Supply Capacitor

Super Capacitors

One of the most critical aspects of an uninterruptible power supply (UPS) is its standby or back-up battery system, where super capacitors are now beginning to play a role.

A super capacitor resembles a regular capacitor except that it offers high capacitance in a small package. Energy storage is by means of static charge rather than an electro-chemical process, inherent in lead-acid uninterruptible power supply batteries. Applying a voltage differential on the positive and negative plates charges the super capacitor (this concept is similar to an electrical charge that builds up when walking on a carpet).

Their design makes them ideal for small uninterruptible power supply installations whereby they are being utilised in favour of a battery set or to reduce the potential for battery discharge during momentary mains power failures.

The amount of energy that can be stored depends upon the active material used in the design of a super capacitor. Potentially, it can achieve up to 30kW of stored energy.

A super capacitor (also referred to as electric double layer capacitor, electrochemical double layer capacitor or ultra capacitor) consists of two electrodes constructed from a highly activated carbon material, which may be woven. Whereas a regular capacitor consists of conductive foils and a dry separator, the super capacitor crosses into battery technology by using special electrodes and some electrolyte. There are three types of electrode materials suitable for the super capacitor: high surface area activated carbons, metal oxide and conducting polymers. The high surface electrode material, also called Double Layer Capacitor (DLC), is least costly to manufacture and is the most common. It stores the energy in the double layer formed near the carbon electrode surface.

The carbon activated electrodes provide a large reticulated area upon which an active material such as Ruthenium Oxide is deposited. The material provides an enormous area, for example, 1000 square meters per gram of material used. Cellulose paper with polymeric fibers to provide reinforcement is typically used as the separator between the electrodes. Electrolyte is usually diluted Sulphuric Acid. Ruthenium Oxide is converted into Ruthenium Hydroxide by a chemical reaction and this enables energy to be stored.

To operate at higher voltages, super capacitors are connected in series. On a string of more than three capacitors, voltage balancing is required to prevent any cell from reaching over-voltage.

Energy within a super capacitor is quickly available – and this is one of its greatest advantages. When coupled to an existing battery set, they can inhibit battery cycling for momentary interruptions, which helps extend the working life of the set. A super capacitor’s working life is typically ten years (double that of an average UPS battery). They can also operate over a wide temperature range (minus 30 to 45 degrees centigrade).

Other Advantages

• Virtually unlimited cycle life – can be cycled millions of times.
• Low impedance – enhances load handling when put in paralleled with a battery.
• Rapid charging –super capacitors charge in seconds.
• Simple charge methods – no full-charge detection is needed; no danger of overcharge.

Limitations

• Linear discharge voltage prevents use of the full energy spectrum.
• Low energy density – typically holds one-fifth to one-tenth the energy of an electrochemical battery.
• Cells have low voltages – serial connections are needed to obtain higher voltages. Voltage balancing is required if more than three capacitors are connected in series.

• High self-discharge – the rate is considerably higher than that of an electrochemical battery.

Whereas the electro-chemical battery delivers a steady voltage in the usable energy spectrum, the voltage of the super capacitor is linear and drops evenly from full voltage to zero volts. Because of this, it is unable to deliver the full charge. If, for example, a 6V battery is allowed to discharge to 4.5V before the equipment cuts off, the super capacitor reaches that threshold within the first quarter of the discharge cycle. The remaining energy slips into an unusable voltage range. A DC-to-DC converter could correct this problem but such a regulator would add costs and introduce a 10 to 15 percent efficiency loss.

The charge time of a super capacitor is about 10 seconds. The ability to absorb energy is, to a large extent, limited by the size of the charger. The charge characteristics are similar to those of an electrochemical battery. The initial charge is very rapid; the topping charge takes extra time. Provision must be made to limit current when charging an empty super capacitor.

In terms of charging method, the super capacitor resembles the lead-acid battery. Full charge occurs when a set voltage limit is reached. But unlike the electrochemical battery, the super capacitor does not require a full-charge detection circuit. Super capacitors take as much energy as needed. When full, they stop accepting charge. There is no danger of overcharge or ‘memory’.

Super capacitors are relatively expensive in terms of cost per watt. Some design engineers argue that the money would be better spent providing a larger battery by adding extra cells. But the super capacitor and chemical battery are not necessarily in competition. They enhance one another.

About the Author

Robin Koffler is the General Manager for Riello UPS Ltd the UK subsidiary of Riello UPS (RPS S.p.A) a leading European manufacturer of Uninterruptible Power Supplies and a co-author of The Power Protection Guide(ISBN 978-0-9554428-0-3)- available from Amazon.com

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LED power supply

An LED has diode like voltage versus current characteristics. Current is approximately an exponential function of voltage, so a small voltage change results in a large change in current. That is why it is important that the power supply gives the right voltage. If the voltage is below the threshold no current will flow and the result is an unlit LED. If the voltage is too high the current will go above the maximum rating, heating and potentially destroying the LED. As the LED heats, its voltage drop decreases further increasing current. Consequently, LEDs should only be connected directly to constant-voltage sources if special care is taken.

Series resistors are a simple way to stabilize the LED current, but wastes energy in the resistor. A constant current regulator is commonly used for high power LEDs. Low drop-out (LDO) constant current regulators also allow the total LED string voltage to be a higher percentage of the power supply voltage, resulting in improved efficiency and reduced power use. Switched mode power supplies are used in some LED flashlights, stabilizing light output over a wide range of battery voltages and increasing the useful life of the batteries. Miniature indicator LEDs are normally driven from low voltage DC via a current limiting resistor. Currents of 2 mA, 10 mA and 20 mA are common. Sub-mA indicators may be made by driving ultra bright LEDs at very low current. Efficiency tends to reduce at low currents. Strings of LEDs are normally operated in series LEDs, with the total LED voltage typically adding up to around two-thirds of the supply voltage, with resistor current control for each string. LEDs, by nature, require direct current (DC) with low voltage, as opposed to the mains electricity from the electrical grid which supplies a high voltage with an alternating current(AC). A capacitor and resistor followed by full-wave rectification is the usual electrical ballast with series-parallel LED clusters. A single series string minimizes dropper losses, while paralleled strings increase reliability. In practice usually three strings or more are used. An advantage of a capacitor is that it can reduce the high line voltage to an appropriate low voltage, without wasting power, with a very simple circuit; a disadvantage is that there may be a high surge of current for a short time when it is first turned on. Multiple LEDs can be connected in series with a single current limiting resistor provided the source voltage is greater than the sum of the individual LED threshold voltages. Parallel operation is also possible but can be more problematic. Parallel LEDs must have closely matched forward voltages (Vf) in order to have equal branch currents and, therefore, equal light output. Variations in the manufacturing process can make it difficult to obtain satisfactory operation when connecting some types of LEDs in parallel. To increase efficiency (or to allow intensity control without the complexity of a DAC), the power may be applied periodically or intermittently; so long as the flicker rate is greater than the human flicker fusion threshold, the LED will appear to be continuously lit.

LED power supply

About the Author