this post was submitted on 04 Jul 2023
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Help finding value of burnt resistor. (lemmy.nz)
submitted 1 year ago by WaltzingKea@lemmy.nz to c/askelectronics@discuss.tchncs.de
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Mates bread maker stopped working so I had a look inside and saw this burned resistor.

I'm guessing the heat changed the colors a bit so wondering if anyone has experience in reading cooked resistor values.

I removed it from the PCB and measured it at 403 Ohms.

Thanks for any help.

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[โ€“] rouqee@discuss.tchncs.de 1 points 1 year ago (1 children)

Thank you for the detailed insight! I miss some basics in electronics but am eager to learn how to test and fix circuits.

Years ago I tried to repair an old keyboard/synthesizer by cleaning it and replacing leaked/bloated capacitors. Unfortunately the onboard sound memory could not be loaded anymore or was wiped entirely as far as I understood. But due to lack of knowledge (me and community that time) it was too complex to got the keyboard up and running again. It's sometimes sad to loose good hardware...

Back to the resistor/thread: I can't imagine a resistor to be the source of the problem. Isn't it more possible that a capacitor wears out or a transistor cooling fails?

[โ€“] Saigonauticon@voltage.vn 3 points 1 year ago

Those things are indeed more common!

However, if the circuit was in an abnormal state (e.g. the contact with the case), then a resistor could very well blow. It would not be surprising if it took some other components down with it, and that this damage is not obvious yet. "The transistor blows to protect the fuse" is a common fail-state, facetiously stated.

Another possibility is just... bad design. You could call me adequate at circuit design (I mostly design prototypes, not finished systems that have to last thousands of hours), but regularly see commercial products designed poorly with some stupid point of failure. For example, using a 1 watt resistor that is dissipating close to 1 watt, instead of designing a more efficient system that doesn't require dissipating heat at all.

I spend a lot of time answering questions for people just getting started. Probably 75% of them boil down to a few things. Here is that list in case amusing / useful:

  1. Relays are not a great solution in general, and there are many better alternatives (MOSFET, SSR, etc).
  2. Output impedance matters: you can't power a huge motor off a microcontroller pin.
  3. Back-EMF from inductive loads can burn out your control system unless you add a protection diode.
  4. Lead acid batteries aren't a magic solution to power everything. Especially automotive ones. Understand and use lithium ion.
  5. Connecting LEDs in parallel then adding a single resistor will lead to failure pretty quickly.
  6. Generally, don't pass significant power through a switch. Use the switch to control the state of a power MOSFET or similar.
  7. Button debouncing.

Most of the rest is refusing to do other people's homework, help people build weapons, or do unwise things with mains power / high voltage / centrifuges. Occasionally people ask me really interesting questions though, so I don't mind that the interactions are a bit scripted the rest of the time! I've noticed on Lemmy I've gotten much more interesting questions so far!