Inside a vehicle relay tester with a polarity issue

This is a 12/24V automatic relay tester for vehicles that cycles relays ten times while looking at their contact status and gives an indication of their functioning and reliability. Note that the unit tests the contacts with a current of 100mA and they may not perform the same under high load current.

This one has an interesting quirk that I only spotted because I tested it with some relays that have internal LED indicators. They didn't light while the relay was being tested, and it turns out that the unit does not comply with the DIN 72552 standard for vehicle wiring. The polarity across the coil. is wrong, and while this is not an issue with most ordinary relays, some have built in diodes across the coils to clamp the high voltage spike caused by the collapsing field when they are de-energised.

If a relay with one of those diodes is tested in this, then the result will be a short circuit and possible damage to the unit and the diode in the relay itself. A high current short can cause diodes to fail short circuit, and that could render the relay unusable and potentially cause damage to a circuit it was then plugged into.

The fix on this unit is fairly straightforward if you choose to do it.

If you want to get one of these then this unit was part of a bundle deal on AliExpress (not a sponsor).
Here's an affiliate link that shouldn't affect the price you pay, but as always, shop around for a good deal.
https://s.click.aliexpress.com/e/_c3gYiMvz

If the linked item has sold out and the listing disappeared then some suitable search keywords are "car relay tester".




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