I have a 2.7V 100F super-capacitor. I am going to be charging it with a 6V 1W solar panel. Now the solar panel only puts out 6V when it is receiving the best sunlight so this means the output from the solar panel can be lower. What is the best way to go about charging the super-cap. I was thinking of just using a voltage divider with resistors but I wanted to get some other opinions.
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1\$\begingroup\$ "using a voltage divider with resistors" - take a few searches around this site and convince yourself that's not a great idea. \$\endgroup\$– Greg d'EonMay 7, 2015 at 1:19
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2\$\begingroup\$ The easiest way is to charge the cap directly from the panel, with a circuit to disconnect the cap when its voltage reaches about 2.5 volts. \$\endgroup\$– WhatRoughBeastMay 7, 2015 at 1:31
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\$\begingroup\$ A simple 2.5V zener diode regulator would probably be as effective as anything. If you a blocking diode as well to prevent current flowing back into the panel then you will be pretty much set. Unless you plan on doing max power point tracking (for solar panels the peak voltage is not necessarily where they deliver the peak power), then the zener diode approach is probably the simplest. \$\endgroup\$– Tom CarpenterMay 7, 2015 at 3:08
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Even for the simplest schematics, simple resistor is not enough. You definitely want a series diode to prevent capacitor discharging INTO solar panel when it is dark, as well as some way to prevent capacitor from being overcharged -- either zener, red/yellow LED, or a shunt regulator. In fact, once you have a protection circuit you no longer need a resistor divider -- just charge at max power and let regulator dissipate any extra power. This gives the following schematics:
"OVP" may be:
- a 1W, 2.0 V zener (zeners generally have high voltage tolerances)
- 1W red/yellow LED
- 2 regular diodes in series (each has about 0.7v-1v voltage drop, two would regulate to 1.5-2V)
- diode + transistor shunt regulator
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\$\begingroup\$ @Nick As theamk says, the diode preventing current flow into the panel is important. Be aware that this diode is built-in to many solar panels, so you might not need to add an extra one. Check the datasheet, or do some experimenting :) \$\endgroup\$– bitsmackMay 7, 2015 at 4:16
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\$\begingroup\$ So let me try to figure this out. The output of the solar panel would go to a diode to prevent back flow, then that would connect to the capacitor along with the OVP in parallel? \$\endgroup\$– NickMay 7, 2015 at 20:40
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\$\begingroup\$ any links for 1W 2v zener? I only found 0.5W \$\endgroup\$– NickMay 8, 2015 at 0:21