Michael Berg
- May 21, 2017
- 9
- Joined
- May 21, 2017
- Messages
- 9
Hello Forum,
This is my first posting so be gentle.
I'm looking for a way to design a circuit that would allow a solar panel to charge a capacitor to a specific voltage, and then bleed off additional extra current through a resistor in order to maintain the cap at that specific voltage.
Let me briefly explain the background for that. Solar panels are funny, in the sense that they produce most of their power at a specific voltage. You can read this voltage on the back of a panel, under the label Vmp. Vmp is around 80% of the Voc of the panel - for example it could be around 18.5V for a 21V panel. Basically, if you are able to present the panel with a load that consumes all the current at 18.5V, you get the maximum power out of the panel. This is also why most "12V battery charging" solar panels you find on ebay are around 18V if you just measure the open circuit voltage with a multimeter - they provide most of their current at 12-14V which the battery pulls the panel down to.
So I connect the panel to a capacitor with a sufficiently high voltage rating. As the sun illuminates the panel, the cap begins to charge, which can be measured directly with a multimeter on the cap ends. Left unattended the voltage would rise to the panel's Voc, or 21V. But I don't want this, I need the cap to charge to 18.5V and no higher, even if this means bleeding off solar power through a resistor. This way I can connect my load to the cap and have the maximum power from the panel available to me because the cap holds the voltage at the Vmp of the panel.
My idea is to use two transistors. The first transistor decides, based on the cap voltage, if power from the panel should go to the cap or not. A second transistor is wired in the opposite way, i.e. to open when the voltage exceeds 18.5 and allow current to flow from the cap + to the cap - through a resistor. Like a safety valve of sorts. As the voltage rises, these two transistors work together to keep the cap at 18.5 by either charging it or discharging. The bleed-off resistor probably needs to be able to handle quite a bit of power.
The important thing to keep in mind is that when I measure with my multimeter what the voltage is across the panel, it must read 18.5. This is why a simple voltage divider is unsuitable for this project.
I have already come up with a design that uses an arduino and a voltage sensor to control a relay to achieve something like this, but it is entirely overengineered, too power hungry and just too elaborate. It feels to me as if this MUST be possible with discreet components, I'm just not good enough to come up with a design.
Can anyone help with specific designs or just suggestions on how to approach this problem?
PS - yes I realize the Vmp changes in different lighting conditions and that an MPPT controller handles this by continuously scanning the IV curve for the optimal Vmp. For this project, simply assume the Vmp is known and fixed.
This is my first posting so be gentle.
I'm looking for a way to design a circuit that would allow a solar panel to charge a capacitor to a specific voltage, and then bleed off additional extra current through a resistor in order to maintain the cap at that specific voltage.
Let me briefly explain the background for that. Solar panels are funny, in the sense that they produce most of their power at a specific voltage. You can read this voltage on the back of a panel, under the label Vmp. Vmp is around 80% of the Voc of the panel - for example it could be around 18.5V for a 21V panel. Basically, if you are able to present the panel with a load that consumes all the current at 18.5V, you get the maximum power out of the panel. This is also why most "12V battery charging" solar panels you find on ebay are around 18V if you just measure the open circuit voltage with a multimeter - they provide most of their current at 12-14V which the battery pulls the panel down to.
So I connect the panel to a capacitor with a sufficiently high voltage rating. As the sun illuminates the panel, the cap begins to charge, which can be measured directly with a multimeter on the cap ends. Left unattended the voltage would rise to the panel's Voc, or 21V. But I don't want this, I need the cap to charge to 18.5V and no higher, even if this means bleeding off solar power through a resistor. This way I can connect my load to the cap and have the maximum power from the panel available to me because the cap holds the voltage at the Vmp of the panel.
My idea is to use two transistors. The first transistor decides, based on the cap voltage, if power from the panel should go to the cap or not. A second transistor is wired in the opposite way, i.e. to open when the voltage exceeds 18.5 and allow current to flow from the cap + to the cap - through a resistor. Like a safety valve of sorts. As the voltage rises, these two transistors work together to keep the cap at 18.5 by either charging it or discharging. The bleed-off resistor probably needs to be able to handle quite a bit of power.
The important thing to keep in mind is that when I measure with my multimeter what the voltage is across the panel, it must read 18.5. This is why a simple voltage divider is unsuitable for this project.
I have already come up with a design that uses an arduino and a voltage sensor to control a relay to achieve something like this, but it is entirely overengineered, too power hungry and just too elaborate. It feels to me as if this MUST be possible with discreet components, I'm just not good enough to come up with a design.
Can anyone help with specific designs or just suggestions on how to approach this problem?
PS - yes I realize the Vmp changes in different lighting conditions and that an MPPT controller handles this by continuously scanning the IV curve for the optimal Vmp. For this project, simply assume the Vmp is known and fixed.