# Low voltage AC to DC for energy harvesting

For an energy harvesting application, I need to charge a capacitor using induced voltage due to the movements of a magnet inside a coil. The generated voltage is typically low frequency AC (4-5 Hz) with maximum 0.1 Vpp. This makes it extremely inefficient to use a traditional bridge rectifier, as the voltage is well below the active treshold of the diodes. What circuit could I use for the purpose?

I tried stepping up the voltage from the coils via a transformer (25:1) with no success. There also seems to be readily available ICs for the purpose, but they are unavailable at this time in my location. Also, it is worth noting that the capacitor should charge solely using the output of the coil, no external voltage supply is available.

• Define source impedance and MPPT Power, The low f may not be worth the trouble. MPPT is a matched load to source impedance. Commented Nov 29, 2017 at 23:45
• You will most likely need to use a full wave rectifier to bootstrap your energy harvesting application. Just saying. - Is it possible for you to increase the number of turns on the coil? Commented Nov 29, 2017 at 23:46
• @HarrySvensson It is possible to increase the turns in the coil, but not tenfold, as space is a limitation as well. So even if I push to squeeze as many coil turns as possible into the system, the input voltage will be still too low to simply use a bridge rectifier. Commented Nov 30, 2017 at 0:01
• LT4320 looks nice BUT needs >=9VDC which may eb a problem here.
– user16324
Commented Nov 30, 2017 at 10:35
• You could use a much stronger magnet and a larger number of turns, then use a suitable transformer to step up the voltage. You could also rotate the coil instead of the magnet, and use a commutator, to obtain DC. Commented Dec 1, 2017 at 8:39

Starting from 0.1vpp, you will need an external power source to bias up your active rectifier / converter IC. This source can be as simple as a capacitor, which stores charge from last time the converter operated, but it does need to be charged initially. That voltage is simply not sufficient to make even sub-threshold circuits work.

There are a couple of options for a circuit topology that would convert a very low voltage up to a reasonable voltage, a boost converter, or a transformer. There are plenty of application notes for energy harvesting ICs that spell out the techniques.