Technical Field

The present invention relates to a shortcircuit limit¬ ing protector, and more specifically to an electronic fuse which has minimal power losses under normal operating conditions and which will be triggered rapidly and reliably at a predetermined current level.

Background Art

There is found in different electronic circuits and units for supplying working voltages to such circuits a need for safety devices in the form of fuses which will trigger when the current in the circuit protected by the fuse becomes excessively high, for instance due to a short-circuit.

The most common type of fuse is the thermal fuse which normally consists of a glass tube which contains a thin filament or wire which melts or burns when a given current passes through the filament, because the product of filament resistance and current generates heat which, when the current is excessive, melts the filament.

A fuse of this kind will function satisfactorily in the event of a complete short-circuit, whereupon the fuse is triggered relatively quickly. In the event of a short-circuit, the fuse permits a high short-circuit current to pass through which is limited solely by the impedance of the supply source. Such high short- circuiting currents are liable to damage conductors, electric contacts and other electronic devices, and may disturb parallel-supplied electronic devices. On the other hand, if the current is high and lies close to or immediately above the rated value of the fuse, it may take considerable time for the fuse to trigger,

which in some cases can cause considerable problems due to the overloading of other circuit components. Furthermore, a fuse of this kind can be made either slow or quick. Furthermore, when subjected to loads that are close to its rated value, a fuse of this kind may undergo changes caused by aging, such that after a longer installation period the fuse may trigger with¬ out the original rated value of the fuse being exceed¬ ed.

Up-to-date voltage supply units often have some form of current limitation which permits maximum power to be taken-out by a circuit. This is often achieved by allowing the current to obtain a given maximum value and then reducing the voltage so that the current will never exceed the current limiting value. When the voltage is reduced, this may render the function of many circuits unsafe because a voltage level becomes critical, which may jeopardize the application as a whole.

In order to overcome this drawback, it is necessary to monitor voltage in addition to monitoring current, so that the supply will be broken and a fault indicated when the voltage falls beneath a predetermined value. This results in a particularly complex safety system, which may be more comprehensive than the detail to be monitored or supervised. This is a problem, for in¬ stance, in equipment which serves many users and where there is a natural desire that a fault that occurs in respect of one user will not interfere with the other users. An example of this application is found in a telephone network. For instance, if a subscriber suf¬ fers a fault, the fault must in no way influence any of the remaining, possibly hundreds of thousands of subscribers. It is important in applications of this nature that the current can be monitored in the sim¬ plest but most effective manner, so as to keep instal¬ lation costs down to a reasonable level. Many similar

applications.are found within the fields of electron¬ ics and data technology where an overload protector is desirable and which is rapid and precise without the protector generating radio disturbances and being too complicated. Another desire is that a protector of this kind need not necessarily be replaced with a new component, but can be readily re-set, either manually or automatically, after eliminating the source of the fault.

US-A 4,531,083 and US-A 4,531,084 teach a current regulating circuit for a direct-current mains unit which is intended to charge rechargeable batteries without the use of transformers, capacitors, or coils. The emitters of a first and a second transistor are connected via a fifth current limiting PTC-resistor which is able to function as a fuse. Instead of using a plurality of transistors in a Darlington circuit, there can also be used MOS field effect transistors. The PTC-resistor is intended to operate at a nominal temperature of 125°C during a battery charging pro¬ cess, wherein the charging current will decrease by 0.45% with each degree of increased working tempera¬ ture for this resistor. The resistor is also prefera- bly obtained by metallizing with a material which will enable the resistor to function as a fuse.

DE-B 2,533,182 teaches a circuit arrangement for indicating a triggered or blown fuse in a current supply arrangement, wherein an optoswitch is used to indicate to a display device that its fuse has been triggered.

There is thus a need to provide a current limiting device which is both rapid and precise, and in which the power generated under normal operating conditions will be very small.

Disclosure of the Invention

Accordingly, it is an object of the present invention to provide a short-circuit limiting protector which will fulfil the following specifications: a) will be triggered quickly and immediately at a chosen current value; b) will have minimum power losses at normal current values; c) will have a non-linear characteristic at trigger¬ ing currents, so that triggering of the device will be accelerated; d) that the short-circuiting current will be limited to a desired level, irrespective of the internal impedance of the supply source; e) that the device will not generate radio frequency disturbances in operation; and f) when breaking erroneous currents, the device will limit the induced overvoltage to a predetermined value.

