Question

In this procedure, you get to write and execute an experimental procedure to find a Thévenin equivalent model for a Light Emitting Diode (LED) The simplest model for a diode is as a switch that is on (short circuit) when current tries to flow in the direction of the arrow in the diode symbol, and off (open circuit) when the current ties to flow in the direction opposite to the arrow. This model doesn't work all that well for an LED because the LED emits more light as the current passing through it increases. The light requires power, and short circuits do not model power dissipation. So we need a model with a resistance and/or source to dissipate the power that goes into the light. A resistance and a voltage source is a Thévenin Equivalent. a. (20 points) Write an experimental procedure to determine a Thévenin equivalent model for an LED that is in the On (light emitting) state. Include a circuit diagram and the steps of your procedure b. (5 points) Execute your procedure, and determine the parameters of your model. Draw a circuit diagram of your model with the parameters. Check your model against measured values. Check your model when the diode is in the off state. Comment on how good a model you have. Bring the circuit you used to find the model to class and show it to your TA NOTE: If you do not show your circuit to your TA, you will get a zero on this Procedure A few possibly helpful remarks The LED leads are indicated by both the length of the leads and the flat spot on the rim of the diode. The short lead is next to the flat spot, and is the cathode (K), the lead at the head of the arrow, which is also the end of the diode symbol with the bar (Figure P4-1). The long lead, the one further from the flat spot, is the anode (A), the lead at the tail of the arrow of the diode symbol. The diode will allow current to flow from the anode to the cathode Figure P4-1 Diode circuit symbol As the voltage vak from anode to cathode is slowly increased from 0 to 2.0 V, the LED will glow dimly and then suddenly become much brighter. This sudden transition marks the turn-on point of the diode, called the turn-on voltage. The model you are looking for is for the On state, i.e. for values of vak greater than the turn-on voltage It's a bad idea to have the diode across the battery by itself. It WILL burn out. Keep at least a 1 kS2 resistance in series with the diode Hint: setup the circuit as following, find the turn-on voltage first. Next is to find Rth and voc from the Thévenin equivalent model for the LED. You need two equations of KVL to solve them. th 9 V 9 V 0

Answer 1

The simplest model for a diode is as a switch that is on when current tries to flow in the direction To find the evening equivalent model first draw a circuit diagram for both the original so, V_ak = V_th + i^* Rth connect the Led with the battery v & a reservation R series use an of more than v ohm for get a set of values sigma i, limiting v_ak to more than 2.0 v \r\n change the values of R to get another set of values v_ak limiting v_ak to more than 2.0 v so that (epsilon) is in the ON state use the v_ak & z gotton in state 2 & 3 to solve for vth, Rth sigma get the quitrent therefore v_ak = Vth + i^* ak = V_ak - 2.04 v = 7.45 mA V_esistor of 1.1 k & a battery of eqn V_ak = 2.024 v T = 6.798