MINIPROJECT

52 Elektor Electronics 10/2003

PP3 (6F22) type 9 V batteries are often usedin small portable equipment that require verylittle current and may only be used intermit-tently. Under these conditions its often thecase that the battery is flat just when youurgently need to use the equipment. NiCdrechargeable cells are not a good choice inthese applications because their self-dis-charge characteristics are much worse thandry cells and often there is no charge leftafter a long time in storage. Capacitors offeran interesting alternative power storagedevice, they can be charged very quickly andthey retain their charge for years. SuperCapsare small outline capacitors offering hugestorage capacity measured in farads ratherthan the microfarads (µF) that we are used to.The maximum supply voltage of 2.3 V meansthat some form of voltage multiplier is neces-sary to increase voltage up to a more useful9 V. Using a 10 F, 2.3 V SuperCap specifiedhere, the complete circuit can be built into apackage the same size as a PP3 cell.Compared to rechargeable cells a capacitorcan handle a very high peak current for bothcharging and discharging and has a very lowself-discharge characteristic (it keeps itscharge). A capacitor is an almost ideal energystorage medium as can witnessed for exam-ple by its universal use in camera flash equip-ment. There are no batteries (dry or recharge-able) of the equivalent size that can deliverso much energy in such short a time. Alsothere are very little loses involved in thecharge/discharge cycle. Now the bad news;unlike a battery a capacitor has a very simpleformula relating its voltage to charge storedso that when charge is removed, output volt-age falls. In this respect a rechargeable cell is

much more user-friendly, it keepsthe output volt-age relativelyconstant over thedischarge cycleuntil the chargeis used upwhen the out-put voltage fallsmore sharply. Acapacitor will there-fore need some form ofvoltage regulation circuit before itcan act more like a battery.

The switching regulatorThe circuit shown here in Figure 1uses a small outline SuperCap witha capacitance of ‘just’ 10 F (yesthat’s 10,000,000 µF!) with a maxi-mum operating voltage of 2.3 V. Aswitching voltage regulator is usedto pump up the capacitor voltage to9 V. A small switch or jumper is fit-ted to the circuit to ensure that thecapacitor retains its charge overtime. An LM317T voltage regulator(IC1) is used on the input to ensurethat the capacitor can be rechargedfrom a wide range of voltage sourceswhilst protecting the capacitor fromover-voltage. This IC has built-inover-current and over-temperatureprotection. The circuit can becharged from a standard mainsadapter where the internal resis-tance of the adapter will limit the

charging current. Withan input current of 1 A the capacitorwill be fully charged in 20 s!The rest of the circuit is a switchingregulator containing a small ferritecoil or choke (L1). The 470 µH chokeconsists of approximately 20 turns ofenamelled copper wire woundaround a ferrite core. The resistanceof the winding should be less than1 Ω. The two ends of the winding aresoldered to connecting pins in thebody of the choke. Figure 2 showsthe second winding of 20 turns thatneeds to be wound over the top ofthe existing winding to provide feed-back for the oscillator (wind in thesame direction as the first winding).Alternatively the existing windingcan be removed from the choke andreplaced by a 40 turn centre-tappedwinding of enamel coated copperwire (0.5 mm diameter or 24/26 SWGis suitable). The choke has now beenconverted into a small transformer.Oscillator operation has proved to bereliable and runs with an input volt-age as low as 0.5 V.

SuperCap BatteryPower from a GoldCap By B. Kainka

GoldCap capacitors offer an interesting alternative power source whencompared to conventional disposable or even rechargeable batteries.They can be charged very rapidly and can also deliver a high peak outputcurrent. Their voltage rating however is quite low so a little electronicassistance is necessary to raise the output voltage to a more useful level.