In accordance with the present invention, there is provided a device according to the above specifica¬ tions which when triggered can be easily reset, after establishing and eliminating the cause of the current surge.

In accordance with the present invention, there is also provided a short-circuit limiting device which utilizes a combined electronic and thermal feedback principle.

The present invention also provides a device which when nominal or rated current is reached exhibits a carefully established delay and is quickly triggered, or "blown", immediately after this delay has lapsed. Thus, in the event of a fault in the load, the errone¬ ous current is limited to a much lower value than is a conventional fuse.

In accordance with the present invention, there is also provided a device according to the above specifi¬ cations which can be reset automatically or manually and which can also be set to an inactive state without needing to remove the device.

Brief Description of the Drawings

The present invention will now be described with reference to a number of exemplifying embodiments thereof and also with reference to the accompanying drawings, in which

Figure 1 is a circuit diagram of a basic illustrative embodiment of an inventive short-circuit limiting device;

Figure 2 is a circuit diagram of a second embodiment of an inventive short-circuit limiting de¬ vice;

Figure 3 is a circuit diagram of a third embodiment of an inventive short-circuit limiting de¬ vice;

Figure 4 is a circuit diagram of a fourth embodiment of an inventive short-circuit limiting de¬ vice; and

Figure 5 is a circuit diagram of a fifth embodiment of an inventive short-circuit limiting de¬ vice.

Description of Exemplifying Embodiments

Figure 1 is a circuit diagram which illustrates by way of example a circuit which utilizes an electronic and thermal feedback principle in accordance with the present invention. The circuit diagram includes a

number of resistors R2, R3, R4, R7, R8, R9 and a non¬ linear temperature-dependent resistor R5. The circuit also includes a field effect transistor Ml in series with the non-linear temperature-dependent resistor R5, and a silicon diode Dl and two zener diodes Zl, Z2, and a bipolar NPN-transistor Ql and two capacitors Cl and C2. According to the present invention, the main functional part of the illustrated circuit is com¬ prised generally of the components Ml, R5, Ql, R4, R3 and R8. In the case of the illustrative embodiment, the resistor R5 is a PTC-resistor of the type RUE 800 produced by Raychen Pontaisc SA, France. The field effect transistor Ml is a power MOSFET, for instance of the type MTW 45N10E produced by Motorola, and the transistor Ql is, for instance, a BC 847B transistor produced by Motorola, for instance. Remaining resis¬ tors, capacitors and diodes are comprised of generally available standard components. The zener rated voltage of the zener diode Zl is 12 volts, while the zener diode Z2 has a rated voltage of 90 volts.

The reference SW1 identifies a switch which has three positions, OFF, ON and RESET. When the switch is in the OFF position, sufficient potential is applied via the resistor R2 for the transistor Ql to conduct, therewith blocking the field effect transistor Ml, which consequently allows no current to pass through. In other words, the load L is disconnected.

If the switch SW1 is briefly switched to the RESET position, Ql is activated and the voltage on the gate of Ml will rapidly increase to the level determined by the zener diode Zl and the voltage divider consisting of resistors R4 and R7, where R4 is connected to the positive terminal of the voltage source that supplies the load L. The field effect transistor Ml will then conduct current and the load L is thus activated. The gate on Ml thus obtains a potential at which it is fully conducting, wherewith the voltage drop across Ml

in the case of moderate currents will be practically negligible (one or some tenths of a volt). When the switch SW1 is then left in its neutral position, i.e. its ON position, the circuit will operate in its operational mode.

The non-linear temperature-dependent resistor R5 is connected in series with the thus created circuit, at the collector connection of the transistor Ml. The resistance of R5 is small at normal working tempera¬ tures, in the same order of magnitude as the input or ON-resistance of Ml. The two components Ml and R5 of the preferred embodiment are mounted so that when operating at normal current, thermal equilibrium is obtained between the dissipated power generated and the heating power cooled-off from Ml and R5. These components are mounted in good thermal contact with the component carrying board (fiberglass board, ceram¬ ic substrate or a metal board, provided that the components are insulated from the board). The non¬ linear temperature-dependent resistor is also conve¬ niently mounted so as to be also influenced by the thermal energy developed by the transistor Ml. In the case of an embodiment which includes discrete, encap- sulated components, the transistor Ml is appropriately provided with a cooling fin. The circuit created in series with the load L and the positive and negative terminal connection of the voltage source therewith operates in its operational mode when the switch SW1 is in its ON position, so as to monitor the state of the circuit in readiness to immediately break the circuit of the load L at a predetermined overcurrent.

