DEVICE FOR LIMITING SURGE CURRENT

Abstract: A device for limiting surge current comprises a resistor (R₂) for limiting the surge, current which may arise in an incandescent lamp (L). The (R₂) is connected in series with the lamp (L) and a power source (D). A first controlled rectifier (SCR₂) has its main current path connected in parallel with the resistor (R₂). A second controlled rectifier (SCR₁) has its main current path connected to the gate of the first controlled rectifier. A delay circuit, for example, a RC-time constant circuit (R₅,C₂) has its output connected to the gate of the second controlled rectifier such that the power source supplies the lamp through the resistor (R₂) for a time period determined by the time constant of the delay circuit.

Description: The present invention relates to a device for limiting surge current, for example, for limiting the surge current which may arise in a lamp on starting. The resistance of a cold filament is generally one-tenth of that of the filament in its incandescent state. Energization of the filament at its rated voltage therefore causes a high inrush or surge current into the filament and this may damage the filament and/or the power source. It is disclosed in Japanese Laid-Open Patents Nos. 215,697/84, 215,696/84 and 230,298/84 that such surge current can be effectively limited using a device including a controlled rectifier whose main current path is connected in parallel with a surge current limiting resistor. The controlled rectifier is connected in series with the lamp and its conduction is controlled by way of a delay circuit such that the power source energizes the lamp by way of the resistor for a time period predetermined by the time constant of the delay circuit. The triggering voltage of a controlled rectifier varies very much with changes in the junction temperature, for example, from 0.9 to 0.6 volts in the temperature range -40°C to +40°C. Thus, the operation point of the controlled rectifier is dependent upon the ambient temperature, and, at a relatively high ambient temperature, destruction by overheating may shorten the life of the controlled rectifier. It is an object of the invention to seek to stably control the controlled rectifier without causing overheating even when there are wide variations in the ambient temperature. According to the present invention there is provided a device for limiting surge current in an incandescent lamp, which device comprises a first resistor connected in series with said incandescent lamp in a power supply circuit for limiting surge current in said lamp, a gate controlled rectifier circuit having a main current path connected in parallel with said resistor and a delay circuit connected to said power supply circuit for operating said controlled rectifier circuit after a time delay, characterised in that to stabilize the operation of said controlled rectifier circuit with variation in ambient temperature and avoid any gate overheating in said controlled circuit, said circuit comprises a first controlled rectifier providing said main current path across said first resistor and a second controlled rectifier having a gate connected to said delay circuit and an output connected to a gate of said first controlled rectifier, said second controlled rectifier being operable at a relatively small current with respect to said first controlled rectifier so as to thereby avoid unwanted heating of the gate of said first controlled rectifier. The present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: FIG. 1 shows the circuit of one embodiment of a surge current limiting device, FIG. 2 shows the circuit of a prior art current limiting device, and FIG. 3 shows the circuit of another embodiment of a device of the invention. Throughout the accompanying drawings, symbol R is used to represent a resistor; C is a capacitor; Z is a Zener diode, S is a switch; D is a diode; SCR is a controlled rectifier; and L is a lamp. In the circuit illustrated in FIG. 1, the output terminal of a rectifier bridge D is connected in series with a lamp L and a resistor R₂. The resistor R₂ acts to limit surge current and is connected in parallel with a main controlled rectifier SCR₂. A secondary controlled rectifier SCR₁ which operates with a relatively small current is connected by way of a resistor R₃ between the anode and the gate of the main controlled rectifier SCR₂. A resistor R₅ and a capacitor C₂ form a delay circuit having a time constant, and, on closing a power switch S₁, conduction of the secondary controlled rectifier SCR₁ is delayed in accordance with the time constant. The filament of lamp L is preheated during this delay period, and the resistance of the filament increases to its steady state level by the time when the resistor R₂ will be short-circuited. At the end of the delay period, the voltage on the capacitor C₂ triggers the secondary controlled rectifier SCR₁. The conduction current of the secondary controlled rectifier SCR₁ instantly energizes the main controlled rectifier SCR₂ which short-circuits the resistor R₂. The output of the rectifier bridge D is smoothed by a capacitor C₁, and supplied to the lamp L instantly on short-circuit of the resistor R₂. After conduction of the main controlled rectifier SCR₂, the voltage across the controlled rectifier energizes the delay circuit to keep the main and the secondary controlled rectifiers SCR₂ and SCR₁ conductive. Thus, the surge current which may arise on initially switching on the lamp can be effectively limited. In this embodiment, by setting the time constant to between 0.1 and 0.01 second, with a CR02AM (a controlled rectifier having a rated voltage of 0.8 volts, rated current of 0.3 milliamperes, and averaged driving power of 0.24 milliwatts,) as the main controlled rectifier SCR₂, the resistor R₅ of 100 kiloohms, the capacitor C₂ of 220 microfarads and the resistor R₇ of 10 kiloohms, the gate current of the secondary controlled rectifier SCR1 can be suppressed to 1 milliampere or less. Even if the gate current thermally varies up to 50%, the maximum gate current will be 2 milliamperes and this current will never overheat the gate of the main controlled rectifier SCR₂. In this case, the power consumption of the delay circuit is 0.17 watts (= 130 volts x 130 volts/100 kiloohms) which is very much lower than that consumed by a conventional device. The time constant of a few one-tenths of a second can be obtained by omitting the secondary controlled rectifier SCR₁ as shown in FIG. 2, and using resistors R₅ and R₆ (total resistance of 3 kiloohms) and capacitor C₃ (2,000 microfarads). However, this arrangement renders the operation of the controlled rectifier unstable when used in a cold environment, and thus the total resistance of the resistors R₅ and R₆ must be decreased to 2 kiloohms. For this reason, on closing the power switch, a current of 65 milliamperes (= 130 volts/2 kiloohms) instantly flows, and the power consumption in the delay circuit is 8.45 watts (= 65 milliamperes x 130 volts). This is 50 times higher than that of a device of the invention. By transmitting the output of the delay circuit to the main controlled rectifier by way of the secondary controlled rectifier, the conduction of the main controlled rectifier can be stably controlled without causing gate overheating even as the triggering voltage of the main controlled rectifier varies with the ambient temperature. Thus, with the invention, a device directed for use in a cold environment can be stably used at a relatively high temperature. In this embodiment, the resistor R₁ connected at the ac side of the rectifier bridge D is generally set to about 0.5 to 3 ohms to limit any arc discharge current which may arise on the outage of lamp L during dc illumination. This prevents the possibility of damage to circuit elements such as the diodes and the controlled rectifiers. FIG. 3 illustrates another embodiment of a device of the invention. In this embodiment, the positive output terminal of the rectifier bridge D is used as the source to charge the delay circuit. As in the preceding embodiment, this embodiment is arranged to cause a loss to the arc discharge current that may arise on the outage of lamp L during dc illumination by connecting low resistor R₁ in series with the ac side of rectifier bridge D in order to prevent a possible damage of the circuit elements such as diode and controlled rectifiers. As described above, in a device of the invention, surge current into the lamp can be limited without causing overheating of the controlled rectifier even when the triggering voltage of the controlled rectifier may vary with the ambient temperature. Additionally, in a device of the invention, when the ambient temperature increases and the gate trigger voltage of the main controlled rectifier decreases, the conduction current of the secondary controlled rectifier never increases to a level which overheats the gate of the main controlled rectifier to cause its destruction. Furthermore, since a device of the invention is simple but very effective in the limitation of surge current, the device can be used advantageously in illumination using an incandescent lamp or a power source therefor.