The output voltage is regulated toprovide approximately 9 V. Transis-tor T3 uses its reverse biased base-emitter junction to act as a zenerdiode, providing a voltage referenceto regulate the output. The zenervoltage of a small signal NPN tran-sistor is around 8 V in this configu-ration (an 8 V zener diode could besubstituted for T3). The circuit pro-duces an output of 8.5 V with aninput voltage of 2.3 V while at 0.7 Vthe output falls to 8.4 V.Two of the most important criteriafor any switching regulator are itsoutput current capability and its effi-ciency. A 1 kΩ load resistor wasused for testing. The circuit took50 mA with an input voltage of 2.3 Vgiving a power consumption figureof 115 mW. The output voltage of8.5 V is the same as the output volt-age with no load. Current in the loadis 8.5 mA giving an output power of72.3 mW and this equates to an effi-ciency of 63 %.Output current is about 1 mA with aload resistance of 8.2 kΩ. The outputvoltage remains constant until theSuperCap voltage falls to 1 V by

which time the capacitor will havegiven up 80 % of its stored energy. Asthe voltage continues to fall the out-put also sinks until at 0.6 V the out-put voltage is 4.8 V.As with any switching regulator, theinput current increases as the inputvoltage falls. This type of regulator ismore efficient than a standard linearregulator so that smaller loads resultin lower power consumption. Underno-load conditions the converterconsumes just 2 mA with an inputvoltage of 2.3 V. Lightly loaded, thecircuit will run for up to three hoursbefore it needs recharging.The SuperCap used in this circuit isproduced by the Korean companyNuinTEK and has a package outline10 mm diameter by 30 mm. Farnellstock a similar SuperCap producedby Panasonic, this device is a littlelarger with a body diameter of18 mm and will not unfortunately fitinto a PP3 outline. Alternativelythere are 10 F GoldCaps rated at2.5 V, these are more generally avail-able but the case size is again big-ger than the NuinTEK capacitor.

(030109-1)

MINIPROJECT

5310/2003 Elektor Electronics

LM317T

IC1

D1

1N4004

R2

18

0Ω

R1

24

0Ω

S1

C1

10F2V3

C2

100µ16V

T1

BC337

T2

R3

1k

T3

L1

470µH

D2

1N4148

C3

100µ16V

+9V

+4V...+12V

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3x

Figure 1. Circuit diagram of the Goldcap rechargeable.

Figure 2. Coil L1 winding details.

Capacitance measurementMost capacitance meters are not capable ofmeasuring capacitors of 1 F and more, sowhat can you do when you need to measurea SuperCap or GoldCap capacitor? Well theanswer is not that difficult, all you need is atimer (a watch will do), a voltmeter and aload resistor.The voltage stored on a capacitor does notfall linearly when it is connected to a con-stant resistance but exponentially. The curvegets flatter with time because as the voltagefalls so the current in the load gets smallercausing less and less charge to be taken fromthe capacitor. Theoretically it will never betotally discharged, with time the dischargecurve can approach but never actuallyachieve zero volts. This simplecapacitor/resistor circuit is said to have atime constant τ = R⋅C. Where τ is seconds,R ohms and C farads. After each time con-stant the output voltage falls by 37% of itsstarting value and in practice falls to within1% of its final value after 5 time constants. A10 F SuperCap together with a 100 Ω resis-tor has a time constant τ = 100 Ω × 10 F =1000 s (16 Minutes and 40 Seconds). Armedwith this information we can now start mea-suring!Starting with the capacitor charged up to 2 Vwe can work out the value of capacitance bymeasuring the time it takes for the voltage tofall by 37% (37% of 2 V equals 0.74 V so thevoltage across the capacitor will be 1.26 Vafter one time constant) now plugging thistime into the formula C = τ/R will give usthe value of C. In our test the voltage took1200 s to fall to 1.26 V so the capacitancemeasured is 12 F.

Don’t be tempted to use too low a value ofload resistance to speed up the test; youcould end up burning your fingers (literally).The SuperCap can deliver a peak current ofaround 6 A, this equates to a lot of power(heat) dissipated in low resistance loads. Thecapacitor is also not a perfect device and likeall capacitors has a certain amount of internalseries resistance. The smaller the value ofload resistance the greater will be the effectof dissipation in this internal resistance.

time[s]

UC

discharge

RC0 2 RC 3 RC

charge[V]

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