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LED’s are always biased

A. Inc. G. LED’s are always biased. This presentation is partially animated. Only use the control panel at the bottom of screen to review what you have seen. When using your mouse, make sure you click only when it is within the light blue frame that surrounds each slide.

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LED’s are always biased

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  1. A Inc. G LED’s are always biased This presentation is partially animated. Only use the control panel at the bottom of screen to review what you have seen. When using your mouse, make sure you click only when it is within the light blue frame that surrounds each slide.

  2. Pre-presentation Self Assessment Activity The comfort level with your response to the following two question assessment tool should indicate if the presentation that follows will increase you knowledge base on the topic outlined by the questions in this tool.

  3. TSH PSL PSH LSL TSL FSL FSH LSH Pre-presentation Self Assessment Activity 1. Include an LED in the control scheme such that it will be ON when all of the sensors are in alarm. 2. Size the current limiting resistor for this application to prevent the LED from burning out if the controller is a TTL system.

  4. anode cathode Note: Anode terminal The terminals of a LED are identified as an anode and a cathode. Cathode terminal A useful light bulb No light when reverse biased. The Light Emitting Diode The light emitting diode is a small semiconductor light bulb that is useful as a status indicator Like any diode, it can be arranged in a circuit so that it is “forward biased” or “reverse biased”. When the LED is forward biased it will emit light. LED’s come in various flavors. You can get one that emits red, green, yellow, or blue light when forward biased. In all of these LED’s they emit no light when they are reversed biased. light when forward biased.

  5. anode cathode R 1 A useful light bulb Possible Light Emitting Diode circuit arrangements A) the LED in its passive state is wired so that anode is more positive than its cathode. 5 volts Light is ON when button is not depressed. LED is forward biased. LED is forward biased. LED is reverse biased.

  6. anode cathode the LED in its passive state wired so that the anode is not more positive than the cathode. B) 5 volts R R 1 1 A useful light bulb Possible Light Emitting Diode circuit arrangements A) the LED in its passive state is wired so that anode is more positive than its cathode. 5 volts Light is OFF when button is not depressed. Light is ON when button is not depressed. LED is forward biased. LED is reverse biased. LED is forward biased. LED is reverse biased.

  7. anode cathode the LED in its passive state wired so that the anode is not more positive than the cathode. B) 5 volts The duel role of R 1 a) Pull up resistor - pulls the LED anode above the voltage at the LED cathode. R R 1 1 b) Current limiting resistor - restricts the current flow through the LED. A useful light bulb Possible Light Emitting Diode circuit arrangements A) the LED in its passive state is wired so that anode is more positive than its cathode. 5 volts Light is OFF when button is not depressed. Light is ON when button is not depressed. LED is forward biased. LED is reverse biased.

  8. anode cathode 5 volts Light is ON when button is not depressed. V 4 R R R 3 2 1 LED is forward biased. A useful light bulb Possible Light Emitting Diode circuit arrangements C) the LED in its passive state is wired so that anode is more positive than its cathode. Note; (a) V4 is also the voltage drop across the LED (b) V4 depends on the equivalent resistance at the node.

  9. anode cathode 5 volts Light is ON when button is not depressed. (d) In most situations, LED is wired in series with single resistor. V 4 R R R 2 3 1 anode anode R R + + LED LED V V cathode cathode LED is forward biased. LED is forward biased. LED is reverse biased. A useful light bulb Possible Light Emitting Diode circuit arrangements C) the LED in its passive state is wired so that anode is more positive than its cathode. Note; (a) V4 is also the voltage drop across the LED (b) V4 depends on the equivalent resistance at the node. (c) Forward biased LED’s has a very low resistance to current flow.

  10. 5 Volts = R R LED LED 0.016 Amps is approximately equal to 330 ohms. R LED LED process status indicator Circuit constraints An LED emits light when it is forward biased and there is enough current flow to allow us to see the light. 5V To protect the LED, the current is always limited. As a good first approximation, the LED is presumed to contribute no resistance to current flow when it is forward biased. Most people can see the light from an LED when the current is limited to 0.016 amperes. TTL devices provide 5 volts at logic “1”.

  11. 11 011 011 FSL PSL TSH PSH TSL LSL 1 1 0 1 1 0 1 1 R LED FSH LSH LED process status indicator Typical indicator application for LED If 333 (base 8) is a code that represents the signals from a set of 8 sensors that indicate that it is important to turn on an active low motor, design a circuit that will turn on an LED when that situation is present. Alarm status code from sensors LED will only be forward biased when 333 (base 8) is represented at the inputs of the logic circuit. LED is ON when it is forward biased.

  12. TSH PSL PSH LSL TSL FSL FSL PSL 0 1 1 1 1 0 1 0 R TSH PSH LED TSL LSL FSH FSH LSH LSH Binary pattern when all sensors are in alarm Post Presentation Self Assessment Activity 1. Include an LED in the control scheme such that it will be ON when all of the sensors are in alarm. LED is ON when it is forward biased.

  13. FSL 5 Volts = PSL 5V 0 1 1 1 1 0 1 0 R R R TSH PSH LED LED LED 0.016 Amps TSL LSL FSH LSH Logic 1 is nominally 5 volts for TTL devices. R is approximately equal to 330 ohms. LED Post Presentation Self Assessment Activity 2. Size the current limiting resistor for this application to prevent the LED from burning out if the controller is a TTL system. 0.016 Amperes is a typical design constraint current limit.

  14. End of Presentation A Inc. G

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