The input impedance is dependent on the programmed sample rate, the transmit rate, and how many and what type of sensors are enabled. As can be seen from the product datasheet, there are 3 different harvester/generator inputs, and a myriad of programmable operating modes to accommodate varied harvesters and energy environments.
The most commonly used harvester input is the Piezo input, an ultra-efficient switchmode converter. Figure 1 below clearly demonstrates the capability of the EH-Link versus different wireless transmit rates. Figure 1 shows test results where the applied voltage was 7.5V, a 1000 ohm Wheatstone bridge is being measured, and the transmit rate and packet payload size were varied. In this test the sample rate is fixed at one sample per second. The number of measurements saved up for transmission was varied from 1 to 30, where at 1 a single measurement was transmitted over the wireless link once per second. At 30 in this test, 30 measurements are accumulated and transmitted every 30 seconds. This is done to demonstrate that much less power is used to sample than to transmit. It is important to note that even at 30, the data sample timing is preserved and no data are missed in the received measurement stream.
Another harvester input, the 'ultra low voltage' input, is very low impedance and is intended for thermoelectric generators and thermopiles, which have very low voltage and relatively high output current. The input impedance on this input is on the order of 6 ohms, and varies a little with operating mode. With our demo TEG harvester is able to sustain continuous sampling at 64 samples per second and one transmission per second with a temperature differential of only 8 degrees Celsius between the mounting surface and the heat sink on the cool side.
The third input is an ultra high impedance AC type intended for use with high voltage piezo materials where the capacitance of the piezo is discharged at a threshold of approximately 130V peak. This is done to maximize the 'V' term in the capacitive energy equation (one half C*V squared). This input is somewhat experimental but has been shown to be very high efficiency and very high (>1 megohm) input impedance before the discharge threshold is reached, making it ideal for use with high voltage piezo ceramics.
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