Abstract:
A capacitor discharging circuit for discharging a potential on a capacitor when the capacitor is decoupled from an AC source. This circuit is particularly useful where the capacitor is unplugged from an ordinary AC power socket.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the field of discharging capacitors, particularly where the charge remaining on the capacitor could have safety implications. 
     2. Prior Art 
     Often in homes and businesses, the power line voltage contains numerous high-frequency components not associated with, for instance, the 60 Hz alternating current (AC power) or its harmonics. These high-frequency components are caused, for example, by switched power supplies, dimmers, motors, and other sources. There are numerous good reasons why these high-frequency components are undesirable and should preferably be removed from the power line. 
     One way of reducing the high-frequency components is to connect a capacitor across the power line to shunt out the high-frequency components. An ordinary plug may be used to connect the capacitor to the power line at a power receptacle. This is convenient since it does not require the permanent or hard-wiring of the capacitor into the power circuit. When the capacitor is unplugged, a charge will typically remain on the capacitor and can cause a shock if, for example, a hand comes in contact with the prongs of the plug. 
     As will be seen, the present invention provides a circuit for discharging the capacitor once it is unplugged from the power line. 
     A circuit similar in structure, but not function, to the circuit of FIG. 1 is sometimes used in AC dimmers. In the dimmer application, the AC signal is phase shifted through a variable resistor and used to trigger a triac. As will be seen with the present invention, the circuit of FIG. 1 is triggered only with DC signals and is used to discharge a capacitor, not to control an AC signal for a light, or the like. 
     SUMMARY OF THE INVENTION 
     A method and apparatus for discharging a capacitor when the capacitor is unplugged from an AC receptacle is described. An attenuation circuit provides attenuation for the AC power signal and substantially less attenuation to a DC signal. When connected to the AC power line, the output of this circuit is low enough to not cause triggering of a triac since the output remains lower than a predetermined threshold voltage. However, when the capacitor is disconnected, the full DC potential remaining on the capacitor is coupled to a triggering mechanism for a triac. A discharge circuit, which includes the triac, causes the capacitor to be discharged when the DC signal is greater than the predetermined voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an electrical schematic showing a filtering capacitor in an embodiment of the invented circuit. 
     FIG. 2 is a graph used to describe the characteristics of a Diac used in the circuit of FIG.  1 . 
     FIG. 3 is an electrical schematic for an alternate discharging circuit that may be used in the circuit of FIG.  1 . 
     FIG. 4 describes the steps implemented by the circuit of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     A method and apparatus is described for discharging a capacitor after it has been disconnected from an alternating current (AC) power line. In the following description, numerous specific details are set forth, such as specific component values, in order to provide a thorough understanding of the present invention. It will be apparent to one skilled in the art that the present invention may be practiced without these specific details. 
     Referring now to FIG. 1, a capacitor  10  is illustrated which in connected directly to a AC power line such as a standard 120V 60 Hz power receptacle or socket through the plug  17 . The capacitor  10  may be directly connected to the power line with an ordinary 2-prong or 3-prong plug commonly used in home and business applications for 120V AC. The function of the capacitor  10  is to filter out high-frequency signals that are often found on the power line. These signals are typically noise from numerous sources such as power supplies, dimmers, appliances, etc., and not typically harmonics associated with a 60 Hz power generation. Often the noise is above 10 kHz and can have a magnitude of {fraction (1/10)} volt (rms). The filtering capacitor  10 , in one embodiment, has a capacitance of 85 μF and is able to withstand a voltage of 220V AC. 
     In an application where there are two phases of the AC power as often is the case, two capacitors are used, including two circuits such as shown in FIG. 1, one being connected to each of the phases. 
     The circuit of FIG. 1 includes an attenuation circuit comprising the resistor  11  and the capacitor  12  coupled to the capacitor  10 . For 60 Hz, the values for the resistor  11  and capacitor  12  are selected such that the AC signal present at the capacitor  12  remains below a predetermined threshold voltage (e.g. 30 volts). For instance, if the resistor  11  is 200 K ohms and the capacitor is 0.1 μF at 60 Hz slightly more than ten percent (10%) of the signal across the capacitor  10  is present over the capacitor  12 . Thus, even if the peak AC power reaches 180V, only approximately 18V of the AC signal appears across the capacitor  12 . 
     The attenuation circuit could be realized using an inductor and resistor or a combination of an inductor, capacitor and resistor. However, these embodiments are not preferred since the inductor required would be larger and more costly than the capacitor  12  and the resistor  11  of FIG.  1 . 
