The document discusses LED characteristics and how to calculate the resistor value for an op-amp circuit to drive an LED. A typical LED has a forward voltage drop of 2V and requires 10mA of current to emit light. The op-amp output can swing between +/-14V with +/-15V supplies. The resistor value for Rlim should be (14V - 2V)/10mA = 1.2kΩ to provide 10mA through the LED when the op-amp output is saturated.

1. A typical LED allows current to flow in only one direction (from anode to cathode - see Figure 1
below), has a voltage drop of around 2V when forward biased, and requires approximately 10mA
of forward bias current to emit light (less current and it will be dim, more current risks damage).
Consider the op-amp circuit of Figure 2 in the lab. If the output of the op-amp is saturated near
either its positive supply voltage (+15V) or negative supply voltage (-15V), what resistor value
should be chosen for R lim such that the current through the LED is near 10mA? Hint: does R lim
= (14V - 2V)/10mA make sense? . Note the +-14V output voltage swing is given in the op-amp's
data sheet when supply voltages of +-15V are used.
Figure 1: LED diagrams
A typical LED allows current to flow in only one direction (from anode to cathode - see Figure 1
below), has a voltage drop of around 2V when forward biased, and requires approximately 10mA
of forward bias current to emit light (less current and it will be dim, more current risks damage).
Consider the op-amp circuit of Figure 2 in the lab. If the output of the op-amp is saturated near
either its positive supply voltage (+15V) or negative supply voltage (-15V), what resistor value
should be chosen for Rlim such that the current through the LED is near 10mA? Hint: does Rlim
= (14V - 2V)/10mA make sense?. Note the +-14V output voltage swing is given in the op-amp's
data sheet when supply voltages of +-15V are used. Figure 1: LED diagrams
Solution
fIG 2 is not given