Cheap AliExpress relay tester risks short circuits with incorrect coil polarity

When you buy a budget Car Relay Tester from AliExpress, you expect it to function within standard automotive specs. However, this specific unit fails to meet the DIN 72552 standard, which dictates terminal polarity for vehicle wiring. Specifically, the tester reverses the polarity on terminals 85 and 86. While a simple electromagnetic coil doesn't care about direction, many modern relays include internal flyback diodes or LEDs. Testing a diode-equipped relay with reversed polarity creates a direct short circuit, potentially frying the tester's drive transistor or the relay itself.

Prerequisites and Tools

To correct this hardware bug, you need a basic understanding of DC circuits and PCB rework. Ensure you have a soldering iron, solder wick or a desoldering pump, a sharp hobby knife for track cutting, and some 22AWG jumper wire. A replacement 1A diode (like a 1N4007) is also recommended if you prefer not to reuse the SMD component.

Component Breakdown and Schematic

The internal logic relies on an 8-pin microcontroller powered by a buck converter. The relay coil is switched via a G122 Darlington Transistor.

// Conceptual logic for the testing sequence
for (int i = 0; i < 10; i++) {
    energize_coil(HIGH);
    check_contact_continuity();
    delay(100);
    energize_coil(LOW);
    delay(100);
}

The device passes roughly 100mA through the relay contacts during the test, which is sufficient for logic-level verification but won't stress-test the relay for high-current automotive loads.

Step-by-Step Polarity Fix

To swap the polarity, you must physically modify the PCB traces feeding the coil.

1. Cut the Tracks: Use a hobby knife to remove a 2mm section of the two main traces leading to the coil pins.
2. Cross the Wires: Solder two jumper wires in a "criss-cross" pattern to swap the positive and negative feeds.
3. Reverse the Diode: The back-EMF clamping diode must also be reversed. Desolder it and flip it 180 degrees, or relocate a new diode across the G122 Darlington Transistor collector and the positive rail.

Tips and Gotchas

Watch out for the "Reset" button; it actually functions by cutting power to the entire unit, forcing a hard reboot of the microcontroller. If you're testing 5-pin changeover relays, ensure the selector switch is in the correct position, or the unit will instantly report a failure.