GB2176956A - Voltage and current transient protection - Google Patents

Abstract

In order to protect an electrical device 40, and especially a remotely located device, a protection circuit 50 is disclosed. A zener diode D1 can be coupled in parallel with the device 40 to be protected, creating a constant voltage across the electrical device for a range of currents, or can be coupled to the base of a transistor T2, coupled in parallel with the electrical device 40. Transistor T2 provides switching apparatus for current values that could overload the electrical device. A surge in current or voltage across a resistor R1 located between the input and output terminals 20, 30 of the protection circuit 50. An increase in voltage across the resistor R1 results in increased conduction by a transistor T1 coupled across the input terminals 10, 20 and in increased conduction in a transistor T2. <IMAGE>

Description

SPECIFICATION Apparatus and method for voltage/current transient protection This invention relates generally to the protection of electronic devices from transient electrical conditions and, more particularly, to an apparatus and method forthe protection of telecommunication equipment that can be located, for example, at remote sites. The location at remote sites restricts the amount of possible unscheduled operator maintenance.

Electronic devices frequently experience voltage or currenttransient surges that can result in damageto the circuit elements. Telecommunication devices are particularly susceptible to overload conditions because ofthe amount of equipment exposed to ambient conditions and arefurthervulnerableto damage as a result of unattended operation which may be required for locating such devices or equipment in isolated areas. In the past, gas discharge devices have been used to protect electronic devices.

The gas discharge devices are characterized by a breakdown voltage that depends on transient risetime. These devices are subject two aging effects and, when these devices fail, the resu It is an open circuit leaving the electronic circuits unprotected.

Another device used in protection of electrical devices against transient overloads is the silicon controlled rectifier (SCR) device. However, these devices can be rest only by removing the current; a feature that limits their usefulness in remote locations.

Zener diodes have also been used as circuit protection devices. These devices operate by clamping vol tage transients ratherthan suppressing them. This overload protection technique can provide unacceptable energy dissipation.

More recently, dedicated solid state transient suppressors have been used to eliminate manyofthe disadvantages of devices described above. However, even the recently developed transient suppressor devices do not effectively handle high current transients when produced buy a low impedance source in which a necessary breakdown voltage is not reached.

It is therefore an object of the present invention to provide an improved apparatus and method forthe protection of electronic circuits from current and voltagetransients.

It is yet another object of the present invention to provide a device and method capable ofthe protection of electrical devices against transient overload conditions at remote locations.

It is still another object ofthe present invention to provide a circuit protection device and method that can be activated by both high voltage transients and by high current surges.

It is a more particular object of the present inven- tion to provide a circuit protection device and method that can be implemented with inexpensive components to provide adjustable voltage and current thresholds.

It is still another particular object ofthe present invention to provide a transient overload protection device and method where the device becomes inactive automaticallywhen the current or voltage overload condition is no longer present.

It isyet another particularobjectofthe present invention to provide a transient overload protection device and method that provides an electrical short circuit in parallel with the protected circuit upon failure ofthe components.

It is yet another particular object of the present invention to provide a circuit and method for protection against current and voltage overload conditions that is relativelyfreefrom aging effects.

The aforementioned and other objects are accomplished, according to the present invention, by a zen erdiodecoupledacrosstheoutputterminalsofthe protection device, by a resistor located between the input and outputterminals of the protection device, by a resistor-transistor network coupled between the input and the common terminal ofthe protection device and by a second transistor network coupled between the output terminal andthecommonter- minal of the protection device. The zener diode provides a bias for the firsttransistor network during normal operation.During transient conditions, the increase in voltage across the resistor causes an in- creased current to flow in the first transistor network and changes the second transitor network bias, causing an increased current to flow in the second transistor network. The increased current in the first and second transistor networks effectively shunt the cur rent that would otherwise haveto be stabilized bythe zener diode. By replacing the zener diode with anotherzener diode having a different operating voltage and replacing the resistor, the breakovervoltage of the protection circuit can be adjusted.

