You are an electrical engineer working on a commercial product. The User interfa

Question

You are an electrical engineer working on a commercial product. The User interface (UI) group would
like you to add an LED to the front panel to indicate the power supply is on. The power supply engineer
said she can provide up to 24 mA from the 15 Volt power supply for the indicator. The manufacturing
engineer said you can use these values of resistors 2.2M Ohm 1/8 Watt, 100 Ohm, 1/4 Watt, 1000 Ohm
1/8 Watt. The manufacturing engineer also said you can use a maximum combination of three resistors
and that the LED you must use has a terminal Voltage of 3 Volts at 20 mA. Design a circuit that fits
within the requirements of the UI, Power supply, manufacturing specs, and wont go up in smoke. For
the answer draw the schematic with the resistor values. Why might the manufacturing engineer care
about limiting the number and values of resistors used to implement this?

Explanation / Answer

We know that the given supply is of 15V, 24mA max and we need to get 3V, 20mA

For reducing current, we need a parallel connection of resistors and for reducing the voltage , a series connection is required.

By hit and trial, we find out that the 1000 ohm, 1/8 watt resistor leads to a voltage drop of 11.18V

For a voltage of 3V, we can use the 100 ohm resistor in parallel with 2.2M ohm resitor, so its wattage will be less than rated and hence safe. So the final circuit is the power supply, 1000ohm in series and 2.2M|| with 100ohm in || with the LED

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