Suppose you're designing a really
low-power telemetry application—
so low-power that you want to run it off free energy that you "harvest" from
the environment. Let's say it's intended
to monitor the vibration signatures of
the wheel trucks in railroad boxcars and send data about those signatures
to trackside collection points via a
wireless link. Suppose further that you
want to power the application from those
same vibrations.
Advanced Linear Devices' ALD EH300
and EH301 EPAD energy-harvesting modules can accept energy from an assortment of widely available waste energy
sources, such as vibration, light, chemical
reactions, fluid and air flow, and environmental heat. The modules then store this
energy to power conventional 1.8- and 5.0-V systems ().
The most thoughtful feature of these modules is that their datasheets tell you
the minimum length of time a useable
voltage will be available on their output
terminals. That's a key design parameter. It lets the engineer writing the
application code know the maximum
number of clock cycles that a complete
loop of wake-up, data acquisition, number-crunching, data output, and shutdown has to fit into. The ALD energy harvesting module may provide more time,
but it will never provide less.
The modules are fairly simple. There is
a growing number of options for the power source. In its examples, ALD uses
piezoelectric fiber elements from
Advanced Cerametrics. These elements
are built using ACI's spun-piezoelectric
Viscose Suspension Spinning Process
(VSSP), which produces more rugged
transducers than monolithic piezos.
The modules' unique elements include
ALD's EPAD MOSFETs, which use
precharged floating gates to precisely
controlled threshold voltages. One EPAD
MOSFET on the module input gates the
voltage from the transducer that charges
the module's storage capacitor from a
trickle of electrons up to a preset maximum voltage. The second EPAD device
controls the output, shutting off discharge
when the voltage on the capacitor reaches a preset minimum.
The EH300 modules provide an output voltage that ranges from a high of 3.6 V to a low of 1.8 V. The output range
on the EH301 modules is 5.2 to 3.1 V.
So for a nominal 3.3- or 5-V rail, the output of the module is always within the
standard operating voltage range of the
microcontroller and associated circuitry
selected for the application.
Given that voltage control, the capacitance on the module determines the
energy available between VMAX and VMIN
and therefore the minimum draw-down
time at any constant current. The usable
energy capacity of the EH300 is 4.6 mJ,
which equates to 68 ms at 25 mA. For
the "A" version of the EH300, the specs
are 30 mJ, or 75 ms at 150 mA.
The 5.2 to 3.1-V EH301 also has a
base version and an "A" version. The
former provides 8.3 mJ, or 80-ms
operating time at 25 mA. The latter provides 55 mJ, or 88 ms at
150 mA.
The ALD EH300 and EH301
EPAD are available now starting
at $36.54 in quantities of 1000,
with higher-volume prices (100,000
units) in the $20 range.
Advanced Linear Devices
www.aldinc.com
Advanced Cerametrics
www.advancedcerametrics.com