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Supply Voltage Monitor(BC558)

2015-01-24 03:37  
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This article describes Supply Voltage Monitor (BC558). The principle is very simple, very practical. The circuit components can help you understand better grasp this principle. For example, in this circuit, you can go to find and buy these components: BC558.

A circuit for monitoring supply voltages of ±5 V and ±12 V is readily constructed as shown in the diagram. It is appreciably simpler than the usual monitors that use comparators, andANDgates. The circuit is not intended to indicate the level of the inputs. In normal operation, transistors T1 and T3 must be seen as current sources. The drop across resistors R1 and R2 is 6.3 V (12 –5 –0.7). This means that the current is 6.3mA and this flows through diode D1 when all four voltages are present. However, if for instance, the –5 V line fails, transistor T3 remains on but the base-emitter junction of T2 is no longer biased, so that this transistor is cut off. When this happens, there is no current through D which then goes out.

Figure:1

Figure 1 

 

 


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