Solid state circuit protector

A solid state circuit protector is comprised of a coil winding, a latching type Hall effect device, and a solid state switching network including an NPN bipolar junction transistor with a current limiting resistor. Other types of solid state switches can be employed instead of or in addition to the bipolar junction transistor.

FIELD OF THE INVENTION 
The present invention relates to solid state circuit protection devices 
that turn off the load current when an excessive load current level is 
reached. A solid state switch, such as a transistor, is controlled by an 
electromagnetic sensing circuit. A coil winding in series with the load 
will produce a stronger magnetic field when an overload occurs. The 
increased magnetic field is detected by a magnetic sensor which causes the 
solid state switch to turn off. 
Different types of magnetic sensors may be employed in this invention. 
These sensors may be, for example, a magnetic Hall effect device, a reed 
switch or an electromechanical switch. The preferred embodiment of the 
invention employs the Hall effect device because the circuit is then 
totally solid state with all the attendant advantages to such circuitry. 
DESCRIPTION OF THE PRIOR ART 
The aformentioned application of Sakatos Ser. No. 878,358, filed June 25, 
1986, now U.S. Pat. No. 4,811,153, issued Mar. 7, 1989, discloses a 
circuit protection device employing a magnetically responsive reed switch 
having at least two sources of magnetic bias, one source to initiate the 
switching of the reeds and the other source to complete the reeds 
switching and maintain the reeds in the switched state. The current flows 
through the protector of Sakatos until a predetermined fault conditions is 
reached at which time the magnetic bias is changed to effect switching of 
the protector. 
SUMMARY OF THE INVENTION 
The solid state embodiment is comprised of a coil winding, a latching type 
Hall effect device (such as is commercially available from Sprague 
Electric Company, type 3075), and solid state switching network including 
an NPN bipolar junction transistor with a current limiting resistor. Other 
types of solid state switches can be employed instead of or in addition to 
the bipolar junction transistor. 
In operation, current is caused to flow through the coil winding, load and 
the solid state switching element. The Hall effect device is in its OFF 
state because the magnetic field generated by the coil is insufficient to 
turn it ON. In this state, the ouput of the Hall effect device appears as 
a very high electrical impedance (as an open circuit). This permits 
current to flow to the base of the transistor. The current, limited by the 
current limiting resistor, is sufficient to saturate the transistor to 
maintain it conducting to carry the load current. When the load draws 
excessive current, the Hall effect switch senses the increased magnetic 
field from the coil and changes to the ON states at its output. The base 
of the transistor is now shorted to ground through the Hall effect output 
device causing the transistor to turn OFF stopping the load current. 
At this point, the coil's field collapses, but the Hall effect switch 
remains on because it is of the latching type. A reduction of the source 
voltage will allow the Hall effect to reset to its normal OFF state, 
thereby resetting the circuit. 
A principal object of my invention is the provision of a solid state 
circuit protector which avoids disadvantages associated with mechanical 
switches. Another object of my invention is the provision of a circuit 
which will respond rapidly to the occurrence of a fault condition in a 
load to be protected. A still further object of my invention is the use of 
a Hall effect device to short the circuit to ground on the occurrence of a 
fault condition. Another object of the invention is the provision of a 
coil to monitor the current flow in the load, the coil generating a 
magnetic field which triggers the Hall effect device.

DETAILED DESCRIPTION OF THE INVENTION 
Hall effect devices serve as contactless magnetically activated switches 
and sensors. The Hall effect device may be biased so as to produce a 
negligible output voltage in the absence of a magnetic field. If the 
biased Hall device is placed in a magnetic field with flux lines at right 
angles to the Hall current, the voltage output is directly proportional to 
the strength of the magnetic field. This is the Hall effect discovered in 
1879. 
FIG. 1 is a schematic diagram of the circuit employing my invention. In 
FIG. 1, terminals 12 and 14 denote the input terminals of the circuit 
protector from a voltage source. Numeral 8 denotes the load to be 
protected. The load 8 is connected between the collector of transistor 10 
and coil 6. A Hall effect switch 2 is connected across terminals 12 and 
14. The output of the Hall effect device 2 is connected to the base of 
transistor 10. A resistor 4 is connected across the input and the output 
of the Hall effect switch 2 and to coil 6. 
FIG. 2 is a schematic diagram of another embodiment of the invention. Like 
numerals have been used to designate like elements in FIGS. 1 and 2. FIG. 
2 includes another transistor 9 connected across the coil 6 and the base 
of transistor 10. Resistor 11 is connected in series between the emitter 
of transistor 9 and the base of transistor 10. 
The transistors 9 and 10 in these embodiments may be NPN bipolar junction 
transistors. The Hall effect device is a Sprague Type 3075, or equivalent. 
FIG. 1 circuit initially operates when current is caused to flow through 
the coil winding 6, load 8 and the solid state switching element 10. The 
Hall effect device 2 is biased in its OFF state because the magnetic field 
generated by the coil 6 is insufficient to turn it ON. In this state, the 
output of the Hall effect device 2 appears as a very high electrical 
impedance (as an open circuit). This permits current to flow to the base 
of the transistor 10. The current, limited by the current limiting 
resistor 4, is sufficient to saturate the transistor 10 to maintain it 
conducting to carry the load current. When the load 8 draws excessive 
current, the Hall effect 2 switch senses the increased magnetic field from 
the coil 6 and changes to the ON states at its output. The base of the 
transistor 10 is now shorted to ground 14 through the Hall effect output 
device 2 causing the transistor 10 to turn OFF stopping the load 8 
current. 
At this point, in coil 6 the coil's field collapses, but the Hall effect 
switch 2 remains on because it is of the latching type. A reduction of the 
source voltage (at terminals 12 and 14) will allow the Hall effect device 
2 to reset to its normal OFF state, thereby resetting the circuit. 
The FIG. 2 circuit operates in a manner similar to that described in 
connection with the circuit of FIG. 1. However, in FIG. 2, transistor 9 
and resistor 11 are employed as a current amplification stage. Transistor 
9 is ON while the Hall effect device 2 is OFF. Transistor 9 permits 
sufficient current flow therethrough to saturate the transistor 10. Higher 
load currents are possible in this configuration if transistor 10 is rated 
at higher power 
The circuits of FIGS. 1 and 2 may employ either a reed switch as the 
magnetic sensor as taught in the aforementioned Sakatos Patent or an 
electro-mechanical switching device as the magnetic sensor as taught in a 
copending application filed of even date herewith Ser. No. 320,158 of 
Mulshine and Sakatos entitled CIRCUIT PROTECTOR. 
In one modification, a reed switch is connected in series with resistor 4 
in FIG. 1, and Hall effect switch 2 is removed. A holding coil is 
connected across the collector and the emitter of transistor 10. This coil 
serves as the holding coil taught in Sakatos patent No. 4,811,153.