Hello,
I'm using this short type solar panel regulator circuit, as shown in the schematic, to charge a 12V SLA battery. However, it's possible that my PIC could become non-functional and I'd like to avoid overcharging the battery by just letting the panel stay connected to the battery 100% of the time. Which is what happens if the PIC doesn't put 5V on gate of the MOSFET (when the gate is at 5V, the MOSFET conducts and shorts the panel).
So I added this 'safety circuit' that is supposed to apply 5V to the gate if the battery voltage ever gets above 15V.
My thinking is that if the battery voltage rises above 15V, the 15V zener will conduct and the voltage divider will provide the ~5V to the MOSFET gate, which will short the panel and hold the battery voltage to 15V. However when I test this circuit and the battery voltage rises above 15, I see only about 3V on the node between the 15V Zener and R2. I would expect to see closer to 15V. Also the gate voltage doesn't get above a volt or so. Any ideas as to why that doesn't seem to work the way I'm expecting?
Also, I'm pretty sure the PIC is not going to like sinking 500mA through that I/O pin but I'd rather ruin a PIC than a battery (it's cheaper and safer).
The calculations for the resistor value for R2 can be found here: https://docs.google.com/spreadsheet/ccc?key=0As0iaN-Y_KkNdHp0X3dvTngwVEVQOW9vNk5IMHBReVE
I'm using this short type solar panel regulator circuit, as shown in the schematic, to charge a 12V SLA battery. However, it's possible that my PIC could become non-functional and I'd like to avoid overcharging the battery by just letting the panel stay connected to the battery 100% of the time. Which is what happens if the PIC doesn't put 5V on gate of the MOSFET (when the gate is at 5V, the MOSFET conducts and shorts the panel).
So I added this 'safety circuit' that is supposed to apply 5V to the gate if the battery voltage ever gets above 15V.
My thinking is that if the battery voltage rises above 15V, the 15V zener will conduct and the voltage divider will provide the ~5V to the MOSFET gate, which will short the panel and hold the battery voltage to 15V. However when I test this circuit and the battery voltage rises above 15, I see only about 3V on the node between the 15V Zener and R2. I would expect to see closer to 15V. Also the gate voltage doesn't get above a volt or so. Any ideas as to why that doesn't seem to work the way I'm expecting?
Also, I'm pretty sure the PIC is not going to like sinking 500mA through that I/O pin but I'd rather ruin a PIC than a battery (it's cheaper and safer).
The calculations for the resistor value for R2 can be found here: https://docs.google.com/spreadsheet/ccc?key=0As0iaN-Y_KkNdHp0X3dvTngwVEVQOW9vNk5IMHBReVE
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