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Circuits designed by David Johnson, P.E.
Last Updated on:
Monday, December 25, 2017 02:09 PM
List of Dave's Circuit Designs
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Super Capacitor Circuits Supercapacitor Voltage Limiting
Circuit
Supercapacitors are working their way into more and more applications where electrical
energy needs to be stored. These robust devices can be charged and discharged
1000s of times and will typically outlast a battery. A supercapacitor is often
chosen to supply power to low current loads for many hours at a time, recharged by a
solar panel. They are also handy when infrequent heavy power demands are needed.
Many supercap manufacturers claim a life span of 10 years or more. But to
insure a long life, the capacitor voltage rating should not be exceeded.
The circuit below limits the capacitor voltage to a safe level.
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All
supercapacitors have a maximum voltage rating. When charging these devices, that
voltage should not be exceeded. Doing so can damage the device or shorten its lifetime.
In many applications several capacitors are wired in series, to produce a capacitor bank
with a higher voltage. But even if the proper charging voltage is used, the weakest
device in the string will charge up first. Without a circuit to limit the voltage
across each part, the weakest part in the series string will be overcharged as the rest of
the parts in the string finish their charge. The circuit below solves this over-voltage
problem by balancing the string with a voltage limiting circuit across each capacitor.
The circuit diverts charging current around each part, when the voltage reaches a critical
point. The circuit is really a classic shunt type voltage regulator. It takes
advantage of a LMP2231 low voltage op Amp from National Semiconductor, a low current LM385
voltage reference, also from National, and a PMV30UN n-channel FET from NXP
Semiconductors, which has a low gate-source threshold voltage. With the components
shown, the circuit limits the capacitor voltage to 2.65v and is perfect for
supercapacitors with a 2.7v rating. By changing R4 to 69.8K, the voltage can be
reduced to 2.5v, for 2.5v rated super caps. The charge current should be less than 300ma. |
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on Drawing Below to view PDF version of Schematic |
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