Design power circuit
Bacause the AirMobile sensor is going to harvest energy from waste heat by means of a Peltier cell, a proper power circuit is required to provide a reliable power supply to the electronic board.
My choice was the Linear Technology LCT3108. This is a highly integrated DC/DC converter ideal for harvesting and managing surplus energy from extremely low input voltage sources such as TEGs (thermoelectric generators), thermopiles and small solar cells. The step-up topology operates from input voltages as low as 20mV.
Using a small step-up transformer, the LTC3108 provides a complete power management solution for wireless sensing and data acquisition. The 2.2V LDO powers an external microprocessor, while the main output is programmed to one of four fixed voltages to power a wireless transmitter or sensors. The power good indicator signals that the main output voltage is within regulation. A second output can be enabled by the host. A storage capacitor provides power when the input voltage source is unavailable. Extremely low quiescent current and high efficiency design ensure the fastest possible charge times of the output reservoir capacitor.
Circuit simulation
Using the power requirements defined in my previous post, I tried to define a power circuit that could meet my requirements. In particular I was interested in determining how much time would be required to charge the super-capacitor that will provide energy to heat up the CO sensor.
LTSpice has been of great help. It can be freely downloaded from Linear Technology web site. Starting from one of the proposed circuits based on LTC3108, I changed the components to meet my scenario.
In particular I assumed that
- the input voltage from Peltier cell is 300 mV (which is typical for a Δt of 20 °C
- the Cstore (i.e. the capacity of the super.capacitor) is 0.1 F
The first thing that I found out was that a 0.1 F super-capacitor would never be charged. So I tried again with a 0.01 F super-capacitor, which is, in any case, 10 times the capacitance required by the sensors
I ran the simulation with this parameters and I got the graph below
I limited to simulation to 10 seconds due to hardware constraints.
It takes about then second to charge the super-capacitor up to 1.4 V. To simplify, let's assume that the charging curve is (almost) linear. I expect the super-capacitor to be fully charged in 50 seconds. However this is the initial condition. During normal operation the super-capacitor is never going to be fully discharged. The voltage drop should be
where
- Vo is the capacitor initial voltage (5V)
- C is 0.01 F
- R is the resistance of the circuit. Because current is 200 mA, the resistance R is 25 Ohms
- Sensor 200 mA for 1 ms to heat up the capsule, to t = 0.001 s
With this values, the above formula tells me that the voltage drops to 4.80 V
Starting from this residual voltage, the super-capacitor is going to be charged at full power in a few seconds
