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university:courses:engineering_discovery:lab_8 [07 Sep 2016 21:25]
Jonathan Pearson
university:courses:engineering_discovery:lab_8 [03 Jan 2018 19:36]
Doug Mercer [Observations and Conclusions]
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   - Repeat the above for the green and red LEDs   - Repeat the above for the green and red LEDs
 ==== Theory ==== ==== Theory ====
-An ideal diode would act as a short circuit to currents flowing in one direction and an open circuit to currents flowing in the other direction. ​ When a diode is conducting current it is said to be //forward biased//, and when behaving as an open circuit it is said to be //reverse biased//​. ​ When forward biased, practical diodes have a current versus voltage (I/V) characteristic that is exponential,​ and beyond the knee of the exponential the slope becomes very steep. ​ Diodes are often forward biased to operate on the steep vertical parts of their I/V characteristics where they exhibit small incremental resistances;​ incremental resistance is defined as the reciprocal of the slope of the I/V characteristic in the immediately ​vicinity of the bias point. ​ When the diode is reverse biased, a very small //leakage current// flows.+An ideal diode would act as a short circuit to currents flowing in one direction and an open circuit to currents flowing in the other direction. ​ When a diode is conducting current it is said to be //forward biased//, and when behaving as an open circuit it is said to be //reverse biased//​. ​ When forward biased, practical diodes have a current versus voltage (I/V) characteristic that is exponential,​ and beyond the knee of the exponential the slope becomes very steep. ​ Diodes are often forward biased to operate on the steep vertical parts of their I/V characteristics where they exhibit small incremental resistances;​ incremental resistance is defined as the reciprocal of the slope of the I/V characteristic in the immediate ​vicinity of the bias point. ​ When the diode is reverse biased, a very small //leakage current// flows.
  
 In the lab we operate three different color LEDs and one 1N914 signal diode in the forward bias mode and examine the exponential current versus voltage characteristics of each.  They all exhibit exponential behavior, with some differences in where the knee occurs and how steep the curve becomes beyond the knee.  The forward bias voltage at the exponential knee is often referred to as the //cut-in// voltage because the diode is just beginning to conduct current and is "​cutting in" from an effective open circuit to a low resistance. ​ Because the slope of the I/V characteristic is so steep beyond the knee, the change in voltage is very small with respect to change in current. ​ Because of this, the diode voltage is often approximated to be a constant over a wide range of currents, which makes designing with diodes simple for many circuits. ​ The cut-in voltage and diode characteristic are generally not tightly controlled, so designing with diodes often involves approximations. In the lab we operate three different color LEDs and one 1N914 signal diode in the forward bias mode and examine the exponential current versus voltage characteristics of each.  They all exhibit exponential behavior, with some differences in where the knee occurs and how steep the curve becomes beyond the knee.  The forward bias voltage at the exponential knee is often referred to as the //cut-in// voltage because the diode is just beginning to conduct current and is "​cutting in" from an effective open circuit to a low resistance. ​ Because the slope of the I/V characteristic is so steep beyond the knee, the change in voltage is very small with respect to change in current. ​ Because of this, the diode voltage is often approximated to be a constant over a wide range of currents, which makes designing with diodes simple for many circuits. ​ The cut-in voltage and diode characteristic are generally not tightly controlled, so designing with diodes often involves approximations.
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   *  Because the diode voltage changes very little for large current changes, the diode voltage can be approximated as constant for a wide range of currents, simplifying design   *  Because the diode voltage changes very little for large current changes, the diode voltage can be approximated as constant for a wide range of currents, simplifying design
   * Diode I/V characteristics are affected by self heating   * Diode I/V characteristics are affected by self heating
 +
 +**Return to [[university:​courses:​engineering_discovery|Engineering Discovery Index]]**
university/courses/engineering_discovery/lab_8.txt · Last modified: 03 Jan 2018 19:36 by Doug Mercer