Abstract:
An exemplary charger control circuit includes an input port having a positive terminal and a negative terminal, an electrode control circuit, and an output port having a positive terminal and a negative terminal. The input port is connected to a source of power. The output port is connected to a load. The electrode switching circuit detects the electrode polarity of the input port and the output port for selectively coupling the positive terminals and the negative terminals of the two ports, and protecting the electrical equipment.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to control circuits, and particularly to a charger control circuit with automatic terminal polarity selection. 
   2. Description of Related Art 
   The battery packs for portable power tools, outdoor tools and certain kitchen and domestic appliances may include rechargeable batteries, such as lithium, nickel cadmium, nickel metal hydride and lead-acid batteries, so that they can be recharged rather than replaced. Thereby a substantial cost saving is achieved. 
   Rechargeable batteries are charged by a DC battery charger. Generally, battery chargers include an alternating current (AC) to direct current (DC) (or DC to DC) adapter containing an AC to DC (or DC to DC) converter for generating a substantially constant current to charge the battery to a nominal battery voltage. The chargers also include an output port with a positive terminal and a negative terminal for connecting the charger to the depleted battery. When connecting the charger to the battery, the positive terminal of the output port must be connected to the positive terminal of the battery, and the negative terminal of the output port must be connected to the negative terminal of the battery or to ground so that the direct current is supplied to the positive terminal of the battery. 
   If the positive and negative terminals of the output port of the battery charger are reversely connected to the battery terminals, the battery charger will supply the direct current to the negative terminal of the battery. This situation can result in overheating of the battery charger, excess arcing between the connection cables and the terminals of the battery, and, in extreme cases, battery explosions. 
   What is needed, therefore, is a charger control circuit which can solve above problem. 
   SUMMARY OF THE INVENTION 
   In one aspect, the present invention relates to a control circuit which is capable of automatic terminal polarity selection for controlling current supplied from a source of power to a load. In one embodiment, the source of power is a battery charger and the load is a rechargeable battery. The charger control circuit includes: an input port with a positive terminal and a negative terminal connected to corresponding terminals of an output port of a battery charger for receiving direct current; an output port with two terminals connectable to the rechargeable battery; and a polarity detecting circuit coupled between the input port and the output port of the charger control circuit, wherein the polarity detecting circuit detects the polarity of the terminals of the output port of the charger control circuit, and provides direct current accordingly to the output port of the charger control circuit. 
   In another embodiment, the source of power is a DC power supply and the load is a battery charger. The charger control circuit includes: an input port with two terminals connected to the DC power supply; an output port with a positive terminal and a negative terminal connected to corresponding terminals of an input port of a battery charger for providing direct current; and a polarity detecting circuit coupled between the input port and the output port, wherein the polarity detecting circuit detects the polarity of the terminals of the input port, and receives direct current from the DC power supply. 
   Other advantages and novel features will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram of one embodiment of a charger control circuit in accordance with the present invention; and 
       FIG. 2  is a circuit diagram of another embodiment of a charger control circuit in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , a charger control circuit  10  in accordance with a preferred embodiment of the present invention includes an input port  12 , a polarity detecting circuit  14 , and an output port  16 . The input port  12  defines a positive terminal + and a negative terminal −, and the terminals + and − are connected to corresponding terminals of an output port of a battery charger for receiving direct current. The output port  16  defines two terminals a and b, connected to a rechargeable battery. The polarity detecting circuit  14  is coupled between the input port  12  and the output port  16  for detecting the polarity of the terminals a and b, and providing direct current accordingly. 
   In this exemplary embodiment, the polarity detecting circuit  14  includes a relay-switch  142  with a switch element and a relay element, and a control circuit. The switch element of the relay-switch  142  defines two pairs of switch leads A 1 , A 2  and B 1 , B 2  in two opposite sides of the switch element, and the relay element of the relay-switch  142  defines two relay leads C 1  and C 2 . The control circuit includes a first transistor Q 1 , a second transistor Q 2 , and a photocoupler  144  with a luminous element and an optical receiving block. Each of the transistors Q 1  and Q 2  has a base, a collector, and an emitter. The first transistor Q 1  and second transistor Q 2  are NPN transistors. 
   The switch leads A 1  and A 2  of the relay-switch  142  are respectively connected to the positive terminal + and negative terminal − of the input port  12 . The switch lead B 1  of the relay-switch  142  is connected to an anode of a diode D of which a cathode is connected to the terminal a of the output port  16 , and the switch lead B 2  of the relay-switch  142  is connected to the terminal b of the output port  16 . An anode  1  of the optical receiving block is coupled to a power source Vcc via the relay element of the relay-switch  142 , and a cathode  2  of the optical receiving block is grounded. An anode  3  of the luminous element is coupled to the emitter of the first transistor Q 1 , and a cathode  4  of the luminous element is grounded. The collector of the first transistor Q 1  is connected to the power source Vcc, and the base of the first transistor Q 1  is connected to the collector of the second transistor Q 2 . The collector of the second transistor Q 2  is connected to the power source Vcc, the base of the second transistor Q 2  is connected to a node between the diode D and the terminal a of the output port  16 , and the emitter of the second transistor Q 2  is grounded. 
