Sunday, December 6, 2015

Electrical theory 1.3c - LEDs leg type

Here I'll explain in simple terms about LEDs with legs or terminal pins.

This type of LED can be:
  • single colour = 2 legs; These are connected in series;
  • multi colour flashing = 2 legs. These are new on the market;
  • multicolour = 3 or 4 legs. These emit different colours depending on which legs are energised and LEDs are connected in parallel. They are made as either common anode or common cathode to suit type of switch. See following section for details.
LED operation:

·    For 3mm, 2 leg LEDs, 15mA current will give long life. For 3mm, 3 or 4 leg LEDs, 10-15mA each primary colour will give long life.
·    For 5mm, 2 leg LEDs, 20 to 25mA current will give long life. For 5mm, 3 or 4 leg LEDs, 15-20mA each primary colour will give long life.
·    Refer to manufacturer’s data for 10mm LEDs and high power LEDs which may have multiple light emitters and current maxima.
·    Higher currents will quickly reduce life and give minor increase in brightness.
.    The reverse (connection) breakdown voltage for a LED is typically 5V - exceed this and bye bye LED!

The table below shows typical voltage drop across each LED colour and brightness (MCD) for 5mm size. This voltage drop is used in calculations. However, the supply voltage must exceed (total LED 'typ' voltage +1.1V) to give good brightness. A resistor is used to limit current and protect the LED. Some cheap lights don’t have correctly sized resistors and LEDs continually fail. We'll look at correct sizing of LEDs and resistors in the advanced theory section.

LED characteristics –

Light Colour
Forward voltage at If= 20mA
Min V
Typical V
Red
 
1.7
2.0
Yellow
 
1.8
2.1
Orange/ Amber
 
1.8
2.1
Green (low)
 
2.0
2.2
Green (high)
 
3.0
3.1 to 4.0
Pink
 
3.0
3.2
Purple
 
3.0
3.2
White
 
3.0
3.3
Blue
 
3.2
3.4

 

Notes:

1 LEDs can have low brightness (< 1000 mCd at 20mA) and angle of light directionality of 85 degrees;

2 LEDs can have high brightness (> 5000 mCd at 20mA) and angle of light directionality of 40 degrees;

3 Green LEDs can be either low or high brightness and have a range of forward voltages as shown.  

4 Reverse breakdown voltage is about 5V. Exceeding this usually damages the LED;

5 LEDs with internal flashing capability may have higher forward voltages than above;

6 Always use manufacturer's data if available.



NEXT >> LED Connections
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