Electronic – Causes for a burned up flyback diode on a relay

circuit analysisdiodesrelayrepair

I'm repairing a control board for a premium washing machine (model #s at bottom for anyone that's interested). The machine was displaying an error code for relay failure. I took the board out of circuit, injected voltage on the DC bus, and detected two shorted 1N4148 "T4" diodes, each in parallel to a 12v SPST-NO relay (marked with arrows below). I replaced them. Each relay switches one side of the water heating element (+ or -). I have not tested the repair yet, but the machine does work without issue with a substitute board.

Trying to figure out what may have caused the diodes to short?

Note: After repair

Machine: MHW6000X
Relays: Omron 12V G2RL-1A-E
Control Board: W10406607 / W10406604 (alternates, W10342325 / W10342327, W10388205, W10427972, W10384506, W10354088, W10406635)

I found these short diodes using isopropyl alcohol (99%) and a microscope (loupe or optivisor works for less $$). Here's a video from my microscope camera showing the alcohol


This is one of the larger "neighboring" diodes, MURS160T3G "U1J" in SMB package. Same relay, so perhaps a strange bit of design here.

neighboring diodes

Best Answer

enter image description here

Figure 1. Relay datasheet extract.

Checking the relay datasheet we find that the coil current should be about 33 mA. At the instant of switch-off the coil inductance will maintain that 33 mA through the flyback diodes.

enter image description here

Figure 2. Extract from the Vishay 1N4148 datasheet.

Checking the Vishay (chosen at random) datasheet we see that 33 mA is well within specification.

I would conclude that the designers did their job OK but that the purchasing department may have found some bargain diodes.

Update for "T4" diodes:

The Diodes Incorporated datasheet rates their diode at 250 mA continuous. That isn't the problem either.