GB2291547A - Overload protection circuit for electronic apparatus - Google Patents

Abstract

Rectified mains voltage Us is supplied to a television receiver, video apparatus or the like 2 by way of a regulating transistor TF compares the supply voltage with a reference voltage Uref. If the supply voltage is low because of a short circuit fault, a signal is passed to a microprocessor 3 causing transistor TR to be turned off. A plurality of operating voltages may be monitored by a common circuit, and an indicator lamp may be included. <IMAGE>

Description

Overload protection circuit for a device of entertainment electronic The invention relates to an overload protection circuit according to the introductory part of claim 1. Such a circuit may be used in all types of electronic equipment using a pP as system controller and where a circuit protection is necessary, especially in a television receiver and a video recorder.

In practice strong safety legal requirements have to be met (i.e. VDE) in case of a short circuit on the switch power supply lines.

According to prior art, so-called fusible resistors are used to open the current path in case of an overcurrent during a short circuit. Said fusible resistors have a number of drawbacks. In some cases it is a very difficult problem to find a compromise value for said resistor. It is often a trade off between the safety and circuit requirements. Said resistor undesirably introduces an additional resistance within the circuit and may influence in some designs the efficiency of the voltage regulation. The resistor requires high costs and needs a special amount of power before releasing. Because said resistor only releases by warming up, there is an inertia in release in emergency case which can be a danger for special circuit elements.

Said resistor must be easily accessible for service. As a safety part it must be specially inserted and cannot touch any other part when bended.

It is an object of the invention to provide a simple circuit for overload protection which meets the safety legal requirements in case of a short circuit without having the drawbacks of fusible resistors.

The invention concerns an overload protection circuit for a device of entertainment electronic including a mains power supply unit for providing an operating voltage and a microprocessor (3) generating according to command of the device a switching voltage (Uv) switching off said power supply.

According to the invention, said operating voltage and a reference voltage are fed to a comparator having an output connected to a control input of said microprocessor for switching off said power supply in case of said operating voltage assumes a dangerous value defined by said reference voltage.

The circuit according to the invention has a number of advantages. As no fusible resistor is needed, most of the drawbacks described are no longer of concern. The circuit is rather simple and cheap, especially because circuit elements already available such as said microprocessor are used for overload or short circuit protection. As the circuit does not operate with warming up an element, it works nearly without inertia so that in case of overload emergency a very rapid switch-off of the operating voltage is achieved. There is no part damage in case of short circuit. The circuit can be extended to an unlimited number of switched power supply lines.

Preferably said power supply unit includes a power transistor within the main current longitudinal path having it's base connected to ground via a zener diode. Said power transistor has three different functions, that is stabilization of the operating voltage, switch-off of the operating voltage according to command of the device, preferably for complete switch-off or standby operation, and switch-off of the operating voltage in case of short circuit or any other emergency case.

Preferably the output of said microprocessor is connected via a control circuit to the base of said power transistor. In one embodiment, the output of said microprocessor is connected to the base of a first transistor having it's emitter connected to ground and it's collector connected to the base of a second transistor having it's emitter/collector-path connected between a source of dc-voltage and the base of said power transistor.

According to a further embodiment of the invention said operating voltage is fed to the base of a comparison transistor having its emitter connected to a tap of a series circuit of resistors connected to a source of dc-voltage.

According to a further embodiment means is provided for displaying at the cabinet of the device the emergency case wherein output of the comparator effects switching off said power supply unit.

According to a further embodiment a plurality of different operating voltages are connected each to a separate comparator, said comparators being connected to a common hard error stage having its output connected to the input of said microprocessor.

In this case, the circuit can supervise a higher number of operating voltages with different amplitude and polarity which may be generated by the same switch mode power supply.

In order that the invention may be more clearly understood, the invention will now be described with the accompanying drawing.

Within the drawing Fig. 1 shows the principle of the invention in block form, Fig. 2 a detailed circuit of the arrangement according to Fig. 1 and Fig. 3 an example for a further embodiment of the invention.

In Fig. 1, mains voltage UN is fed to a rectifier 1 providing operating voltage Us which is fed via switch S1 to a load 2. A microprocessor 3 is provided for controlling switch S1 according to the command of the device. If the device is switched on, microprocessor 3 provides switching voltage Uv with a value ON which closes switch S1. In case the device shall be switched off, e.g. for complete switch-off or for a standby operation of the device, microprocessor 3 provides switching voltage Uv with the value OFF which opens switch S1 so that Us is no more fed to load 2.

