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Battery operation

The power supply used may be mains driven, or in compact or mobile use battery driven. Alternatively, it can be mains driven with battery back-up to cater for power outages, or battery driven with mains charging. At the same time, a facility to use an automotive/marine/solar supply might be required. The circuitry will then usually be expected to run on voltages that are much lower than 'normal', any amplifiers used then possibly necessitating the use of low-voltage dropout regulators, unless step-up regulators are used. Taking the simplest approach can get a system off a bench and running in the quickest time.

The major advantages of batteries are that they enhance mobility and reduce noise by eliminating mains interference or power supply byproducts (by some 30 to 40dB in some cases). Disadvantageously, they may have to be replaced at the most awkward moment. With any battery system, a duplicate back-up is recommended since, just like other components, it is not a question of if batteries fail, but when. Also worth considering can be the need to safely change a supply in a wet environment, for example, by integrating back-ups (tagged types) into the same watertight enclosure used by the system. Being electro-chemical devices, low temperatures can adversly affect performance. By the same token, a low battery can be 'revived' by warming it. Some specialist types with wide temperature ranges have been used, eg; Gates 'Cyclon' sealed lead-acid -40°C to +65°C (2V, 2.5-25Ah, max 2,000 cycles).

The most mobile systems are light, the bulk of the deadweight being batteries. Where it is possible, solar-charging can reduce the size of a 'pack' considerably. The capacities of some common types are compared below.


Specifications and prices can vary considerably between manufacturers. Rechargable types will generally maintain a higher output voltage and despite a lower capacity can offer a considerable saving in cost over time (say, recharge x25 for rechargable alkaline, x500-3,000 for nicad and x500-1,000 for NiMH). Some considerations to be made include a long shelf-life (say 10 years), service life and capacity versus weight and cost. The energy density and discharge/time characteristics can vary greatly between types. For example, nicads have a memory effect in that, if not discharged completely before recharging, they will 'lose' their full capacity. NiMH types do not have this disadvantage. However, for a given application, a number of systems will be suitable. Lead-acid can offer high currents, but also a weight disadvantage (sealed types are maintenance-free and can operate in any position, even upside down). Li-Ion types intended for mobile phones and digital cameras can produce very light and compact low current solutions, albeit at a much higher price.

Separate test units, like multimeters, run from 9V batteries can benefit from an auto-power-off circuit, such as those below, which prevent batteries running down when an unused unit is left unintentionally switched on.

    
Both 8.4V nicad PP3s and substantially more powerful model racing car packs (multiple tagged Cs) have been used, but NiMH can now offer longer life and are environmentally more acceptable. Be mindful that, if abused, a high energy battery pack can represent a very real fire hazard.

A good combination for ±7V2 rails used 12 x AA NiMHs with an in-built TEA1101 charger. This gave about 50 hours use with a 21mA drain, flat batteries charging in 3 hours at 500mA (then trickle-charging) so that temperature monitoring of the batteries was not required. An LED indicated mains connection and a bi-colour LED provided status (off, battery, battery + charger). Such an arrangement can be useful for noise measurement systems as well.

Tailoring the frequency response of a system to the target's bandwidth can extend battery life, the system then not having to process unwanted signals.

Some specialised circuits that have proved useful for battery operation are shown below. In some cases regulators with low drop-out voltages are a prequisite. The LM2931 (5V) series low dropout regulators give extended supply life, although discrete designs can give an even lower drop-out,eg:


A more complex version runs at 50µA with a 0.5V drop-out. Another design, below, consumes even less.


As can be seen, solutions can be implemented using standard components.


For monitoring purposes, head or earphones may be required.


IC options to drive headphones include the TDA2822, LM386, TDA7052, TBA820M , LM380 or the class D MAX4297 (>83% efficient). A discrete low-power design intended for solar use can be found here, a simple but adequate design is shown below.


A suitable tone control for this can be found here.

The need to change batteries during use, without interrupting operation, will doubtless occur. This can be accommodated by fitting an extra battery socket to the amplifier enclosure with a low forward-drop diode in series with one of the leads, as shown below. This will prevent a fully charged battery discharging wastefully into a used or failed one when paralleled, whilst protecting from an accidental reversal of polarity as well.


If a high efficiency LED is used to indicate that power is on, the circuit below will cause it to flicker at low battery levels.


Alternatively, the LED can be fed to the collector of the output transistor, then operating only to indicate low levels. Here's another whose output can be used to trigger a back-up system.


The next circuits can extend the operating life of a small battery.

    
The LED output of the RC4190N has been used with a decade counter to sequentially switch in fresh batteries, although a comparator (or indicator, see above) can be used instead.


Dedicated automatic battery back-up switches are available, like the ICL7673. Micropower switched regulators can be used to supply a negative rail for dual rail supplies from a single supply with efficiencies in excess of 90%. These include the ICL7660 series, MAX665, SI7660CJ, MAX680CPA, etc.

More complex arrangements may require dual rail operation at higher voltages, although a larger power source than a PP3 is recommended for this supply, given that losses will invariably be greater. Improvements can, however, be made.


Care must be taken with switching regulators and DC-DC converters to isolate any transient byproducts from the signal path with plentiful regulation, smoothing and screening (particularly of inductors). Similarly, some 78x/79x series regulators can be very noisy compared to adjustable types like the LM317/37 types. A post filter of a 10R resistor and a 470µF-1mF electrolytic can help alleviate this. Discrete formats can, arguably, outperform IC types in terms of noise.

Higher outputs

Many proprietary solutions are available and often the main consideration is what form a power amplifier, say, will take to accommodate the given supply. Taking 12V supplies as an example, a mains inverter may be used to re-create a mains supply in which case extra capacity will be required to cope with inevitable losses arising from the transformers used. Cheaper models can cause problems with transient noise, although uninterruptible power supplies intended for PCs have been pressed into service (with additional capacity).

Some amplifiers may feature an in-built switched mode power supply to raise the voltage levels and provide, say, dual-rail operation. The transformers for these can interest; the primary sometimes consisting of a single turn of copper sheet, the secondaries being made up of layers of paralleled lengths of 'litzendraht' (see Speaker cable), for example.

Another approach is to examine the efficiency of the system run. Taking a PA or music system, speaker placement can be important, but so can the choice of amplifier, class D offering the most efficient format. Many 12V designs suited to low-impedance loads are available. Class H designs switch in a higher voltage supply rail to cope with peaks. A good example is the Philips TDA1560Q IC, intended for automotive use.

As mentioned previously, advantage could be taken of means of recharging a battery supply. This can be done during operation and when the system is dormant. An average adult male on a bicycle-type generator will, flat-out and therefore intermittently, produce an output of only about 100W. Bad news if the amplifier/s chosen are thermionic or class A.

If high-current, say, automotive batteries are paralleled, some degree of isolation should exist between them. Ideally, a low-loss rectifier and fuse will be placed between individual batteries and the common load. However, high-output light-bulbs can suffice. In one arrangement, four batteries were continuously trickle-charged by a single charger (via smaller bulbs), with no significant supply noise, the batteries acting as very effective smoothers.

Concerns can be raised, in terms of toxicity and thus environmental impact with larger sized lead-acid batteries, like the submarine batteries that were used in the Netherlands to produce ball-lightning, although these cannot really be deemed, like milk-float batteries, to be portable. At the same time, with non-sealed types, there are dangers like the leakage of acid and hydrogen to be considered.

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