   When the terminals a and b of the output port  16  are respectively connected to the positive terminal and the negative terminal of the rechargeable battery, the dump energy of the rechargeable battery turns on the second transistor Q 2 . The first transistor Q 1  is turned off, and the optical receiving block of the photocoupler  144  is open. Therefore, the relay-switch  142  is turned off, and the switch leads A 1  and A 2  are respectively connected to B 1  and B 2 . Then, the direct current flows in the proper polarity to the rechargeable battery. 
   When the terminals a and b of the output port  16  are respectively connected to the negative terminal and the positive terminal of the rechargeable battery. The second transistor Q 2  is turned off. The first transistor Q 1  is turned on, and the optical receiving block of the photocoupler  144  is shorted. Therefore, the relay-switch  142  is turned on, and the switch leads A 1  and A 2  are respectively connected to B 2  and B 1 . Then, the direct current still flows in the proper polarity to the rechargeable battery. 
   Referring to  FIG. 2 , a charger control circuit  20  in accordance with another embodiment of the present invention includes an input port  22 , a polarity detecting circuit  24 , and an output port  26 . The input port  22  defines two terminals a′ and b′ connected to a DC power supply. The output port  26  defines a positive terminal + and a negative terminal −, and the terminals + and − are connected to corresponding terminals of an input port of a battery charger for providing direct current. The polarity detecting circuit  24  is coupled between the input port  22  and the output port  26  for detecting the polarity of the terminals a′ and b′, and receiving the direct current in the correct polarity from the terminals a′and b′. 
   The polarity detecting circuit  24  includes a relay-switch  242  with two pair of switch leads A 1 ′, A 2 ′, B 1 ′, B 2 ′, and two relay leads C 1 ′ and C 2 ′, a first transistor Q 1 ′, a second transistor Q 2 ′, and a photocoupler  244 . The switch leads A 1 ′ and A 2 ′ of the relay-switch  242  are respectively connected to the terminals a′ and b′ of the input port  22 . The switch lead B 1 ′ of the relay-switch  142  is connected to an anode of a diode D′ of which a cathode is connected to the positive terminal + of the output port  26 , and the switch lead B 2 ′ of the relay-switch  242  is connected to the negative terminal − of the output port  26 . An anode  1  of the optical receiving block is coupled to a power source Vcc via the relay element of the relay-switch  242 , and a cathode  2  of the optical receiving block is grounded. An anode  3  of the luminous element is coupled to the emitter of the first transistor Q 1 ′, and a cathode  4  of the luminous element is grounded. The collector of the first transistor Q 1 ′ is connected to the power source Vcc, and the base of the first transistor Q 1 ′ is connected to the collector of the second transistor Q 2 ′. The collector of the second transistor Q 2 ′ is connected to the power source Vcc, the base of the second transistor Q 2 ′ is connected to a node between the terminal a′ of the input port  22  and the switch leads A 1 ′ of the relay-switch  242 , and the emitter of the second transistor Q 2 ′ is grounded. 
   Therefore, when the terminals a′ and b′ of the input port  22  are respectively connected to the positive terminal and the negative terminal of the DC power supply, the second transistor Q 2 ′ is turned on. The first transistor Q 1 ′ is turned off, and the optical receiving block of the photocoupler  244  is open. Therefore, the relay-switch  242  is turned off, and the switch leads A 1 ′ and A 2 ′ are respectively connected to B 1 ′ and B 2 ′. Then, the direct current flows in the proper polarity to the battery charger. When the terminals a′ and b′ of the input port  22  are respectively connected to the negative terminal and the positive terminal of the DC power supply. The second transistor Q 2 ′ is turned off. The first transistor Q 1 ′ is turned on, and the optical receiving block of the photocoupler  244  is shorted. Therefore, the relay-switch  242  is turned on, and the switch leads A 1 ′ and A 2 ′ are respectively connected to B 2 ′ and B 1 ′. Then, the direct current flows to the in the proper polarity to the rechargeable battery. 
   The foregoing description of the exemplary embodiments of the invention has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to explain the principles of the invention and their practical application so as to enable others skilled in the art to utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those skilled in the art to which the present invention pertains without departing from its spirit and scope. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description and the exemplary embodiments described therein.