Additionally, operating voltage Us and a reference voltage Uref are fed to a comparator 4. In case operating voltage Us is higher than reference voltage Uref, that means there is no short circuit or overload or emergency, then comparator 4 provides voltage Ue with a value "Yes" for Uref c Us. This value does not effect operation of microprocessor 3. If comparator 4 detects "No" for Uref c Us, then voltage Ue which is fed to an input of microprocessor 3, controls said microprocessor in such a way that Uv assumes value "OFF" so that switch S1 is opened thereby effecting the desired overload protection.

In Fig. 2 which shows a more detailed circuit of Fig. 1. Power transistor TR represents switch S1 whereas R6 represents a resistor and a coil within the current path. Transistor TF represents comparator 4 whereas reference voltage Uref is derived from a tap of resistor divider R1/R2 connected to dc operating voltage Uever. Output of microprocessor 3 is connected via resistor R4, transistors T1, T2 and resistor RB to base of power transistor TR, the base being connected to ground via a zener diode D2. Diode D2 provides reference voltage for stabilizing function of TR. In the following the two modes "normal operation" and "emergency case" will be described.

Normal operation Voltage Us fed to base of TF is higher than reference voltage Uref. This is a case of no overload and no emergency. Transistor TF is cut off and no voltage is developed across R5. Microprocessor 3 is not influenced by voltage Ue so that the circuit works in the normal way feeding Us to load 2 the power transistor TR stabilizing value of Us at load 2.

Overload or short circuit or emergency case In this case Us fed to base of TF drops below Uref. In this case, transistor TF is turned on providing a voltage at R5 which is fed to input of microprocessor 3. Microprocessor 3 is controlled in such a way that voltage Uv provided by microprocessor 3 switches off power transistor TR via R4, T1, T2 and RB.

Thereby Us is no more fed to load 2 so that an effective overload protection in this emergency case is provided.

In Fig. 3 three different operating voltages Usl, Us2 and Us3 are fed to three separate comparators 4a, 4b, 4c each working in the same manner as comparator 4 in Fig. 1. Outputs of said comparators 4a - 4c are connected via equal resistors R to a base of transistor T3 providing at its output voltage Ue which is used like in Fig. 1, 2. In this way, three operating voltages are supervised by a common circuit and each of said voltages can in an OR-mode switch off voltage Us in case of short circuit, overload or emergency.

Additionally, voltage Ue is fed to an indicator 5 like a lamp or an LED indicating that due to a short circuit or overload or emergency voltage Us is switched off by switching power transistor TR.

Claims (7)

Claims

1. Overload protection circuit for a device of entertainment electronic including a mains power supply unit for providing an operating voltage and a microprocessor (3) generating according to command of the device a switching voltage (Uv) switching off said power supply, characterized in that said operating voltage (Us) and a reference voltage (Uref) are fed to a comparator (4) having an output connected to a control input of said microprocessor (3) for switching off said power supply in case of said operating voltage (Us) assumes a dangerous value defined by said reference voltage (Uref).

2. Circuit according to claim 1, characterized in that said power supply unit includes a power transistor (TR) within the main current longitudinal path having it's base connected to ground via a zener diode (D2).

3. Circuit according to claim 2, characterized in that the out put of said microprocessor (3) is connected to the base of said power transistor (TR).

4. Circuit according to claim 3, characterized in that the out put of said microprocessor (3) is connected to the base of a first transistor (T1) having it's emitter connected to ground and it's collector connected to the base of a second transis tor (T2) having it's emitter/collector-path connected between a source of dc-voltage and the base of said power transistor (TR).

5. Circuit according to claim 1, characterized in that said op erating voltage (Us) is fed to the base of a comparison tran sistor (TF) having it's base connected to the tap of a series circuit of resistors (R1, R2) connected to a source of dc voltage (Uever).

6. Circuit according to claim 1, characterized in that means (5) is provided for displaying at the cabinet of the device the emergency case wherein output of the comparator (4) effects switching off said power supply unit.

7. Circuit according to claim 1, characterized in that a plural ity of different operating voltages are connected each to a separate comparator (4a - 4c) said comparators being con nected to a common hard error stage (T3) having it's output connected to the input of said microprocessor (3).