Current Overload Protection Circuit

1. Field of the Invention

This invention relates to a current overload protection circuit which will interrupt current flow from a power source to a load operable with both AC and DC power sources.

2. Background of the Invention

The problem of current overload protection is typically solved by the use of fuses or circuit breakers which suffer from the disadvantage in some applications in that they cannot be automatically reset. Resettable fuses are available, but such fuses usually employ temperature coefficient resistors which increase their resistance with current increases. Such resettable fuses do not provide precise repeatable performance.

When the power source is limited to a DC power source, current overload protection has been achieved by means of a series pass transistor. However, to use this solution for an AC power source, all of the circuitry of the protection circuit must be duplicated. A relay operated in response to current detection circuitry has been used in some applications. However, relays require a lot of circuit board real estate. Solid-state relay circuits for current overload protection suffer from the disadvantage of being susceptible to operation in a linear region between full on and full off causing excessive heating and power dissipation. In addition, they require excessive packaging volume when several isolated loads have to be protected due to the requirement for a dedicated power supply for each protection circuit to maintain isolation.

The present invention improves on prior art solutions by requiring a minimum number of components, all of which are readily available, to protect both AC and DC high voltage loads and the system is readily expandable to protect multiple, isolated loads without requiring isolated power sources for individual protection circuits protecting different isolated loads.

In accordance with the invention, a current sensing resistor is connected with a bridge rectifier to receive load current flowing in one direction for both polarities of current flowing through the load. The voltage developed across the current sensing resistor is connected to the reference input of a shunt regulator. When the voltage across the sensing resistor increases to a predetermined value representing a fault condition, the input voltage to the regulator turns the regulator on. This action energizes a light emitting diode in an optical isolator, turning on the transistor of the optical isolator. The output current from the optical isolator transistor actuates an inverter circuit to discharge a capacitor. The discharging of the capacitor triggers a Schmidt trigger connected to the input of a solid-state relay and causing the solid-state relay to turn off. The output circuit of the solid-state relay is connected in series with the load and, thus, the current to the load is interrupted in response to the current overload. The circuit automatically resets after a time delay determined by an RC timing circuit.

The single FIGURE of the drawing illustrates the current overload protection circuit of the present invention.

As shown in the drawing, a power source 11 supplies AC or DC power to a load 13. The power is supplied through a full wave bridge rectifier 15 which is connected with its AC terminals in series with the load to supply AC or DC power from the source 11 to the load 13. The output circuit of a solid-state relay 17 is connected in series between the full wave rectifier bridge circuit 15 and the load 13. A resistor 19 is connected across the DC terminals of the bridge 15 between circuit junctions 21 and 23 so that both cycles of AC current from the power source 11 will flow in the same direction through the resistor, from circuit junction 21 to circuit junction 23. The resistor 19 is the current sensing resistor and a voltage is developed across the resistor 19 proportional to the amount of current flowing through the load 13. A voltage divider formed by resistors 25 and 26 is connected between the circuit junctions 21 and 23 to apply a portion of the voltage developed across the resistor 19 to the reference terminal input of a shunt regulator 27. The circuit junction 21 is also connected through a resistor 29 and a light emitting diode 31 of an optical isolator 33 to the cathode of the shunt regulator 27. The anode of the shunt regulator diode 27 is connected to the circuit junction 23. The shunt regulator 27 may be implemented by the Motorola TL431,A integrated circuit.

Normally, the shunt regulator 27 will be nonconducting. When the voltage across the sensing resistor 19 rises above a predetermined threshold value selected to indicate an overload or fault condition, the voltage applied between the reference input terminal and the anode of the shunt regulator will cause the regulator 27 to become conductive so that current will flow through the light emitting diode 31. The light emitting diode 31 upon being energized will then irradiate the light responsive element 35 of the optical isolator 33 causing the light responsive element 35 to turn on. The light responsive element 35 is a transistor and when it turns on, current will flow from a 5 volt source applied at terminal 37 through the transistor 35 and through a resistor 39 to ground. The junction between the transistor 35 and the resistor 39 is applied to the input of an inverter 41 and the current flowing through the resistor 39 will cause the voltage at the input of the inverter 41 to rise. As a result, the voltage at the output of the inverter 41 will drop, which action will discharge a capacitor 43 by current flowing from the capacitor 41 through a resistor 45 and a diode 47. The capacitor 43 is connected in series with a resistor 53 between ground and the input of a Schmidt trigger inverter 49, which is triggered in response to the discharge of the capacitor 43 and causes its output to go high. The input circuit of the solid-state relay 17 comprises a light emitting diode 55, which is connected in series with a resistor 53 between the output of the Schmidt trigger 49 and a source of +5 volts applied at a terminal 51. When the output voltage from the Schmidt trigger is low, current will flow from the terminal 51 through the resistor 53 and the input circuit of the solid-state relay maintaining the output circuit of the solid-state relay 17 conducting. When the output of the Schmidt trigger 49 goes high in response to the discharge of the capacitor 43, the input circuit of the solid-state relay 17 is rendered nonconductive, thus shutting off current flow in the output circuit of the solid-state relay 17 and disconnecting the power source 11 from the load 13.

