Connecting LEDs in parallel, but in opposite directions, allows you to visually see which direction (polarity) the power supply is attached to the circuit at any given time.
- Forward biased (FB) LED lights up.
- Reverse biased (RB) LED blocks current. Don’t apply much more than 9V to a RB LED
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- Forward biased (FB)- Anode more positive than cathode.
- Reverse biased (RB)- Cathode more positive than Anode.
- As long as it hasn’t been trimmed, the anode is usually the longer lead of the LED while the cathode is the shorter lead. Cathode may also have a flat edge along the rim.
- LEDs are a type of diode (Light Emitting Diode). – Diodes conduct relatively easily and light up while forward biased. They do not conduct while reverse biased unless you exceed their breakdown voltage, which will destroy the LED. I don’t think the breakdown voltage is much higher than 9V for the commonly used indicator LEDs.
- Protective resistor is needed. The value depends on the voltage that will be applied and the forward voltage of the LED. I usually use at least 220 Ω for a 5V supply, 470Ω for a 9V supply and 1k (1,000Ω) for a 12 volt supply. You can go as high in resistance as you want, but the LED will get dimmer the higher you go.
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