In accordance with one embodiment of the present invention, a device for protection of an electrical circuit against voltage and current overload transients is disclosed. The protection device includes constant voltage means coupled across the circuit for main taining a constantvoltagefor a range of currentflow- ing therethrough. The protection device further includes adjustable current means coupled in parallel with the constant voltage means. The adjustable current means conducts a current depending on a first signal. Signal means, coupled across input terminals of the device for providing thefirstsignal, are provided. The first signal being related to a voltage applied to the signal means.

In accordance with another embodiment ofthe present invention, the method of protecting an electrical circuit against excessive voltage and currenttransient conditions is disclosed. The protection method includes applying a current to a load impedance through a resistor and diverting a portion ofthecur- rent, a constant voltage device and an adjustable current device. The current th rough the adjustable current device is increased when the resistor current increases above a preestablished level.

According to still another embodiment of the present invention a device for the protection of an electrical circuit against excessive voltage and circuit transient conditions includes constant voltage means coupled across the electronic circuit and current means coupled in parallel with the constantvoltage means for conducting current determined by the ex cessive voltage or the excessive currenttransient overload condition.

According to yet another embodiment ofthe present invention, apparatus for protecting electronic circuits against excessive voltage and current overload conditions are disclosed. The protection apparatus includes first transistor means coupled between an inputterminal and a common terminal, second transistor means coupled between an outputterminal andthecommon terminal, constantvoltage means coupled from the output terminal to the second transistor means, and the resistive means coupled be tweentheinputterminal andtheoutputterminal,the firsttransistor means and the second transistor means conducting a portion of current through the protection apparatus when an excessive condition is detected.

According to yet another embodiment of the present invention, apparatus for protecting an electrical device from overload voltage and current conditions is disclosed that includes a firsttransistor coupled between an inputterminal and a common terminal.

The overload protection apparatus also includes a second transistor coupled between an outputterminal and the common terminal, a base ofthe second transistor coupled to a collector ofthe first transistor, a resistor coupled between the inputterminal and the outputterminal and a zener diode coupled between the outputterminal and a base of the second transistor. Thefirsttransistor and said second transistor increase conduction ofcurrent during the overload conditions.

According to still another embodiment ofthe present invention, the method of protecting an electrical circuit against overload voltage or current conditions includes diverting increasing amounts of current from a constant voltage device through a transistor as voltage or current applied to a protection circuit is increased beyond a predetermined value.

These and otherfeatures ofthe instant invention will be understood byreadingthefollowingdescrip- tion along with the drawings.

Figure lisa schematic circuit diagram of the circuit ofthis invention that is used for protection against transient current and voltage overloads.

Figure 2 is a block diagram ofthe circuit of Figure 1 that is used for protection against transient current and voltage overloads.

Referring to Figure 1 ,terminal 20 is an inputtermin al,terminal 30 is an outputterminal and terminal 10 is a common or ground terminal ofthe protection device or circuit 50 of this invention which is contained within the broken lines of the box shown in Figure 1.

Electronic or electrical device 40 that is to be protected is coupled between the output terminal 30 and thegroundterminal 10or60. Powerfortheelectrical device 40 is received from the inputtermina 1 20 and the common terminal 10. Terminal 20 is coupled to a firstterminal or resistor Era, and to an emitterterminal of PNP transistorT1. A base terminal of the transistor T, is coupled to a firstterminal of resistor R2, whilethe collector terminal ofthe transistor Ta is coupled to a firstterminal of resistor R3.A second terminal ofthe resistor R3 is coupled to a base terminal of NPN tran sistorT2 and coupled through resistor R4to the ground terminal 10. The outputterminal 30 is coupled to a second terminal ofthe resistor R1,to a second terminal of resistor R2, to a collector terminal ofthe transistorT2 and to a cathode terminal of zener diode D1. The anode terminal of zener diode D1 is coupled to the base of transistor T2 in the preferred embodiment, but can alternatively be coupled to terminal 60 as shown by the broken line. The emitterterminal ofthe transistor T2 and the anodeterminal of the zener diodeD1 are coupled to the common terminal 10.

