POWER CONDITIONING AND STORAGE A dual-cell CAP-XX supercapacitor consisting of a pair of HW109 cells forms the heart of the power-conditioning and storage block. This supercap, which was chosen because it’s small and thin, has two cells, each measuring 28.5 by 17 by 1.1 mm. It provides high energy storage—140 mF at 5.5 V = 2.1 J.
The supercap’s high power delivery is only 120-mO equivalent series resistance (ESR), so its max power transfer is 63 W. Operation is in the industrial temperature range of –40°C to 85°C. It can be charged with low current down to 50 µA. And its very low leakage current ranges down to about 3 µA with an active balance circuit.
The PMG17 produces ac, which is full-wave-rectified by the diode bridge D1-D4 (Fig. 4). To maximize efficiency, the diodes should have low forward voltage and low reverse leakage current. The BAS16 diodes from ON Semiconductor provide good characteristics from –40°C to 85°C.
Based on Figure 3, the PMG17 operating voltage should remain in the range of 4 to 6 V. There’s approximately a 1-V drop across the diode bridge, so VCAP will be in the 3- to 5-V range. VCAP must be greater than 3.2 V to supply a buck converter with a 3.0-V output that drives the data-gathering and transmission circuits.
D5 has a reverse voltage of 2 V at a reverse leakage current of 3 µA. R2 and D5 ensure that Q1 doesn’t turn on until VCC approaches 5 V. As Q1 turns on, VCC drops as charge current flows to the supercapacitor, turning Q1 off again. The PMG17 then charges C9, C11, and C12 until VCC is sufficient so that reverse current flows through D5 and the voltage across R2 reaches VGS of Q1 to turn it on again.
With a few tens of microamps, the combined capacitance of C9, C11, and C12 = 66 µF can be charged to 5 V in less than 30 seconds. In this manner, R2, D5, and Q1 regulate VCC to approximately 5 V, ensuring maximum power transfer from the PMG17 microgenerator to the supercapacitor. At 120 µA, it will take about an hour and a half to charge the supercapacitor to 4 V, where it can support a data-gathering and transmit cycle.
The two supercapacitor cells need to be balanced so that neither goes over voltage. The balancing circuit connects to the node labelled “Active balance” in Figure 4. The balancing circuit current + supercapacitor leakage current must be much smaller than PMG17 output current ˜ 100 to 200 µA. A high-impedance, low-power operational amplifier, as shown in the balancing circuit of Figure 5, will only draw approximately 3 µA, including the supercapacitor leakage current.
The operational amplifier chosen needs to be rail-to-rail. It only draws less than 1-µA supply current and can source or sink up to 11 mA to bring cells quickly into balance. Once the supercapacitor is charged, the op amp only supplies or sinks the difference in leakage current between the two supercapacitor cells in series to maintain voltage balance.
DETERMINING SUPERCAPACITOR SIZE Figure 6 shows the load supported by the supercapacitor, and the table details the energy required. With dc-dc converters that are 75% efficient, the supercapacitor must deliver 83/0.75 = 111 mJ. The supercapacitor energy delivered equals:
Allow for loss of capacitance due to supercapacitor aging, so start with double the capacitance. Therefore, initial C > 36 mF. Select the smallest CAP-XX part that operates over the industrial temperature range, which is the HW209 with C = 140 mF and ESR at room temperature range = 120 mΩ.
ESR at –40°C is approximately 3 × room temperature ESR. Check the suitability of the HW209: peak current = (28 mA × 3/3.2 V + 3 mA × 18/3.2 V)/0.75 = 58 mA. As a result, voltage drop due to ESR at –40°C = 0.36 Ω × 58 mA = 21 mV << voltage drop from capacitance discharge due to supplying load energy.
Here I am again. In my case the battery in the car would be the best power source for these sensors as it is already installed in the car. It is true that my car sensors are malfunctioning and that the sensors are stating that as a fact as well I do have the evidence of the car not accelerating properly.
Anonymous -September 23, 2009
I am thinking about my car sensors? The fuel sensor and the air sensor. They are not functioning properly so that when I drive up a mountain the car sometimes looses power. If I press the gas pedal down to the floor I can make it up the mountain but until I found this to be the case I had to pump the gas pedal and then only do spurts of power which kept on falling back to almost crawl time. Would this be the same thing as what you are describing here. Thanks for your answers
Anonymous -September 23, 2009
I am thinking about my car sensors? The fuel sensor and the air sensor. They are not functioning properly so that when I drive up a mountain the car sometimes looses power. If I press the gas pedal down to the floor I can make it up the mountain but until I found this to be the case I had to pump the gas pedal and then only do spurts of power which kept on falling back to almost crawl time. Would this be the same thing as what you are describing here. Thanks for your answers
Anonymous -September 23, 2009
I am thinking about my car sensors? The fuel sensor and the air sensor. They are not functioning properly so that when I drive up a mountain the car sometimes looses power. If I press the gas pedal down to the floor I can make it up the mountain but until I found this to be the case I had to pump the gas pedal and then only do spurts of power which kept on falling back to almost crawl time. Would this be the same thing as what you are describing here. Thanks for your answers
Anonymous -September 23, 2009
HTTP://WWW.CN486.COM
Anonymous -March 22, 2009
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