This project, an intrinsically safe battery circuit from 1986,? shows how to ensure the safe operation of a battery in an unsafe environment i.e. where there could be a risk of explosion.
An intrinsically safe battery circuit for connection between a battery and a
load, such as a hand held transceiver, referred to as "HT's". The circuit
includes a minor current path through a switching transistor and a major
current path through a SCR.
The circuit is a hybrid circuit which includes an integrated circuit timer which provides a delay in establishing the major current path subsequent to the minor current path being established, and mounts on a small printed circuit board which results in a battery that is intrinsically safe.
1. Field of the Design
The present design pertains to an intrinsically safe battery circuit, and more particularly, to an intrinsically safe battery hybrid circuit which prevents arcing and allows full current from the battery during operation.
2. Description of the Prior Art
An intrinsically safe battery is defined as a cell pack whose energy is limited to a point so as not to cause ignition as properly used in various atmospheres. The atmospheres considered dangerous are divided into several groups, and the groups are determined by the amount of energy required to cause ignition.
A regulatory agency most recognized in the United States is Factory Mutual ("FM"). FM in an insurance company specializing in hazardous applications. All testing is dictated and conducted on site at FM. Any damage or injury occurred in an accident involving a battery or other equipment not "FM Approved" voids Factory Mutual's liability. For this reason, customers insist on "FM Approved" batteries which are intrinsically safe.
Testing involves a variety of dielectric and physical construction checks. These checks are intended to insure the stability of the components used or limit the energy from the cells. Electrical testing is conducted on a spark generator.
A chamber is constructed which is air tight and includes a viewing window. Inside, a random spaced circular set of contact pins, resembling nails, are attached to one terminal of the battery. The other terminal attaches to a rotating arm whose tip contacts the contact pins. This essentially makes and breaks the circuit to the battery under test. Arcs are created inside the chamber, and these arcs are representative of the energy available from the battery. The different atmospheres are then pumped into the chamber and checked for ignition.
In the prior art, the most common method of making an intrinsically safe circuit for a battery has been to simply add enough series resistance to limit the energy to a safe level. This has worked well except for higher voltage cell packs such as 12 volts and over. These batteries of a larger voltage require a large value resistance at a high wattage that is physically difficult, and not acceptable to locate internally in the battery.
The present design overcomes the disadvantages of the prior art by providing a hybrid circuit to prevent arcing, which passes all electrical tests and allows full current from the battery.
The general purpose of the present design is to provide an intrinsically
safe battery circuit which can be a hybrid circuit, and placed inside
nickel-cadmium ("nicad") batteries, also referred to as nicad packs or nicad
batteries. The circuit includes a minor current path and a major current path.
According to one embodiment of the present design, there is provided an intrinsically safe battery circuit for connection between a battery and a load, a circuit including a minor current path through a transistor and a major current path through a SCR. A timing circuit provides that when a load in place, the timing circuit provides an output signal after a predetermined time which switches a gate of a SCR to apply full power through the major current path. If a connection is broken such as removal of the battery, then another timing delay is again occurred. This avoids arcing which causes ignition.
Significant aspects and features of the present design is an intrinsically safe battery circuit which is a hybrid circuit and can be easily located in a nickel-cadmium battery cell housing.
Another significant aspect and feature of the present design is a hybrid circuit where the delay, when full power is applied, can be predetermined by selecting the value of the "RC" circuit across a timing integrated circuit.
A further significant aspect and feature of the present design is a hybrid circuit which is cost effective and provides 100% reliability while also allowing full power from the battery.
Having thus described principal embodiments of the present design, it is the principal object hereof to provide an intrinsically safe battery circuit.
One object of the present design is to provide an intrinsically safe battery circuit which allows full current from the battery and can be physically located within the battery housing.
Another object of the present design is to provide an intrinsically safe hybrid battery circuit.
FIG. 1 illustrates a plan view of an intrinsically safe battery circuit 10.
A printed circuit board 12 supports conductors 14, 16 and 18, and components
20-34. A battery 36 with a plurality of cells in series such as nickel-cadmium
cells, and a thermal activated switch 40 such as a low resistance device which
breaks the circuit at high current connects between conductors 14 and 16. A
load such as a hand held transceiver or a charger connects between conductors
16 and 18.
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