This invention relates to overload protection, and more particularly to the protection of an output amplifier driving capacitive loads from damage which may be caused by current demands in excess of acceptable amounts.
Output amplifiers find wide applications in supplying output signals with appropriate characteristics to drive loads, in response to input signals which are otherwise inappropriate to drive such loads. In particular, output amplifiers are used to generate appropriate output signals in response to a given input signal with substantially different voltage, current, or impedance characteristics. This frequently results in the generation of an output signal representing a greater amount of energy than was present in the input signal.
In dealing with output signals having large voltage or current levels, short circuits existing on the load can result in a number of serious consequences.
Short circuits can place additional current or voltage demands upon the various components associated with the generation of the output signal. This can lead to the damage or ultimate destruction of the various components through such factors as excessive heat generation, or internal breakdown within the device. In addition to the demands placed upon the individual components, short circuits may simultaneously subject the system power supply to demands beyond which it was intended to operate, resulting in failures of a similar nature.
In addition to the undesirable consequences to which short circuits subject the various components comprising the output amplifier circuitry, such conditions can also simultaneously have undesirable effects on the condition responsible for the improper load. One example of this is inadvertent human intervention wherein a human being has unintentionally become a part of the load to which the output amplifier is supplying substantial voltage or current, such as by touching an exposed conductor with a screwdriver during a component adjustment, resulting in a potentially serious shock hazard.
For the above reasons, various protection schemes have been employed to guard and protect against improper load conditions on the output of an output amplifier. These schemes generally fall into one of two catagories: limiting the current or voltage supplied, or complete shut-down of the output amplifier.
In those schemes directed to limiting the voltage or current present on the output signal, generally the limiting mechanism acts to limit the voltage or current to a specified value. However, this specified level is generally the maximum level which the output amplifier is capable of delivering under normal operating conditions.
This situation results from the fact that if the limiting circuit of the output amplifier were designed to act at a level lower than the maximum level necessary to drive the load, the limiting circuit would prevent the output amplifier from delivering the necessary maximum level to the load. Consequently, limiting circuitry normally associated with an output amplifier generally acts to limit the quantity of interest, e.g., voltage or current, to the predetermined maximum level.
There is necessarily associated with such limiting action component stress. This stress may result in heat build up or other undesirable conditions internal to the components comprising the output amplifier. However, component stress is not necessarily limited to those components associated with only the output amplifier circuit; to the contrary, component stress may also extend to the power supply components as well. Additionally, limiting at such a level will not likely protect against shock hazard problems.
In an alternate approach, an improper operating condition results in a complete shut down of the output amplifier. In such an arrangement, a fuse or similar electromechanical protection device is often employed. Such a device is frequently placed in series with the power supply and the output amplifier. An improper load condition will then result in the complete interruption of the operation of the output amplifier. While such an approach is a solution, it does have a number of disadvantages, including requiring a finite amount of time before corrective action is effective. Perhaps the most troublesome aspect in such an approach is the necessity of operator intervention, either in resetting a circuit breaker, or having to physically effect fuse replacement. From a practical standpoint, this is often further compounded by the problem of the fuse being inaccessible or a replacement fuse not being readily available.
One such example of the above discussed problems will illustrate the foregoing. In dealing with helical scan video recorders, the positionable magnetic heads are located within a rotating structure, known as a scanner, and require several driving potentials in excess of several hundred volts. Due to the limited amount of space, the positionable magnetic head assembly is somewhat inaccessible. Consequently when operator intervention becomes necessary, operator shock due to exposure to the high potential, as well as component failure and ultimately protective fuse replacement, frequently results. These are usually due to inadvertent shorts from hand tools or the operator himself coming into contact with the output of the output amplifier. In such a situation, an overload protection device to limit the hazards present would clearly be desirable.