Overload protection circuit

The overload protection circuit 12 comprises a resistor bridge 20 with a low impedance sensing resistor r and high impedance resistors R1 to R4 which generates voltages Va and Vb at the inputs 32 and 34 of a comparator 22. Voltage Vb depend upon the value of the load current so that the comparator provides at its output 36 a control signal for opening the switching device 24 comprising a field effect transistor when the load current exceeds a maximum value. Resistor R5 reinforces the effect of the control signal. This circuit can be used in a system which comprises a central power supply unit 8 which powers a plurality of devices such as 4. The status of the circuit 12 is reported to the system 8 though line 14 and circuit 12 comprises a control circuit 26 which is responsive to set and reset control signals on lines 14 and 18 from the unit 8 to close or open the switch 26.

DESCRIPTION OF THE INVENTION 
The subject invention relates to an overload protection circuit and more 
particularly to such a circuit which can be used for protecting devices 
powered from a unique remote unit. 
BACKGROUND ART 
Electronic circuits for protecting devices against overloads due to short 
circuits or any type of defaults already exist on the shelves, but none of 
them present a negligible resistance when in set status (closed) as a 
result; they can not be used to protect devices powered from a unique 
remote unit because the resistance induces power losses which are 
detrimental to the operation of the remote devices. 
Mechanical overload protection circuits exist and might be appropriate in 
such an environment but these circuits are cumbersome and cannot be 
integrated within a VLSI chip. 
In addition there is a requirement at the remote powering unit to be aware 
of the status of the overload protection circuit in any device in order to 
take appropriate actions depending upon the reported status. 
SUMMARY OF THE INVENTION 
Consequently, an object of the invention is to provide an overload 
protection circuit which presents a negligible resistance. 
Another object of the invention is to provide such an overload protection 
circuit which is simple and can be easily integrated in a chip. 
Another object of the invention is to provide such an overload protection 
circuit which is able to report its status to a remote unit and to be set 
or reset under control of this unit. 
The overload protection circuit according to the present invention is used 
for protecting a device powered from a power supply unit which provides a 
supply voltage and a load current to the device. It comprises: 
an overload detecting circuit which includes: 
a resistor bridge having two legs connected in parallel between the supply 
voltage and a reference voltage (ground) and a first and second output 
terminals, and generating a fixed voltage on the first output terminal and 
a variable voltage which depends upon the load current value on the second 
output terminal, 
a comparator having first and second inputs connected to the first and 
second output terminals of the resistor bridge, respectively and an 
output, said comparator providing on its output a control signal which 
indicates whether an overload condition exists or not, 
a switch having an input receiving the load current, an output connected to 
the device and a control input responsive to the control signal for 
closing or opening said switch, whereby the load current is provided to 
the device or not. In a preferred embodiment of the invention the switch 
comprises a field effect transistor the first electrode of which receives 
the load current, the second electrode of which is connected to the device 
and the gate electrode of which is connected to the output of the 
comparator. 
The type of the field effect transistor depends upon the polarity of the 
supply voltage, if it is positive the field effect transistor is of the P 
enhancement type and if it is negative the field effect transistor is of 
the N enhancement type. 
According to another characteristic of the present invention a feedback 
resistor is connected between the output of the switch and the second 
output terminal of the resistor bridge to reinforce the effect of the 
control signal on the closing and opening of the switch. 
According to a further characteristic of the present invention the output 
signal at the output of the comparator is indicative of the status of the 
switch and is reported to the power supply unit. 
In addition, the protection circuit can be used for protecting a device 
which is powered from a power supply unit which comprises a first and a 
second control lines and provides on these lines switching control signals 
for closing and opening the switch, respectively. The overload protection 
circuit comprises a switching control circuit connected to the first and 
second control lines and responsive to the switching control signals on 
these lines to provide control voltages to the first and second inputs of 
the comparator, whereby the comparator generates closing or opening 
control signals on the control input of the switch. The overload 
protection circuit according to the subject invention can be implemented 
in a system comprising a central power supply unit which powers a 
plurality of devices, wherein each device is protected against overloads 
by an overload protection circuit so that an overload in any one of the 
devices does not impair the operation of the other devices.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 represents a system incorporating overload protection circuits 
according to the subject invention. This system is only shown for the, 
sake of illustration and it will be obvious to the person skilled in the 
art to implement the overload protection circuit in any other system from 
the description of the overload protection circuit. 
The system shown in FIG. 1 comprises a central unit 2, which can be any 
type of information processing unit or any other central control unit 
attached to remote devices 4-1 to 4-n through busses 6-1 to 6-n 
respectively. Busses 6-1 to 6-n carry the control and information signals 
which are used for the operation of the remote devices, these signals are 
specific to the system and do not interact with the operation of the 
overload protection circuit of the present invention. 
The remote devices 4-1 to 4-n do not have their own power supply and are 
powered from the central power supply unit 8. 
Each remote devices 4-1 to 6-n is powered from the power supply unit 8 
through lines 10-1 to 10-n respectively. An overload protection circuit 
12-1 to 12-n respectively, is associated to each remote device 4-1 to 4-n, 
to protect the device against overcurrents due to short circuits or any 
other failures. For example, the overload protection circuit 12-1 for 
device 4-1 is connected to line 10-1 and prevents the remote devices 4-1 
from being powered through line 11-1 if it detects an overcurrent due to a 
short circuit and it reports its status to the power supply system through 
line 14-1. The overload protection circuits can be set or reset by the 
power supply unit through lines 16-1 and 18-1. 
