Abstract:
A dissipator useful for discharging provides a continuously variable high-voltage load under control of a computer or other device capable of providing an analog control voltage. Known dissipator arrangements use multiple high voltage switches and resistors to attempt to dissipate power. However, the disclosed dissipator uses the resistive property of a matrix of photo resistive cells, typically cadmium sulphide arranged on a circuit board. These photo resistive cells are series connected and controlled optically to vary their resistances. An optical control signal is provided for each photo resistive cell by a corresponding light emitting diode (LED). The amount of light emitted by each LED is analog controlled.

Description:
RELATED APPLICATIONS 
   The instant application is a continuation of and claims priority from Provisional Application Ser. No. 60/720,051 filed Sep. 26, 2005. 

   BACKGROUND AND SUMMARY 
   A dissipator provides a continuously variable high-voltage load under control of a computer or other device capable of providing an analog control voltage. It is useful for discharging a capacitor such as a high voltage cable under test at a precisely-controlled rate. It is particularly useful as a component of a very low frequency high voltage test set to test a length of shielded power cable. 
   Known arrangements for discharging a capacitive load use multiple high voltage switches and resistors to attempt to dissipate power. However, there are operational difficulties associated with such known arrangements. They can only control the load in coarse steps. They do not provide control over a continuous analog range. Mechanical high voltage switches are prone to wear. Load control is inaccurate. 
   In order to overcome these operational difficulties, this patent document describes a novel arrangement for a dissipator. Rather than use switched resistors to dissipate power, there is provided a matrix of photo resistive cells, typically cadmium sulphide, arranged on a circuit board. These photo resistive cells are series connected and controlled optically to vary their resistances. An optical control signal is provided for each photo resistive cell by a corresponding light emitting diode (LED). The amount of light emitted by each LED is linearly controlled. Analog (linear) control is provided by an operational amplifier circuit that is driven by a control voltage of zero to 4 volts DC. This control voltage can be easily scaled to any value. It may also be digitally controlled. In the presently preferred embodiment, the dissipator can withstand up to 50 Kilovolts and will handle substantially 100 milliamps peak. It is not polarity sensitive and is easily scalable to higher or lower voltages and power levels. The output can be floating in reference to ground. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of an embodiment of the dissipator according to the inventions. 
       FIG. 2  is a diagram of a circuit board containing 128 cadmium sulphide photocells. 
       FIG. 3  is a drawing of a presently preferred embodiment of a complete dissipator assembly. 
       FIG. 4  is a view of the dissipator assembly showing components on circuit board  130 . 
       FIG. 5  is a view of the complete assembly showing how it is immersed in oil. 
       FIG. 6  is a view of the complete assembly showing the relationship of respective components. 
       FIG. 7  is a view of the complete assembly showing external connections. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a schematic diagram of an embodiment of the dissipator according to the inventions. 
   The dissipator, according to the inventions, is generally denoted by reference numeral  100 . A control voltage is applied at a control terminal  102 . Typically, the control voltage is zero to 4 volts DC. However, the control voltage can be scaled to any value required by a system in which the dissipator is used. Also a digital control signal can be applied, such as, for example, from a computer based control system. 
   The control signal is applied to a resistive divider including resistors R 6  and R 7 . In the presently preferred embodiment R 6  has a value of 10 KΩ and R 7  has a value of 100 KΩ. A node  104  of the resistive divider is coupled to the [+] input of an operational amplifier  106 , preferably an MCP6021 or equivalent. Of course other types of operational amplifiers can be used with appropriate design adjustments, as necessary. 
   A series of 4 Light Emitting Diodes (LED) D 1  . . . D 4  is coupled from a first voltage source V 1  to the collector of a drive transistor Q 1 , preferably a 2N4124. The emitter of transistor Q 1  is coupled through a resistor R 5  to ground. In the presently preferred embodiment R 5  has a value of 120 Ω. The [−] input of operational amplifier  106  is coupled to the emitter of transistor Q 1  and to resistor R 5 . The components described thus far including control input, resistive divider, LEDs, operational amplifier and driving transistor together constitute a “low voltage side” of the dissipator. 
   For convenience, only 4 LEDs D 1  . . . D 4  are shown in the drawing. Actually, the number of LEDs in series is a matter of design choice and can be varied as needed. In the presently preferred embodiment, there are 128 green LED&#39;s arranged in 32 series strings of 4. Power to each string of 4 LEDs is provided by a constant current driver. The current to all LED&#39;s in the array is modulated by a zero to four volt control voltage. In the voltage modulation range, the amount of light output by an LED is a function of the control voltage. By varying the control voltage, the amount of light output can be linearly controlled. At the time of this writing, it is preferred to use green LED&#39;s because the emitted wavelength most closely matches the spectral response of the photo resistive cells used on the “high voltage side” of the dissipator. 
