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# Lu Jonathan Lab 1

last edited by 6 years ago

a. What resistance do you need to limit current to 30 mA (if using red LED) or 25 mA (if using yellow or green)? Be sure to state which color LED you are using. This resistance refers to the total resistance in series with the LED.

Hint: Make sure that you account for the forward voltage drop (Vf) of the LED that you're using.

Max forward drop is 2.5V. But typical is 1.85V. To be safe and conservative in our resistance value, we should assume to drops 2V. However, we are reckless, so lets say we push for a 2.5V drop across the LED. The max current through a red LED is 30mA. So if the diode is on, it eats up 2.5 V. So we have 5-2.5V= 2.5 that must drop across the resistor.  Since the max current it handles is 30mA, we would want a resistance value of approximately 2.5/30mA=83.3 ohms.

✔   Use 1.85V for VLED would be fine.

b. Is the resistance from question (a) a maximum or minimum resistance? That is, in which direction if you change the resistance (higher or lower) would the LED likely fail.

This is a minimum resistance value we got in the last problem. If we went lower, we would be getting larger current going through the diode and the circuit component will fail.

c. What is the resistance range of the potentiometer?

Around 0.2ohm to 9.8kohms is the range of the potentiometer. This was measured with the multimeter.

a. Does it matter what order the components of your circuit are arranged between power and ground? Why or why not?

No order in this case does not matter, because the current is still set by the resistor and the components are all in series. If the button is not pressed, it doesn’t matter where it is in a series circuit, the circuit will be open.

a. Using this battery, what is the minimum resistance required for use with your LED?

Absolute theoretical minimum resistor to add is (9-2.5)/0.03A=216.7ohms. To be conservative, lets add a few more ohms to say 220 ohms or something, though that is still cutting it kinda close. 