Energy harvesting is all the rage, so bring on your lemon batteries, thermocouples, minuscule solar cells, dirt batteries, and tree batteries. This circuit can convert them all to useful voltages.
Energy harvesting is all the rage, so bring on your lemon batteries, thermocouples, minuscule solar cells, dirt batteries, and tree batteries. This circuit can convert them all to useful voltages.
This is still a new, cutting-edge circuit. There’s only one commercial integrated circuit out there that will do the same thing: the LTC3108. But, if you build your own, it will be cheaper and maybe even more efficient.
Without further ado… here’s the circuit. Vs is the supply voltage. That’s where your solar panel, thermocouple, potato battery, or tree leads attach (if you think I’m kidding about using a tree for power, check out voltreepower.com)
C2 D1 D2 and C3 make a voltage doubler, to help get the voltage as high as possible.
C1 is optional. It just helps smooth out the power supply.
You can solder everything together, or use a breadboard like I did here.
All of these parts except the transformer can be bought on mouser.com. The transformer comes from coilcraft.com. All told it’s going to cost you less than $5. That’s one reason to build your own. If you buy the LTC3108, it’s going to set you back at least $7 or $8.
I bought this solar panel on eBay for $3. It’s going to be my power supply. The specs on it are 0.5V and 15mA. That’s the open-circuit voltage and the short-circuit current.
I probed the gate of the transformer to show you what’s going on. The circuit is a blocking oscillator. It oscillates at about 25 kHz and 1V peak-to-peak when Vs is 100mV.
Just so you can appreciate what you just built… Here’s an efficiency plot of the fancy LTC3108 integrated circuit that I mentioned. This is a cutting-edge circuit which came out in 2010.
Well, look at how the efficiency drops off at higher voltages. At 500mV it’s only about 5% efficient. Uh-oh! The solar cell I wanted to use puts out up to 590mV.
What if I wanted to go the extra mile, and make my circuit extra-efficient in the voltage range of the solar panel? That’s about 200-590mV. How would I do that…
I’m going to use that simple circuit to bootstrap another voltage converter. “Bootstrap” is a way of saying, “Get it started.”
Here’s the circuit I’m going to bootstrap. It’s a Texas Instruments TPS61200, and it’s about 50% efficient. But it can’t get itself started below 300mV. That doesn’t help us if we want to use our solar panel on a cloudy day. Let’s hook up the bootstrap to get it going.