   The low and high voltage sides of the dissipator are isolated in part by a piece of ⅛″ thick clear acrylic plastic barrier  110 . Further isolation is provided by oil  190  (see  FIG. 5 ) into which all components are immersed. The oil  190  also provides a mechanism for heat dissipation. 
   On the high voltage side of the dissipator, there are multiple photo resistive cells, typically cadmium sulphide. In the drawing, only four photo resistive cells PC 01 , PC 02 , PC 03  and PC 04  are shown for convenience. In the presently preferred embodiment there are 128 photo resistive cells arranged electrically in series on a circuit board  126  (see  FIG. 2 ). These photo resistive cells are used as variable resistors. The amount of resistance presented by a photo resistive cell is a function of the amount of light received by it. By controlling the amount of light output by the various LEDs, the amount of resistance of the photo resistive cells is controlled. Photo resistive cells PC 01  . . . PC 04 , along with the other components are immersed in oil and are controllably illuminated by the green LED&#39;s controlled by a variable constant-current driver. Photo resistive cells PC 01  . . . PC 04  are preferably 0.4″ diameter cadmium sulphide photo resistive cells, selected for their high power and high voltage capabilities. 
   Varistors V 1  . . . V  128  are 390 volt surface-mount varistors soldered in place directly behind photo resistive cells PC 01  . . . PC 128 , respectively. Each varistor is electrically connected in parallel with its respective photo resistive cell. The varistors ensure that the voltage divides equally across each cell and protects the entire unit from overvoltage. Typically the photo resistive cells are rated for operation at 300 V. In the preferred embodiment, these cells are operated at 400 V. It is possible to operate the cells beyond their normal rated voltage due to the electrical insulating and heat dissipating properties of the oil. The varistors help to evenly divide the voltage and prevent burnout of individual photo resistive cells. 
   In the presently preferred exemplary embodiment, the output can withstand up to 50 Kilovolts and will handle about 100 milliamps peak. It is not polarity sensitive and is easily scalable to higher or lower voltages and power levels. The output can be floating in reference to ground. 
   Voltage to be dissipated is connected between terminals  120  and  124 . Typically the high voltage side of the voltage to be dissipated is connected to terminal  120  and the low voltage side of the voltage to be dissipated is connected to terminal  124 . 
     FIG. 2  is a diagram of a circuit board  126  containing photo resistive cells PC 01  . . . PC 128 . Varistors V 1  . . . V 128  are mounted respectively behind corresponding photo resistive cells. 
     FIG. 3  is a drawing of a presently preferred embodiment of a complete dissipator assembly. In this embodiment there are two identical assemblies back to back. Each assembly includes a circuit board  130  containing the LEDs, a circuit board  126  containing the photo resistive cells and a clear acrylic plastic barrier  110 . Additional structural components  200  allow the entire assembly to be conveniently immersed in a tank of oil which dissipates heat generated by the various elements. The oil can be circulated to dissipate even more heat. 
     FIG. 4  is a view of the dissipator assembly showing components on circuit board  130 . In this view, the components shown have not yet been installed into tank  160  and immersed in oil  190 . Fans  150  circulate oil  190  after this assembly has been placed into tank  160  which is then filled with oil. 
     FIG. 5  is a view of the complete assembly showing how it is immersed in oil. The photo resistive cells and varistors are immersed in insulating oil  190  (see  FIG. 5 ) to carry away heat and to increase the high voltage properties of the cells. The oil is circulated using two 40 mm DC fans  150  immersed in the oil. The oil is enclosed in an aluminum tank  160 . 
     FIG. 6  is a view of the complete assembly showing the relationship of respective components before being placed inside tank  160 . The assembly shown includes two sets of LEDs Dn, barrier  110  and photo cells PCn (back to back). 
     FIG. 7  is a view of the complete assembly showing external connections. The dissipator provides a continuously variable high-voltage load under control of a computer or other device capable of providing an analog control voltage. The outside of tank  160  is fitted with six 90 mm fans  170  on finned heat sinks  180  to dissipate heat to the outside. 
   Alternative embodiments within the spirit of this disclosure are intended to be covered by the claims. For example it possible to utilize more or less LEDs and photo cells. Specific component values are a function of the characteristics of the energy to be dissipated.