Solar Piston V1
One of the aspects of Micro Energy Harvesting that I particularly like is the intermittent nature of machine behavior that it necessitates. To take the solar example: It’s not always sunny. Sometimes it’s downright dark! If you’re working with a power hungry machine, you would expect it to work on the occasion that there is enough power to do the work. Likewise, if you’ve got a low power machine, you may be able to expect that it will hum along a while before it needs to pause for recharge (if ever). Autonomous sensing devises, passively moving robots (wind, tide) start to look like interesting avenues for exploration. Eventually, any MEH system is going to need to take a break and recharge, and that presents an interesting design problem. First things first though, how are we going to charge that battery? With a small to medium sized solar panel and intermittent sunlight, we have to have some control circuitry to manage the power that the solar panel is outputting. If there is enough light to power the solar panel, but not enough to output a voltage above the battery voltage, no charging takes place, and we have wasted time and energy. Being able to store a charge over time, and then dump that charge into the battery is what these Piston circuits do. I’m using a large capacitor to store up the charge, and then a Voltage Detector to release it into the battery. I’ve got two circuits to discuss, so let’s get started.
The central element of both of these Solar Piston versions is the S-80835 from Seiko Instruments. S-808 is the generic part number, and the 35 indicates the voltage trigger point: 3.5V.
The thing to keep in mind during this explication is that the S808 has an Open Drain output. That means when it is active (ON), the OUT pin is connected to GND. When it is inactive (OFF), the OUT pin is in High Z, or High Impedance, or Not Connected to Anything. Hence the pull up resistor. Also, the diode on the top left is there to prevent current draining through the solar panel when there is little or no light. It is a Schottkey diode, and has very low forward voltage drop (0.12V by my measurement). Here we go.
When charge on the 6800uF capacitor is below the voltage trigger of the S-808, let’s say 3.o0V, the OUT pin is ON and connects the base of the NPN transistor (2N3904) to GND turning it off. While the NPN is in the off state, the gate of the P-channel MOSFET (BS250) feels +V through the 10M pull up resistor and it is also off. No current is allowed to flow from the capacitor to the + battery terminal. As the solar panel works, the charge on the capacitor will rise up to the S-808 trigger threshold (on my multimeter, the cap never rises above 3.67V). When that happens, the OUT pin is turned OFF, the base of the NPN feels +V through the 10M resistor, and turns on, connecting the gate of the MOSFET to GND turning it on, and allowing current to flow from the cap to the Battery. As the capacitor drains and the voltage drops, the S-808 is still in operation. When the capacitor voltage falls to 3.50V (by my multimeter and the stated trigger voltage) it will become active again, connecting the NPN base to GND, turning it off, and also turning off the MOSFET. Charge will build up on the capacitor again, and we’ll take another spin on the tilt-a-whirl. Here’s a video.
It helps to note that we are using the S-808 in a kind of ‘backwards’ way. It is designed to provide a RESET signal to a microcontroller, or other hardware, when the voltage falls below a certain level, in this case 3.5V. The S-808 datasheet states the hysteresis for this part as 0.175V. Which gives me a good sense that my multimeter is calibrated at least as perfectly as the Seiko Instruments factory!
This version is slightly flawed in that it requires the NPN transistor to act as an inverter to turn on the P-MOSFET. An N-channel MOSFET won’t work alone, because it would need a higher voltage on the gate than I can provide. With a voltage difference of 0.5V max, there is not enough potential to turn on an N channel MOSFET, so switching the charger on the high side requires this kind of circuit. Next up in version 2, I will show you a simpler way to get this charger working with a P-MOSFET switching on the low side. Stay tuned!