led power output indicator

[tshwary]

hi,

i have designed power supply with out parallel with zener diode of 5.1v,i need to put led for indating that there is load connected but i cant put it series with load since i really need 5.1v and putting series with load may cause a problem since i will no longer get maximum current since the more my load draw lot of current the voltage will decrease accross load since will be droping on led,

May you pleased help me with simple circuit i can put on output of my supply to show that load is connected and is not connected.

 

[jiripolivka]

As the load VOLTAGE is regulated by the 5.1V Zener, you need to indicate load CURRENT. You can do it before the Zener, by making the LED a part of the dropping resistor. If the load current is larger than ~10 mA (most LEDs run at ~10 mA), then connect a shunt in parallel with the LED, to adjust LED current while leaving the Zener at its operation point.
The LED will stay lit even without the load due to Zener current but with the nominal load its light will increase.

If you need a better indication than such simple circuit, make a comparator to sense voltage changes before the Zener, due to load current. The comparator threshold can be set so that a LED on its output will light up at a nominal load.

 

[tshwary]

Thanks for your suggestion