The solid-state relay 17 is an optically isolated type and provides for bidirectional current flow in the output circuit in response to current flow in the input circuit and may be implemented by a Teledyne Model C60 or SC60 solid-state relay. The input circuit of the solid-state relay comprises a light emitting diode 55 which, when energized, emits light to the gates of field effect transistors 57 and 59 connected in series with reverse polarities. The field effect transistors are connected with diodes 61 and 63 so that one cycle of AC current will pass through the field effect transistor 57 and diode 63 and the other cycle of AC current will pass through the field effect transistor 59 and diode 61 when the field effect transistors 57 and 59 are rendered conductive in response to receiving light from the energized light emitting diode 55.

When the output of the Schmidt trigger 49 goes high interrupting current flow through the input circuit of the solid-state relay 17 and thereby interrupting current through the load 13, current flow through the sensing resistor 19 will cease and the shunt regulator 27 will again become nonconducting. As a result, the transistor 35 will cut off, the input to the inverter 41 will go low, and output of the inverter 41 will back bias the diode 47.

The ungrounded plate of the capacitor 43 is connected to the 5 volts applied at terminal 51 through a resistor 65 so that when the diode 47 is cut off, the capacitor 43 will begin to recharge through the resistor 65, which has a substantially greater resistance than the resistor 45. The resistor 65 and the capacitor 43 form an RC timing circuit. As the capacitor recharges the voltage applied to the input of the Schmidt trigger inverter 41 will rise. When the potential has risen to a predetermined level, the voltage will activate the Schmidt trigger 49, which will then switch its output low causing current to again flow through the diode 55 in the input current of the solid-state relay 17 so that the output circuit of the solid-state relay 17 is again rendered conductive and current will again flow through the load 13. In this manner, the circuit automatically resets itself after cutting off current through the load in response to a current overload condition. The resetting occurs after a time delay determined by the RC time constant of the RC timing circuit.

The capacitor 43 and the resistor 65 as described above comprise an RC timing circuit, which is set by being discharged by the inverter 41 allowing current to flow through the diode 47 and then is activated to start its time delay by current through the diode 47 being cut off. The Schmidt trigger cuts off current flow through the input circuit of the solid-state relay 17 in response to the setting of the timing circuit and starts current flow through the input circuit of the solid-state relay 17 in response to the expiration of the time delay, which is started by the activation of the timing circuit.

A diode 67 is connected to allow current flow from the ungrounded plate of the capacitor 43 to the terminal 51 to provide a rapid discharge path for the capacitor 43 during start-up to remove any residual charge on the capacitor 43 so that the solid-state relay will not be energized until the 5 volt power supply is up to its operating value.

The circuit operation described above has assumed an AC power source 11, but it will be apparent that the circuit will also operate in response to a DC power source applied to the load 13 of either polarity. It will be noted that the power supply for the circuit applied at terminals 37 and 51 is completely isolated from the load 13. Accordingly, a common power supply can be provided to multiple overload protection circuits for different isolated loads.

The above-described circuit will survive a dead short and handle signals with several hundred volt peaks, has minimum power dissipation, has a small footprint to facilitate replication for multiple loads and is of relatively low cost involving no custom parts. In the system as described above, the reference load is determined by the resistance of the current sensing resistor 19 and the response time is immediate. It will be apparent that resistors and capacitors could be readily added to the circuit to vary the threshold load current at which the circuit is triggered as well as the response time of the circuit. Capacitors and/or Zener diodes may be added to the circuit for voltage spike protection across the bridge rectifier or across the shunt regulator and optical isolator circuit. These and other modifications may be made to the above-described invention without departing from the spirit and scope of the invention which is defined in the appended claims.