The powering lines 11-1 to 11-n from circuits 12-1 to 12-n to remote 
devices 4-1 to 4-n are partly dotted in FIG. 1 to symbolize the fact that 
they can be quite long, more than 100 meters. 
In another implementation, the overload protection circuits 12-1 to 12-n 
can be close to the remote devices 4-1 to 4-n and lines 10, 14, 16, 18 can 
be quite long, and the provision of an overload protection circuit 
associated to each device allows the operation of the system not to be 
disturbed by an overload due to a short circuit in any device, since when 
a short circuit occurs in one device, the device is isolated from the 
other ones, from a power standpoint. 
The overload protection circuit 12 for any device 4, according to the 
subject invention is shown in FIG. 2. 
Its major components are a resistor bridge 20, a comparator 22, a switch 
24, a control circuit 26 and a feedback path 28. 
The power supply system 8 generates at node 30 in the resistor bridge a 
voltage V. In the implementation shown in FIG. 2, the voltage V is 
positive. 
One leg of the resistor bridge 20 comprises a low value sensing resistor r 
in series with resistors R1 and R2 and the other leg comprises resistor R3 
and R4. Resistor r, R1 and R2 in the first leg are serially mounted 
between node 40 and node 30 which is connected to the ground. 
Resistors R3 and R4 in the second leg are serially mounted between nodes 30 
and 40. 
The common point A of resistors R3 and R4 is connected to the plus input 32 
of comparator 22 and the common point B of resistors R1 and R2 is 
connected to the minus input 34 of comparator 22. 
The signal which is generated at the output of comparator 22 as will be 
described later on, is provided to the switch 24 in order to control its 
status, thus the status of the overload protection circuit is indicated by 
the signal at the output of comparator 22 which is connected to STATUS 
line 14. 
In a preferred embodiment of the invention, the switch comprises a field 
effect transistor FET which presents a very low resistance when closed. In 
the circuit of FIG. 2, the transistor FET is of the P enhancement type. 
The source connected to the common part 38 between resistor r and resistor 
R4, the drain is connected to node 42 which is connected to the remote 
device 4. When the switch is closed a current I (load) is provided by 
overload protection circuit to the remote device 4 which is thus powered. 
The feedback path comprises resistor R5 which is connected between node 42 
and node B. 
The control circuit 26 comprises two switches 44 and 46 which are operated 
by signals provided by the power supply unit 8 through, lines 14 and 18 to 
respectively set and reset the overload protection circuit, as will be 
described later on. 
The circuit shown in FIG. 2 operates as follows. 
Resistor r is a sensing resistor which has a low value. The supply current 
I (load) depends upon the supply voltage +V and the load in the remote 
device. 
The resistor bridge generates a fixed voltage which is derived from +V at 
node A and a variable voltage which is derived from +V-rIs, where Is is 
the current in sensing resistor at node B. 
The resistances of resistors R1, R2, R3 and R4 are chosen in order that 
when I (load)=0, the voltage Va at node A is higher than the voltage Vb at 
node B, which causes the comparator to provide an output signal to switch 
OFF the field effect transistor 24. 
The field effect transistor 24 is switched ON through the power supply unit 
8 which provides a SET signal on line 14 to close switch 44. Thus, voltage 
Va is lower than voltage Vb, and comparator 22 provides a zero level 
signal on its output 36, which switches ON field effect transistor 24. 
When field effect transistor 24 is switched ON, resistor R5 is parallely 
mounted with resistor R1. 
As long as Is remains lower than a threshold current It, Va is lower than 
Vb and the field effect transistor remains ON, 
When Is becomes higher than It, Va becomes higher than Vb and the field 
effect transistor switches OFF, and the remote device is protected against 
the overcurrents higher than It. 
Due to the feedback path arrangement comprising the resistor R5, when the 
field effect transistor 24 is switched ON, voltage Vb decreases and when 
the field effect transistor is switched OFF, voltage Vb increases. This 
results in an hysteresis effect which reinforces the control effect of the 
sense current Is upon the switching ON and OFF of the field effect 
transistor. 
Note that feedback resistor could be implemented differently, for example, 
between FET gate and input A. 
Consequently, the sensing resistor can have a low value. In a preferred 
embodiment of the invention the resistors have the following values: 
R1=54.9 Kiloohms 
R2=R3=R4=49.9 Kiloohms 
R5=274 Kiloohms 
r=1.6 ohms 
The capacitor C connected across A and B prevents the overload protection 
circuit from being sensitive to transient overload conditions. Other 
locations for capacitor C will provide unequal switch on and switch off 
same constants. 
The functions of switches 44 and 46 can be implemented by any type of 
switching devices, such as field effect transistors or bipolar 
transistors. In a preferred embodiment of the invention this function is 
performed by TTL open-collector gates. 
FIG. 3 represents the overload protection circuit when implemented to 
protect a remote device operating with a negative supply voltage -V. The 
components of this circuit are referenced by the reference numbers of FIG. 
2 added to 100, i.e. switch 124 in FIG. 3 has the same function as switch 
24 in FIG. 2. In that implementation, switch 124 comprises a FET 
transistor of the N enhancement type. 
The resistors and capacitor in FIG. 3 have the same references as in FIG. 2 
with a prime sign. 
The operation of circuit of FIG. 3 is similar to the operation of the 
circuit of FIG. 2, so that it is not described again. 
Only the operation of the control circuit 126 is different. Both switches 
144 and 146 are closed in normal operation mode. They are open one at a 
time to set or reset the circuit which causes the voltages at node A' or 
B' to be changed by means of resistors 100 and 102 connected between a 
positive reference voltage +Vref and node 140.