Claims: A device for limiting surge current in an incandescent lamp, which device comprises a first resistor connected in series with said incandescent lamp in a power supply circuit for limiting surge current in said lamp, a gate controlled rectifier circuit having a main current path connected in parallel with said resistor and a delay circuit connected to said power supply circuit for operating said controlled rectifier circuit after a time delay, characterised in that to stabilize the operation of said controlled rectifier circuit with variation in ambient temperature and avoid any gate overheating in said controlled circuit, said circuit comprises a first controlled rectifier providing said main current path across said first resistor and a second controlled rectifier having a gate connected to said delay circuit and an output connected to a gate of said first controlled rectifier, said second controlled rectifier being operable at a relatively small current with respect to said first controlled rectifier so as to thereby avoid unwanted heating of the gate of said first controlled rectifier. A device as claimed in claim 1, wherein said power source (D) comprises: rectifier means (D), the ac side of said rectifier means (D) being for connection to an ac source (AC) and the dc side of said rectifier means (D) being for connection to said incandescent lamp (L); and a second resistor means (R₁) for limiting discharge current of an arc which may arise on the outage of said incandescent lamp (L) when still in connection with said rectifier means (D), said second resistor means (R₁) being connected between said ac source (AC) and the ac side of said rectifier means (D). A device as claimed in either of claims 1 or 2, wherein the first and second controlled rectifiers are reverse-blocking triode thyristors. A device as claimed in any preceding claim, wherein said delay circuit is an RC-time constant circuit (R₅,C₂). A device as claimed in claim 4, wherein said RC-time constant circuit comprises a resistor and capacitor in series, said time constant circuit being connected in parallel with both the first and second controlled rectifiers, the gate of said second controlled rectifier being connected between said resistor and said capacitor of the time constant circuit. A device as claimed in any one of claims 2 to 4, wherein said rectifier means (D) is a full-wave rectifier having smoothing means (C₁) at its dc side.

Inventors: MASAKI, KAZUMI
Applicants: HAYASHIBARA, KEN
Application year: 1987
Main classification: H05B39
IPC codes: H05B39, H02H9