Dimmer Circuit with Improved Inductive Load Imbalance Protection

Abstract

A protection circuit for protecting a dimmer circuit controlling an inductive load including an imbalance detector for detecting an asymmetrical operation in the load and circuit control means for causing the dimmer circuit to reduce a DC component in the load upon detection of the asymmetrical operation. A load imbalance detector is also disclosed having a load DC component detector, a comparator and a signal generating means for generating a circuit shut down signal if the DC component exceeds a pre-set DC threshold.

1 - 9 . (canceled) 
     
     
         10 . A protection circuit in a phase control dimmer circuit that, in use, controls an inductive load, the phase control dimmer circuit comprising a first switch and a second switch, the first switch in use determining a firing angle for the second switch, and the second switch, in series with the inductive load, effecting conduction of the inductive load in response to the first switch, the protection circuit comprising:
 an imbalance detector for detecting an asymmetrical operation in the inductive load by detecting a difference between a conduction period of consecutive positive and negative half cycles and registering a load imbalance if the difference between the consecutive positive and negative half cycles exceeds a preset threshold;   circuit control means for protecting the phase control dimmer circuit by causing the phase control dimmer circuit to reduce a DC component in the inductive load upon detection of the asymmetrical operation, by controlling the first switch.   
     
     
         11 . A load imbalance detector in a phase control dimmer circuit that, in use, controls an inductive load, the phase control dimmer circuit comprising a first switch and a second switch, the first switch in use determining a firing angle of the second switch, and the second switch, in series with the inductive load, effecting conduction of the inductive load in response to the first switch, the load imbalance detector comprising:
 a load DC component detector to detect a DC component in the inductive load by detecting a DC component across the phase control dimmer circuit;   a comparator for comparing a magnitude of the DC component detected by the load DC component detector with a reference voltage; and   a signal generating means for protecting the dimmer circuit by generating a circuit shut-down signal if the DC component exceeds a pre-set DC voltage threshold above the reference voltage.   
     
     
         12 . A load imbalance detector as claimed in  claim 11  wherein said load DC component detector detects a DC sub-component in a positive cycle and detects a DC sub-component in a negative cycle and wherein said magnitude of the DC component is the difference between the respective DC sub-components. 
     
     
         13 . A load imbalance detector as claimed in  claim 12  wherein said load DC component detector includes a first resistor divider chain for detecting said DC sub-component in the positive cycle, and a second resistor divider chain for detecting said DC sub-component in a negative cycle and wherein a divider junction of the first chain is connected to a first side of a capacitor and a divider junction of the second chain is connected to a second side of the capacitor, a voltage across which provides said magnitude of the DC component. 
     
     
         14 . A load imbalance detector as claimed in  claim 13  wherein said comparator includes a first pnp transistor having its base connected to said first side of said capacitor and its emitter connected to said second side of said capacitor, and a second pnp transistor having its base connected to said second side of said capacitor and its emitter connected to said first side of said capacitor, and each respective collector connected to an input of said signal generating means. 
     
     
         15 . A protection circuit as claimed in  claim 10  wherein the circuit control means causes the dimmer circuit to shut down. 
     
     
         16 . A protection circuit as claimed in  claim 10  wherein the circuit control means causes the dimmer circuit to reduce a conduction angle of the dimmer circuit to a point where the DC component reduces to below a preset threshold. 
     
     
         17 . A phase control dimmer circuit comprising a protection circuit that, in use, controls an inductive load, the phase control dimmer circuit comprising a first switch and a second switch, the first switch in use determining a firing angle for the second switch, and the second switch, in series with the inductive load, effecting conduction of the inductive load in response to the first switch, the protection circuit comprising:
 an imbalance detector for detecting an asymmetrical operation in the inductive load by detecting a difference between a conduction period of consecutive positive and negative half cycles and registering a load imbalance if the difference between the consecutive positive and negative half cycles exceeds a preset threshold;   circuit control means for protecting the phase control dimmer circuit by causing the phase control dimmer circuit to reduce a DC component in the inductive load upon detection of the asymmetrical operation, by controlling the first switch.   
     
     
         18 . The phase control dimmer circuit as claimed in  claim 17 , wherein the circuit control means causes the dimmer circuit to shut down. 
     
     
         19 . The phase control dimmer circuit as claimed in  claim 17  wherein the circuit control means causes the dimmer circuit to reduce a conduction angle of the dimmer circuit to a point where the DC component reduces to below a preset threshold. 
     
     
         20 . A phase control dimmer circuit comprising a load imbalance detector that, in use, controls an inductive load, the phase control dimmer circuit comprising a first switch and a second switch, the first switch in use determining a firing angle of the second switch, and the second switch, in series with the inductive load, effecting conduction of the inductive load in response to the first switch, the load imbalance detector comprising:
 a load DC component detector to detect a DC component in the inductive load by detecting a DC component across the phase control dimmer circuit;   a comparator for comparing a magnitude of the DC component detected by the load DC component detector with a reference voltage; and   a signal generating means for protecting the dimmer circuit by generating a circuit shut-down signal if the DC component exceeds a pre-set DC voltage threshold above the reference voltage.   
     
     
         21 . The phase control dimmer circuit as claimed in  claim 20  wherein said load DC component detector detects a DC sub-component in a positive cycle and detects a DC sub-component in a negative cycle and wherein said magnitude of the DC component is the difference between the respective DC sub-components. 
     
     
         22 . The phase control dimmer circuit as claimed in  claim 21  wherein said load DC component detector includes a first resistor divider chain for detecting said DC sub-component in the positive cycle, and a second resistor divider chain for detecting said DC sub-component in a negative cycle and wherein a divider junction of the first chain is connected to a first side of a capacitor and a divider junction of the second chain is connected to a second side of the capacitor, a voltage across which provides said magnitude of the DC component. 
     
     
         23 . The phase control dimmer circuit as claimed in  claim 22  wherein said comparator includes a first pnp transistor having its base connected to said first side of said capacitor and its emitter connected to said second side of said capacitor, and a second pnp transistor having its base connected to said second side of said capacitor and its emitter connected to said first side of said capacitor, and each respective collector connected to an input of said signal generating means.