Table 1: Jumper JP1/JP2 Status and LED Current
LED Current | JP1 Status | JP2 Status |
| | |
350mA | Out | Out |
| | |
700mA | In | Out |
| | |
1A | In | In |
| | |
Refer to the schematic in Figure 3. As the LED current returns back to the power supply, it develops a voltage across the sense resistor(s). Once this voltage reaches the base-emitter junction voltage of the NPN Q2, typically about 650mV, the transistor will conduct collector current. This current also flows through the photodiode inside the optocoupler, IC2. The optocoupler will transfer this feedback information optically across the primary-secondary boundary to the primary side of the supply thereby maintaining the necessary isolation. Thermistor RTH1, is a negative temperature coefficient device and compensates for changes in base-emitter voltage due to temperature change.
© 2007 Fairchild Semiconductor | Page 5 of 17 | Rev 1.1 April 2007 |
