Led s are always biased
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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

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.


Led s are always biased

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.


Led s are always biased

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.


Led s are always biased

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.


Led s are always biased

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.


Led s are always biased

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.


Led s are always biased

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.


Led s are always biased

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.


Led s are always biased

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.


Led s are always biased

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”.


Led s are always biased

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.


Led s are always biased

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.


Led s are always biased

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.


Led s are always biased

End of Presentation

A

Inc.

G


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