Brief polarity indicator LEDs circuit

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.

Simple polarity indicator circuit schematic diagram by electronzap electronzapdotcom
Simple polarity indicator circuit schematic diagram by electronzap electronzapdotcom
  • 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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Quick power supply polarity indicator LEDs circuit schematic to breadboard build

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Main points:

  • 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.

Brief circuit schematics with short video – List of pages

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