If the positive side of a voltage source is connected to the positive end of the diode (the anode), and the negative side is connected to the negative end of the diode (the cathode), the diode will conduct current. If the diode is reversed, the current is blocked (up to a limit).
The arrow indicates the positive side of the diode, while the vertical bar indicates the negative side. You can think of it as the positive side flowing into the negative side, with the arrow indicating the direction of the flow.
If the pins have been trimmed, examine the outer casing of the LED. The pin nearest to the flat edge is the negative, cathode pin.
You can still test the diode without the Diode setting. Set the meter to the resistance (Ω) function.
If your meter has a Diode mode, you will see voltage displayed on the meter if the meter is connected positive-to-positive and negative-to-negative. If its the wrong way, nothing will be displayed. If you meter does not have a Diode mode, you will see very low resistance if the meter is connected positive-to-positive and negative-to-negative. If it’s the wrong way, you will see very high resistance, sometimes express as “OL”.
