Method and apparatus for sensing current and voltage in circuits with voltage across an LED

Abstract

Methods and apparatuses to sense current and voltage in circuits with voltage drop. In one aspect of the invention, a circuit includes a transistor coupled to a voltage reference. A light emitting diode (LED) is coupled to be driven by the transistor in response to a voltage across an output of the circuit. A first diode is coupled to the LED. A first resistor coupled to the first diode and the LED. The LED is coupled to be driven in response to a voltage drop across the first diode and the first resistor to sense a first current directed through the first diode and the first resistor.

1 . A circuit, comprising: 
 a transistor coupled to a voltage reference;    a light emitting diode (LED) coupled to be driven by the transistor in response to a voltage across an output of the circuit;    a first diode coupled to the LED; and    a first resistor coupled to the first diode and the LED such that the LED coupled to be driven in response to a voltage drop across the first diode and the first resistor to sense a first current directed through the first diode and the first resistor.    
   
   
       2 . The circuit of  claim 1  wherein the transistor is coupled to drive the LED in response to the voltage across the output of the circuit exceeding a sense voltage.  
   
   
       3 . The circuit of  claim 1  wherein the transistor comprises a bipolar transistor having a base coupled to the voltage reference, the bipolar transistor to drive the LED when the voltage across the output exceeds a sense voltage.  
   
   
       4 . The circuit of  claim 1  wherein the voltage reference is provided by a Zener diode coupled to the output of the circuit.  
   
   
       5 . The circuit of  claim 1  wherein a sense voltage includes a sum of the voltage reference provided by a Zener diode coupled to the output of the circuit and a forward base emitter voltage of bipolar transistor having a base coupled to the voltage reference.  
   
   
       6 . The circuit of  claim 1  wherein the LED is included in an optocoupler.  
   
   
       7 . The circuit of  claim 1  wherein an optocoupler including the LED is coupled to provide feedback from an output of a power supply, the optocoupler coupled to provide the feedback to a power supply controller included in the power supply.  
   
   
       8 . The circuit of  claim 7  wherein the power supply is a switched mode power supply.  
   
   
       9 . The circuit of  claim 1  wherein the first diode comprises a PN junction diode.