The basic component of an alarm system using a constant current supply are provided . This design is improved over that provided in this lesson in the sense that it is less sensitive to charges in resistance in the circuit due to heating humidity charges in the length of the line to the sensor and so on.
The 1.5killo ohm rheostat is adjusted to ensure a current of 5 mA through the single series security circuit. The adjustable rheostat is necessary to compensate for variation in the total resistance of the circuit introduced by the resistance of the wire sensor sensing relay and milliammeter. The milliammeter is included to set the rheostat and ensure a current of 5 mA.
If any of the sensor open the current through the entire circuit drop to zero the coil of the relay releases the plunger and contract is made with the N/C position of the relay . This action complete the circuit for the bell circuit and the alarm sounds. For the future keep in mind that switch position for a relay are a always shown with no power to the net work, resulting in the N/C position.
When power is applied the switch will have the position indicated by the dashed line. That is various factors such as a charge in resistance of any of the element due to heating humidity and so on ,cause the applied voltage to redistribute itself and create a sensitive situation . With an adjusted 5mA, the loading can charge , but the current will be always be 5mA and the charge of tripping reduced.
Note that the relay is rated as 5v at 5 mA indicating that in the on state the voltage across the relay is 5 V and the current through the relay is 5 mA. It's internal resistance is therefore 5V/5mA= 1killo ohm in the state.
A more advanced alarm system using a constant current is illustrated. In this case an electronic system using a single transistor biasing resistor and dc battery are establishing a current of 4 mA
through the series sensor circuit connected to the positive side of an operational amplifier.
Transistors and op-amp devices may be new to you , but for now you just need to know that the transistor in this amplification is being used not as an amplifier but as part of a design to establish a constant current through the circuit. The op-amp is a very useful component of numerous electronic systems and it has important terminal characteristic established by a variety of component internal to its design.
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