What about the polarity of automotive‑grade diodes? It’s actually quite simple: on an automotive‑grade diode, the longer lead is the positive terminal, and the shorter lead is the negative. If the leads are the same length, the larger lead inside the diode is the cathode, and the smaller one is the anode. When using a multimeter, connect the red probe to the “+” terminal and the black probe to the “–” terminal. When testing an automotive‑grade diode, you won’t get a reading on the low‑resistance range; use the RX10K range instead. Touch the two probes to the diode’s terminals. If the resistance is low, the black probe is connected to the anode; if the resistance is high, the black probe is connected to the cathode. When an automotive‑grade diode is used in conjunction with TTL components, it’s generally necessary to place a 470 Ω current‑limiting resistor in series to prevent damage to the device.