I am trying to understand how this circuit works or to be more specific just the power gathering portion of it. It is part of this project (projects :: hijack) to harvest power from iPhone's headphone jack.
I understand that the right audio coming in is essentially a sin wave at 22kHz and so looking at the bottom left of the schematic I understand the following:
The caps are likely mostly for filtering; I get the LED and current limiting resistor and I get the voltage regulator portion.
What I don't understand is the transformer or the collection of transistors or the diode. Is this one big circuit that rectifies the AC into DC? If so is there a name for that kind of circuit and why choose that over a regular bridge rectifier?
Thanks!
headphone_power.pdf (65.6 KB)
It is essentially a bridge rectifier, the MOSFETs are being used as diodes, but they have very low forward voltage drop and so are very efficient, more so than Schottky diodes would be.
The transformer is either for stepping the voltage up or down, or for providing isolation, or both. The project seems well documented, is this not described anywhere?
Thanks for the input!
It's definitely well documented and on their website it talks briefly about power but I couldn't find anything more specific:
The HiJack energy harvester can supply 7.4 mW to a load with 47% power conversion efficiency when driven by a 22 kHz tone from the output from a single audio channel on the iPhone 3GS headset port, all using electronic components that cost just $2.34 in 10K volumes. We are exploring other approaches for achieving higher conversion efficiencies.
Meant to include this link
Thanks! Looks like it describes exactly what I was looking for.
They state that 7.5 milliwatts can be obtained using this method. On quick look I didn't see the output voltage of the regulator? chip but if it's at 3.3vdc that is around only 2.2 milliamps, minus whatever this module consumes for it'self, the remainder is all that is available to power whatever sensor one is using. They will have to be very creative in design of the sensors.
Ya the power output is definitely minimal. I had two goals in following this project though:
- Learn a little something from this project and just have some fun messing around with components at the same time. (which I have already accomplished)
- I did have a project to power off headphone in mind, but the more I think about the limited power available I think it might be better to just use a couple AAA batteries to power my project but still use the audio out and mic line on the 3.5mm jack to transmits data back and forth