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I have this control board from the charging station of a robot lawnmower. The board broke after a failure with its power supply. The board was powered by a 12v lead-acid battery, with a DC-DC converter connected. The wires from the 12v lead-acid battery were damaged, and shorted, and things started melting.

The broken component has absolutely no markings. Beside the broken component are 2 similar components, also without any markings. I have de-soldered the component, and it has no markings on the underside either.

Can anyone identify the broken component, or alternatively take a guess as to what component it could be based on the location on the board? I would like to try to order a replacement component and see if I can make the board work again.

PCB with broken component

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    \$\begingroup\$ Probably a TVS included for transient protection on the incoming wires. They come in many voltages. It may function without it if nothing else is damaged, but it is intended to provided a level of protection. \$\endgroup\$ Jul 6, 2021 at 17:48
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    \$\begingroup\$ As I said "they come in many voltages". The voltage is important. Maybe it's the same voltage as the others and they could be measured by someone with the required equipment. The other ratings are fairly correlated with the package size. However, if it is not functioning now then probably something else is wrong. Such are the odds, IME. (A TVS is a diode or diode pair which breaks down at a controlled and specified voltage- under normal conditions typically it does little or nothing). The lack of markings on those suggests bipolar TVS. \$\endgroup\$ Jul 6, 2021 at 18:04
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    \$\begingroup\$ It would likely be 24 or 30V, something like that. If the voltage is too low it will burn up. If it's too high it won't do anything. But you might want to find what else is wrong first. \$\endgroup\$ Jul 6, 2021 at 18:08
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    \$\begingroup\$ Most likely the board is damaged and it is possible that it won't work after replacing the burnt component. There is also a chance that the new component burns too. \$\endgroup\$
    – Justme
    Jul 6, 2021 at 19:32
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    \$\begingroup\$ yeah, this looks like there wasn't a single short impulse that the TVS conducted and thus saved the rest of the circuit – there was a massive failure, leading to exploded case, molten solder… this only happens when there's massive current in places it shouldn't. So, this board is with a high probability completely broken, and replacing one thing won't fix that. \$\endgroup\$ Jul 6, 2021 at 23:53

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Without any marking we can't tell what it was. As mentioned in comments, it was very likely a Transient Voltage Suppressor (TVS) diode.

Putting some "random diode" there will do absolutely nothing except burning up too next time something bad happens. You should use a TVS diode, they protect the rest of the board against electrical spikes from motors, static discharge etc. Normal diodes are too slow and not nearly rugged enough for that.

For 12VDC supply you can use this one SM15T18A, which at a glance also appears to be the same SMC package as those already sitting there. Alternatively you could mount a through-hole TVS between the two pins of the connector. Picking one rated at 18V is a sensible choice for 12V supply. (Use something like 33V for 24VDC supply.)

Also please note that a very likely cause for this diode burning up in the first place is a current rush caused by a short circuit somewhere, not necessarily near the diode - it just takes the first blow. So you haven't found the actual error cause, just a symptom.

This could in turn mean that something else on the board is damaged too. Or perhaps that the board got exposed to water, which seems quite likely for a lawnmower. If you manage to repair, I'd recommend to keep it shut off when it's raining or when the grass is wet, or otherwise the same thing might happen again.

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  • \$\begingroup\$ I got the board functioning again with my "random diode" (which I have later identified as a 50V 1N4001) - which means that I have gotten confirmation that the broken component was a diode. My local supplier stocks 33v through-hole TVS diodes, I will replace my quick-n-dirty fix with a more suitable diode. The source of the problem is known and will be fixed. The PCB sits in the charging dock of a robot lawn mower, so it is designed to be operating in outside conditions 24/7, and the board is protected in the enclosure, water/moisture was not the original source of the problem. \$\endgroup\$
    – kalle
    Jul 10, 2021 at 12:20

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