The following takes place when the current through the load L exceeds the chosen overcurrent release value. The temperature of both Ml and R5 will increase mark¬ edly, wherein the increasing temperature of R5 will cause the its resistance to increase rapidly, in an accelerating fashion (non-linear dependency). The

rapidly increasing resistance in R5, and to some extent also in Ml, together with the abnormal current will result in an increasing voltage drop over R5 in series with Ml. As a result of the action of a first voltage divider consisting of the load L and the combination of Ml and R5, the voltage across a second voltage divider consisting of the resistors R3 and R8 supplying the transistor Ql increases. When the poten¬ tial at the point between the resistors R3 and R8 which supply the base electrode on the transistor Ql exceeds a given threshold value, Ql becomes conduc¬ ting, which in a typical case occurs in a very short time, in the order of microseconds, and therewith short-circuits the zener diode SI and the transistor Ml switches to a non-conducting state and the current through the load L is broken.

After breaking the current through the load L, the whole of the supply voltage lies over the second voltage divider consisting of resistors R3 and R8, and Ql is consequently supplied with sufficient base current to be fully conducting and therewith holds Ml in a blocked, non-conducting state.

When a short-circuit occurs in the load L, the short- circuiting current will increase rapidly and therewith also the voltage drop over Ml and R5. A release or turn-off sequence is initiated when the threshold level of the base-emitter voltage for Ql is exceeded. A desired time delay between the time of the non- system current (the occurrence of the short-circuit) and the release of Ml is obtained by suitable selec¬ tion of the value of the capacitor Cl. The peak cur¬ rent through Ml is determined by the value of the capacitor Cl and by the inductance of the load L. If Cl is excluded, the peak current is determined partly by the base-emitter voltage for Ql and partly by the quotient R3/R8.

If the transistor Ml is unable to break the short- circuiting current, R5 will limit the maximum current to a moderate value and then forces the current down close to zero, by increasing its resistance very rapidly to several tens of thousands of ohms.

SI is a typical, standard fuse which has a relatively high value and justified by the requirements placed on fuses by the authorities (the authorities do not accept the use of solely an electronic circuit as a short-circuiting protector), although the fuse SI will never be tripped or triggered in reality.

When breaking an inductive load, the transient voltage is limited by the zener diode S2 in coaction with the field effect transistor Ml. The release time is deter¬ mined by the zener voltage and the amount of inductive energy that must be taken-up by Ml. The diode Dl prevents current flowing through Z2 when Ml is fully conducting. R9 and C2 are constructed to attenuate oscillations when Ml is tripped (so-called snubber circuit) .

Figure 2 illustrates another embodiment of the inven- tion. In the Figure 2 embodiment, the circuit illus¬ trated in Figure 1 has been supplemented with a fur¬ ther bipolar transistor Q2, for instance of the type BC 847B with an associated third voltage divider consisting of the resistors RIO and Rll, a capacitor C3 and a silicon diode D2.

The basic function of the circuit illustrated in Figure 2 is the same as that of the circuit illustrat¬ ed in Figure 1, with the exception that the Figure 2 circuit is reset automatically when a given period of time has lapsed after release. The transistor Q2 is connected in parallel with the resistor R8, between the base and emitter of the transistor Ql. The base electrode on Q2 is supplied through the resistors RIO

and Rll of the third voltage divider. When the tran¬ sistor Ml breaks the current through the load L and the voltage over the second voltage divider R3 and R8 is equal to the supply voltage, this voltage is also obtained over the third voltage divider. Charging of the capacitor C3 then commences via the resistor RIO, to a voltage which corresponds to the dividing ratio of RIO and Rll. When the voltage on the base electrode exceeds the threshold voltage of Q2, after some period of time, Q2 begins to conduct and the transistor Q2 shunts-out the voltage level via the base electrode of the transistor Ql, which therewith ceases to be con¬ ducting. When the transistor Ql becomes non-conduc¬ ting, control voltage again appears on the gate of the field effect transistor Ml, which then again switches to a fully conducting state and current can again flow through the load L. When Ml again becomes conducting, the voltage over the second and the third voltage dividers disappears. The diode D2 then causes the capacitor C3 to be quickly discharged, whereupon the device as a whole is again in its operational mode.

