Electrical transmission or interconnection systems – Plural supply circuits or sources
Reexamination Certificate
1999-12-07
2001-11-06
Tso, Edward H. (Department: 2838)
Electrical transmission or interconnection systems
Plural supply circuits or sources
Reexamination Certificate
active
06313548
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power switching device which switches between power supplied from a main power supply and power supplied from a supplementary power supply to supply the selected power to a main circuit.
2. Description of the Related Art
Any instantaneous power interruptions are not permitted in facilities, such as fire alarm systems. The control panel of such a facility is equipped with a power switching device. The power switching device continuously feeds reliable power to a main circuit in the control panel by selecting between power from a main power supply and power from a supplementary power supply.
FIG. 5
is a block diagram of a conventional power switching device. As shown, the power switching device is housed in the control panel, and switches between power supplied from a main power supply ME (utility service line, for example, a commercial 100-volt power line) external to the control panel and power supplied from a supplementary power supply SE (of 24 VDC, for example) arranged in the control panel.
The conventional power switching device includes an NV (No-Voltage) relay
101
as a switching element, a control unit
102
for controlling the NV relay
101
, and a capacitor C
100
. The control unit
102
includes a voltage detecting unit
121
for detecting the voltage of the power supplied by either the supplementary power supply SE or the main power supply ME, a NOT gate
122
, namely, an inverter, for inverting an output signal of the voltage detecting unit
121
, and a driver
123
for driving the NV relay
101
in response to an output of the NOT gate
122
.
In the power switching device thus constructed, the NV relay
101
is connected to the supplementary power supply SE at its initial state prior to the application of power to the control panel. In the power-on operation of the supplementary power supply and the main power supply, the power supplied by the supplementary power supply is initially supplied to a main circuit MS. When the voltage detecting unit
121
detects that a voltage value of the power supplied by the main power supply ME rises above a predetermined value, the voltage detecting unit
121
outputs a signal indicating a LOW state. The LOW-state signal is inverted by the NOT gate
122
into a HIGH-state signal, which is then fed to the driver
123
. In response to the HIGH-state signal, a transistor in the driver
123
is driven, thereby feeding a voltage to the NV relay
101
. The NV relay
101
is switched to the side of the main power supply ME. In this way, the power from the main power supply ME is fed to the main circuit MS. This switching operation is performed at the power-on operation because the power supplied by the main power supply is not stable immediately subsequent to the power on. The supplementary power supply thus needs to feed power until the main power supply becomes stabilized.
When the voltage detecting unit
121
detects that the voltage value of the power supplied from the main power supply ME drops below the predetermined value as a result of an interruption of the power from the main power supply ME in the above state, the voltage detecting unit
121
outputs a HIGH-state signal. The NOT gate
122
inverts the HIGH-state signal into a LOW-state signal, which is then fed to the driver
123
. With the driver
123
receiving the LOW-state signal, the transistor stops conducting, thereby suspending the voltage application to the NV relay
101
. The NV relay
101
is connected back to the supplementary power supply SE. The power from the supplementary power supply is then fed to the main circuit MS.
The conventional power switching device performs the above switching sequence, thereby reliably feeding power to the main circuit MS.
The operation speed of the NV relay
101
is slow. To avoid a power interruption during the slow power switching operation, power stored in the capacitor C
100
is used to compensate for a lack of power during the power switching operation. Specifically, the capacitor C
100
is charged with the power from the main power supply ME or the supplementary power supply SE, and the capacitor C
100
feeds power to the main circuit MS for a duration of several milliseconds, from the moment the NV relay
101
is driven to the completion of the switching operation, in the switching operation. Because of the need for compensating for the power during the switching operation of the slow-speed NV relay
101
, a high-capacitance capacitor as high as several tens of thousands micro farads is employed for the capacitor C
100
.
In the conventional power switching device, the capacitor C is used to preclude a power interruption during the power switching operation. If the switching operation is carried out before the capacitor C is sufficiently charged, the capacitor C has not enough power to discharge. The use of the capacitor C is not a good instantaneous remedial step. The high-capacitance capacitor of several tens of thousands microfarads is expensive, thereby leading to a cost increase of the entire power switching device. Such a capacitor, in need of ample space for installation, presents difficulty in the miniaturization effort of the entire power switching device.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a power switching device which takes an instantaneous remedial step without the need for a high-capacitance capacitor.
A power switching device of the present invention includes a switching unit for supplying power to a main circuit by switching between power from a main power supply and power from a supplementary power supply, and a control unit for controlling the switching unit. The control unit includes a voltage detecting unit for detecting a voltage value of the power supplied by either the main power supply or the supplementary power supply. The power switching device comprises a first switch assisting unit, composed of semiconductor switching elements, for feeding the power from the main power supply to the main circuit when the voltage detecting unit detects that the voltage value of the power is above a predetermined value, and a second switch assisting unit, composed of semiconductor switching elements, for feeding the power from the supplementary power supply to the main circuit when the voltage detecting unit detects that the voltage value of the power is below the predetermined value.
Preferably. the control unit includes an inverter for inverting an output of the voltage detecting unit. The voltage detecting unit outputs a LOW-state signal when the voltage value rises above the predetermined value, while outputting a HIGH-state signal when the voltage value drops below the predetermined value. The control unit drives the first switch assisting unit when the output of the voltage detecting unit input through the inverter is at a HIGH state, while driving the second switch assisting unit when the direct output of the voltage detecting unit, with the inverter bypassed, is at a HIGH state.
In accordance with the present invention, power supplying to the main circuit is extremely fast carried out through the first switch assisting unit or the second switch assisting unit. This arrangement substantially reduces the need for discharging power stored in a capacitor, permitting the capacitance of the capacitor to be reduced and a cost reduction and a compact design to be implemented in the entire device.
Preferably, the power switching device of the present invention includes one-shot multi-vibrators for outputting one-shot pulses having a predetermined pulse length in response to the output of the voltage detecting unit. The outputs of the one-shot multi-vibrators respectively drive the first switch assisting unit and the second switch assisting unit.
In accordance with the present invention, power supplying through the first switch assisting unit or the second switch assisting unit is performed for only the predetermined pulse length set in the respective one-shot multi-vibra
Hochiki Kabushiki Kaisha
Lackenbach Siegel Marzullo Aronson & Greenspan P.C.
Tso Edward H.
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