     The Diac  13  comprises bilateral snap action diodes the characteristics of which are shown in FIG.  2 . In one embodiment, the positive and negative potential Vs is equal to 30V. The diodes  13  connect the potential across the capacitor  12  through the triac control line  15  to a triac  14 . Thus, when the potential on the capacitor  12  reaches or exceeds 30V for one embodiment (the predetermined threshold voltage) the triac is turned-on. The current through the Diac  13  is sufficient to trigger the triac. The diode  13  and triac  14  may be purchased as a single component referred to as a quadrac and in one embodiment, a 4 amp-200V quadrac is used specifically part number Q2004LT from Teccor Electronics. 
     The triac  14  is coupled in series with the resistor  16  and the capacitor  10 . When the triac conducts, the charge on the capacitor  10  is dissipated in the resistor  16 . Once the charge is dissipated from capacitor  12 , the triac turns-off. 
     Since Vs for the snap action Diac  13  establish a threshold voltage of 30V for one embodiment, the triac  14  will not conduct until the potential on the capacitor  12  reaches 30V. The attenuation provided by the resistor  11  and the capacitor  12  prevents the potential on the capacitor  12  for an AC signal of 120V from ever reaching 30V, as mentioned. However, when the plug is removed from the socket, the charge remaining on capacitor  10  charges capacitor  12  through the resistor  11 . The charge on capacitor  10  from the standpoint of the current produced through resistor  11  and onto capacitor  12  is a direct current (DC) signal. This signal is not attenuated as was the AC signal, that is, the potential on capacitor  12  rises to the same potential as on the capacitor  10  when the plug is removed. (Note in one embodiment capacitor  10  is 850 times larger than capacitor  12 , therefore, very little of the total charge on capacitor  10  is needed to charge capacitor  12 ). 
     The potential on capacitor  10  when the plug is removed from the receptacle may take any value from plus or minus the peak AC value. Only if this value is 30V or greater will the triac  14  conduct since this is the threshold voltage for the Diac  13  for the described embodiment. A 30V potential, however, is low enough that it will not harm a human if it remains on the capacitor  10 . 
     The RC time constant for resistor  11  and capacitor  12  is 20 milliseconds in one embodiment, thus the potential on capacitor  12  rises quickly to the voltage level of capacitor  10 . The RC time constant for the capacitor  10  and resistor  16  allows the charge on the capacitor  10  to be-dissipated in less than 10 milliseconds. Thus, as a plug, is removed from a socket, the charge on the capacitor will be dissipated so rapidly that it will be practically impossible for a shock to be felt from the prongs of the plug connected to the capacitor  10 . 
     In FIG. 3, an alternate embodiment of the discharging circuit is shown. An attenuation circuit is also shown comprising the resistor  40  and capacitor  41 . The leads  39  are coupled to the filtering capacitor, such as capacitor  10  of FIG.  1 . In this embodiment, instead of using the Diac and triac, a biopolar transistor circuit, which includes two diodes is used. The NPN transistors  42  and  43  connected in a Darlington configuration, are coupled to one of the leads  39 . The pair of PNP transistors  44  and  45 , also in a Darlington configuration, are coupled to the same one of the leads  39 . The Darlington pairs are used since they provide a relatively high input impedance. A diode  46  is connected to the collectors of the transistors  42  and  43 , and the diode  47  is connected to the collectors of transistors  44  and  45 . The diodes  46  and  47  on leads  48  can be connected to the discharging resistor, such as the 100 ohm resistor of FIG. 1, or may be directly connected to the capacitor  10  for discharging the capacitor. 
     The values of the resistor  40  and capacitor  41  are selected so that the potential on the base of transistors  42  and  44  remains low enough to prevent the transistors from conducting when an AC signal is applied to the lines  39 . However, with a DC signal, greater than the threshold voltage of the transistors Darlington pairs, one of the Darlington pairs will conduct depending upon the polarity of the DC potential on leads  39 . Once one of the Darlington pairs begins to conduct the capacitor  10  of FIG. 1 is quickly discharged through the collectors of the conducting pair. The diodes  46  and  47  prevent, for instance, conduction through the transistors  42  and  43 , when a negative potential from capacitor  10  is applied to the capacitor  46  when the AC potential is present on leads  48 . Similarly, the diode  47  prevents conduction through transistors  44  and  45  for the opposite polarity condition. 
     FIG. 4 illustrates the method used by the present invention. Step  20  is the sensing of the DC potential on the filtering capacitor. Step  21  is the activation of a discharge circuit. This occurs when the DC potential on the filtering capacitor reaches or exceeds a predetermined threshold voltage such as 30V for the embodiment of FIG.  1 . Note as mentioned earlier, the voltages on capacitors  10  and  11  are equal once capacitor  12  is charged. Finally step  22  is the discharging of the filtering capacitor. This is done by shorting or placing a resistor having relatively low resistance across the capacitor. 
     Thus, a circuit and method has been described which allows a capacitor to be coupled to an AC line with an ordinary plug and to be automatically discharged when the plug is removed from a socket.