If the fault (the short-circuit) remains, the device again takes a blocking state after a given time lapse, the duration of which depends, among other things, on the time constants of the combinations R9 and C2, and R3, R8 and Cl. The device will then break and make the current at a given repetition frequency which will depend on the time constants chosen in addition to the aforesaid combination of RIO, Rll and C3.

The circuit illustrated in Figure 2 includes two conventional light-emitting diodes D3 and D4 connected in series with a respective current limiting resistor Rl and R6. When supply voltage exists and the transis¬ tor Ml is conducting, the diode D3 will shine to indicate an ON-state. If the supply voltage remains but no current flows through the load L and sufficient voltage lies over the second (and third) voltage

divider, the diode D4 will shine (radiate) indicating a triggering of the circuit. When the device in the Figure 2 embodiment repeatedly breaks and makes the current circuit, the diodes D3 and D4 will flash alternately and therewith indicate this state. When the repetition frequency is sufficiently high, the eye of a viewer will experience the two diodes as radiat¬ ing a marking which indicates a continued fault in the load L.

Figure 3 illustrates a third embodiment of the present invention, in which the circuit shown in Figure 1 is supplemented with the diodes D3 and D4 and also an optoswitch 0P1, which in the case of the illustrative embodiment is of the type CNY 74-4 produced by Tele- funken. In this case, the light-emitting diode D3 shows when voltage is applied to the circuit and Ml is conducting, while the light-emitting diode D4 shows that the circuit is triggered. The optoswitch 0P1 lies in series with D4, and at the same time as D4 receives voltage and is ignited, the optoswitch 0P1 receives the same current and is used to provide a remote indication that the circuit has been tripped.

The circuit illustrated in Figure 4 is an extension of the circuit illustrated in Figure 3, where the circuit arrangement has been prepared for remote control with the aid of two further optoswitches 0P2 and 0P3 of a type corresponding to 0P1. In this case, the emitter electrode of a phototransistor in the optoswitch 0P3 is connected by means of a further resistor R12 to the collector electrode of a phototransistor in the opto¬ switch 0P2 and also to the base connection on the transistor Ql. The current monitoring circuit can therewith be remote controlled with the aid of the two optoswitches, in addition to the direct control af¬ forded by the switch SW1. When the optoswitch 0P3 receives supply voltage, so that its incorporated light-emitting diode will shine or radiate, the incor-

porated phototransistor opens and current is led-in on the base of the of the transistor Ql via the addition¬ al resistor R12, and said base will then conduct current which, in turn, switches the field effect transistor to a non-conducting state, in accordance with the aforegoing. Thus, the same function is ob¬ tained as that when the switch SI is switched to its OFF position. This enables the load L to be switched on and off by remote control.

Similarly, when the optoswitch 0P2 receives a control voltage, its phototransistor will short-circuit the base electrode of the transistor Ql to the negative supply connection in a corresponding manner, which corresponds to the function when the manual switch is switched to the RESET position. In other words, the circuit can be reset by remote control.

Finally, Figure 5 illustrates an exemplifying embodi- ment in which the circuit according to Figure 3 is provided with an integrated circuit ICl instead of the optoswitches 0P2 and 0P3. In the case of this embodi¬ ment, the integrated circuit is comprised of a 5-volt logic circuit of the TTL type or the CMOS type, pref- erably with an output level having three states. A 74LS123 type IC-circuit is an example of a circuit which can be used conveniently in the illustrative embodiment. In this case, the output of the logic circuit is connected either to minus supply in the case of the RESET function, and to positive supply in the case of the OFF function, wherein the output will normally take its floating state when the circuit is in its ON function and therewith actively monitor the current through the load L. The integrated circuit ICl is controlled in accordance with prior art techniques and enables the circuit to be readily applied in digital applications.