Referring to Figure 2, a block diagram of the circuit according to the instant invention is shown. Power input terminal 20 is coupled to current by-pass circuit 21 and to biasing circuit 22. Terminal 10 is coupled to the current by-pass circuit 21, to the biasing circuit 22, and to terminal 60. Reference voltage and current by-pass circuit 23 is coupled to current by-pass circuit 21 in this preferred embodiment, but alternatively can becoupledtoterminal l0asshownbythebroken line. The output terminal 30 is coupled to the biasing circuit 22 and to the reference voltage and current by-pass circuit 23. The biasing circuit 22 is coupled to the current by-pass circuit 21 and to the reference voltage and current by-pass circuit 23.A load impend- ance 40 can be coupled between the output terminal 30 andtheterminal 60.

Referring to Figure 1 ,the protection circuit 50 is interposed between a power source and the electrical device 40 that is to be protected. Under normal operating conditions, that is when the voltage and current applied to the load impedance 40 are below the threshold values, the protection device 50 remains in the high impedance or OFF state condition. The threshold values for current and voltage are deter mined bythevalues of resistor R and diode D1. The resistor R1 provides a voltage drop during a current overload or currenttransient. When the voltage across the resistor R1 exceeds a pre-determined value, the protection device is switched to an ON state and operates in a mannersimilarto a short-circuited impedance.Therefore, the currente delivered to the load impedance is limited. As described above, the threshold voltage is established by the zener diode D1. The breakdown voltage ofthe zener diode D, is lower than the breakdown voltage ofthe collectorbase junction of transistor and T2 and lowerthan the maximum voltage supplied to the electrical device 40. The threshold voltage and current values can be adjusted by replacing D1 with anotherzenerdiode having a different breakdown voltage and/or by replacing resistor R1 with a resistor having a different value. When a large voltage transient occurs, thereby causing an overload condition, the diode D1 breaks down and currentflows into the base of T2.Therefore, T2 is switched on the causes T2to conduct. When the product of current gains of T1 and T2 equals one, the protection device is operative, i.e. is in an ON condition.

Referring to Figure 2, a block diagram of the protection apparatus or circuit of Figure 1 is shown. The biasing circuit includes the resistor R1 and the circuit path to the base of the transistorT2. The current by-pass circuit 21 and the reference voltage and current by-pass circuit 23 each provide current paths that reduce the amount of current delivered during a tran sient condition. Atransient condition is attenuated through the zener diode D1 for minor perturbations.

For larger overload conditions, the circuit 21 and the circuit 22 provide attenuation paths for excess current. The resistor R1 also serves to provide a voltage transient to the protection device when an overload condition is the result of a low-impedance current source. The voltage transient provides biasing condi tions for the transistors.

The above description is included to illustrate the operation ofthe preferred embodiment and is not meantto limitthe scope of the invention. The scope of the invention isto be limited only bythefollowing claims. From the above discussion, many variations will be apparent to one skilled in the artthatwouldyet be encompassed by the spirit and scope of the invention.

Claims (22)

1. A device for protection of an electrical circuit against voltage and current overload transients comprising: constantvoltage means coupled across said circuit for maintaining a constantvoltagefor a range current applied thereto; adjustable current means coupled in parallel with said constant voltage means, said adjustable current means conducting a current depending on a first signal; and signal means coupled across input terminals of said device for providing said first signal, said first signal being related to a voltage applied to said signal means.

2. The overload protection device of claim 1 wherein said protection device is arranged to have any of a plurality of selected constantvoltage means coupled thereto, each constant voltage means providing a different voltage across said electrical circuit.

3. The protection device of claim 2 wherein said constant voltage means includes a zener diode, said adjustable current means including a transistor coupled in parallel with said electrical circuit, said zener diode coupled two a base of said transistor.

