EECE 312L Notes Experiment 6 Notes, Fall 2013

Experiment 6 LEDs and Zener Diodes Id= Ise(Vd/nVT) VDD=VD+IDR If we replace the diode by its incremental resistance we would have by KVL: ID (R+ = VD- VDD If we know any thing we can solve for R. What happens when we reverse the diode? The power dissipated in the reverse direction doesn’t have any useful purpose. It can be eliminated by putting the diode or LED in the forward direction, so this will rectify the input voltage to only positive cycle and would eliminate the reverse direction power dissipation. We can also eliminate by superimposing the DC component of the voltage source to the AC source. Connect the circuit of Fig. 1 using the red light emitting diode (LED). Starting from a value of 0.5V, we increase V in steps of 0.5 V and we measure the LED voltage and current for each VS. We do not exceed 30 mA of current in the diode. We connect VS power supply starting from the value equal to 0.5 we increase it by 0.1 respectively. Measure VLED and ILED by the digital multimeter.   Diodes operating in the breakdown region can be used in the design of voltage regulators are circuits that provide constant DC output voltage in the changes in this load current and the system power- supply voltage. The Zener diode regulates the load voltage for the range of load resistance where the Zener voltage is constant.  By definition the line regulation is the ratio of the change in VZ to the change in VS, provided that the diode is in the Zener region. S Z V V gulation Line    Re  Its AC component is  By definition the load regulation is the ratio of the change in VZ to the change in ILoad, provided that the diode is in the Zener region. LOAD Z I V gulation Load    Re  The maximum current in a Zener diode is determined by taking the maximum value of the input voltage at no load. So the maximum current in a Zener diode is determined using: Z Z R P I max max   The minimum current in the Zener diode is determined by taking the minimum value of the input voltage with a very small resistance connected to the load. Therefore, the current in the load is at its maximum value thus the minimum Zener current in the diode can be calculated by the equation: Iz min = I total – I loadmax Thus, operating in the region close to the knee region allows us to determine the minimum value of the Zener current. Show that for a Zener voltage regulator, the value of the resistor R should be chosen using the equation: For the Zener diode to conduct VZK, IZK, and VS must be at a certain minimum value. Thus, the load current is at its maximum value. From the Zener voltage regulator circuit we get: VS = RIS + VL VS = R (IZK + IL) + (VZK + rZ IZK) Due to the above discussed minimum current limit for the Zener diode we replace VS by its minimum value and IL by its maximum value and solving for R we obtain the equation:   We connect the circuit and measure VZ when VS is 15 V by a digital multimeter.  Reduce the value of VS in steps of 0.5 V, and measure V Z and I Z VZ is measured by the digital multimeter. To measure the IZ we remove on leg of the resistor and place it somewhere else. Measure IZ by the multimeter placed across the two legs of the resistor.  Measure VZ. What is the DC (average) value of VZ? What is the AC component in VZ? By the function generator:  Press on the offset bottom  Put all the other factors (amplitude…) DC Vz value is measured by the mean value on the oscilloscope.  AC Vz values is the ripple value .We measure it by placing the horizontal axis and adjust the VOLT/DIV and SEC/DIV settings to get stable traces. Joujou