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Circuits designed by David Johnson, P.E.
Last Updated on: Wednesday, October 18, 2017 05:04 AM

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More  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.
     
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.

Click on Drawing Below to view PDF version of Schematic


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