be prepared to be really disappointed!.

Well... i was thinking something along those lines...

aka a resistor in parallel with the main load, a DAC measures voltage across resistor, the microprocessor does the math...

would this work?

0mega said:
Well... i was thinking something along those lines...

aka a resistor in parallel with the main load, a DAC measures voltage across resistor, the microprocessor does the math...

would this work?

There's no need to add an extra resistor (it's just a waste of power, and you can't afford to waste power with solar panels). Simply monitor the voltage across the existing load - the load might not be constant, but it doesn't really matter, if that's the load which it's going to feed you may as well monitor it like that. If you want to measure the current as well as the voltage, insert a low value series resistor in one of the supply rails, and monitor the voltage drop across it (amplifying as required with an opamp).

If you are trying to extract maximum power, you need to have some type of adjustable system on your output. The power curve of a solar panel depends on many things, one being light. As the light increases / decreases, the maximum power point varies. Normally, people use a switching supply and some type of micro to do dithering. In other words, they slightly increase the current draw and see if they get more power, if so, then the system continues to move in that direction until it reaches the maximum. In this way, you always get the max power. The equation governing a solar cell is:

P = V ( Isc + A(1-e^(BV)))

Where P is the power, V is your load voltage, Isc is the short circuit current. A & B you have to solve experimentally for the panel by taking a dozen or so measurements each with a different load and then best-fitting to find the points.

Thx

I have found a range of "current shunts" designed for the purpose. What i am doing now is simply reading the voltage drop (tiny, as the shunt resistance is 50 milli-ohms) with an ADC and using a micro to log it to an eeprom (smart card). The card is taken out of the car (it is a model solar car ) and then stuck into a computer. The current, voltage of solar panel, tilt, etc is logged, then the values are "tweaked" to obtain optimum power for the specific track. In "race mode", the card & reader is removed to use less current & have less weight. Thankx for replying,

John Beaton
MHS

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