4. The protection device in claim 3, further including a resistor coupled between said adjustable current means and said signal means, said resistor providing an increase in voltage to said signal means when current to said device is increased.

5. The protection device of claim 4wherein said firstsignal means includes an NPNtransistorandsaid adjustable current means includes a PNP transistor.

6. The method of protecting an electrical circuit against excessive voltage and currenttransient overload conditions comprising the steps of: conducting currentthrough a resistor, a constant voltage device, an adjustable current device and said electrical circuit; and increasing current through said adjustable current device when said resistor current increases above a preestablished level.

7. The method of protecting an electrical circuit against excessive voltage and current transient overload conditions of claim 6furthercomprising the step of replacing said constant voltage device with a con stantvoltage device of different characteristics to provide a constant predetermined voltage across said circuit.

8. A device for protection of an electrical circuit against excessive voltage and circu it transient overload conditions comprising: constant voltage means coupled across said electrical circuit; and current means coupled in parallel with said con stantvoltage means for conducting a current deter- mined by said excessive voltage or said excessive current transient overload condition.

9. The device for protection of electronic circuits of claim 8wherein said current means includes afirst transistor means, a second transistor means and a resistor, said first transistor means causing a signal to be applied to said second transistor means when said excessive overload transient is detected by current through said resistor, said signal causing said second transistor means to divert current from said constant voltage source and said electrical circuit.

10. The protection device of claim 9 wherein said constant voltage means can be replaced by a different constant voltage means having a different constant voltage value.

11. Apparatus for protecting electrical circuits against excessive voltage and current overload conditions comprising: firsttransistor means coupled between an input terminal and a common terminal; second transistor means coupled between an output terminal and said common terminal; zener diode means coupled between said output terminal and a base element of said second transistor; and resistive means coupled between said inputterminal and said output terminal, said first transistor means causing said second transistor means to conduct a portion of current through said apparatus when an excessive overload condition is detected.

12. The protection apparatus of claim 11 wherein said zener diode means can be replaced by another zener diode means having a different breakdown voltagevalue.

13. The protection apparatusofclaim 12wherein said firsttransistor is a PNptransistorandsaid second transistor is a NPN transistor.

14. The protection apparatus of claim 13wherein said overload conditions cause increased current in said firsttransistor, said increased first transistor current causing increased second transistor current.

15. Apparatus for protecting an electrical device from overload voltage and current conditions comprising: a firsttransistor coupled between an input terminal and a common terminal; a second transistor coupled between an outputterminal and said common terminal, a base of said second transistor coupled to a collector of said first transistor; a resistor coupled between said inputterminal and said output terminal; and a zener diode coupled between said outputtermin- al and said common terminal; said firsttransistor and sadi second transistor increasing conduction of current during any of said overload conditions.

16. The overload protection apparatus of claim 15 wherein said zener diode can be replaced by a zener diode having a different constant voltage value.

17. The overload protection apparatus of claim 16 wherein said apparatus automatically returns to its original operation when said overload conditions are removed.

18. The overload protection apparatus of claim 17 wherein said apparatus provides a short circuit between said input and said common terminal upon failure of said overload protection apparatus.

19. The method of protecting an electrical circuit against overload voltage or current conditions comprising the step of: diverting increasing amounts of currentfrom a constantvoltage device through at least one transistor as voltage or current applied to a protection circuit is increased beyond a predetermined value.

20. The method of protecting an electrical circuit of claim 19 further comprising the step of replacing said constant voltage device with a different constant voltage device for providing a different voltage across said electrical circuit.

21. The method of protecting an electrical circuit of claim 19further comprising the step of: sensing an increase in voltage at an input terminal of said protection circuit and increasing current conduction in said transistor in response to said voltage increase.

22. The method of protecting an electrical device from excessive line current and voltages comprising the steps of: providing a resistor between said line and said electrical device; coupling a firsttransistor in parallel with said electrical device; coupling a zener diode, said electrical device and a base element of said first transistor; and biasing said base element by a second transistor coupled between said line and a common terminal.