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(I'm new to electrical engineering, both the website and the topic, so don't hesitate to correct me)

I'm building a very tiny form factor transformerless power supply with 5 V output.

I was wondering if I could use a voltage regulator as a fuse so if one of the cap or resistor fails and the circuit starts outputting something like 230 V (more than desired volts and more than rated limit of voltage regulator), the voltage regulator will blow and stop the current from passing.

I've heard that normal fuses are temperature sensitive that's why I want to use a voltage regulator as a fuse.

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  • \$\begingroup\$ tiny as mobile charging brick like if one of those mobile charger could charge a laptop computer \$\endgroup\$ – DohnJoe Apr 21 at 8:37
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    \$\begingroup\$ What's the application for this supply? Do you understand that no part of this kind of supply must ever be exposed to human contact, even the 5V output? \$\endgroup\$ – replete Apr 21 at 8:40
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    \$\begingroup\$ @DohnJoe don't you think that decades of engineering could build smaller power supplies than a self-proclaimed beginner? I think you're seriously underestimating the complexity of the problem, and the quality and space-effectiveness of modern day laptop power supplies. \$\endgroup\$ – Marcus Müller Apr 21 at 8:41
  • \$\begingroup\$ I'm not looking forward to making a product or something. i just wanted to do it as a hobby for charging $2 li-ion cells (also found in a laptop battery) \$\endgroup\$ – DohnJoe Apr 21 at 8:49
  • \$\begingroup\$ Getting 5V is only part of the solution if you're looking to charge Li-ion batteries. Just buy a charger. \$\endgroup\$ – Finbarr Apr 21 at 9:36
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I'm building a very tiny form factor transformerless power supply with 5 V output.

Oh-oh. This could be bad.

I was wondering if I could use a voltage regulator as a fuse ...

No. You can't predict whether the device will fail open-circuit or short-circuit. The design would not be designed to "fail safe".

I've heard that normal fuses are temperature sensitive that's why I want to use a voltage regulator as a fuse.

The principle of operation of a fuse is thermal. As the current increases the fuse wire temperature increases. At some current for a certain time the fuseewire will melt and the output will be disconnected. This is an extremely simple and well-proven technology that is guaranteed to fail safe.


Please, please be careful with this project and have the sense to walk away from it if you realise that you do not have the expertise required. The output must be considered to be at mains voltage. The project seems pointless when you can purchase a 5 V, 2 A mobile phone charger with full mains isolation so cheaply.

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A voltage regulator is not a fuse; it doesn't work like one in any way. Even the ones with overcurrent shutoff aren't meant to protect against arbitrary overcurrent, just against the narrow range of current defined in the datasheet.

So, no.

The fact you're asking this might mean you really should think twice before working on mains voltage. It's dangerous. Also, modern switch-mode power supplies are very space-efficient beasts: without years of engineering experience, you really won't be building anything similarly small.

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  • \$\begingroup\$ Thanks, i just wanted to charge $2 li-ion batteries, but they explode if something goes wrong but also is using transformerless power supply worth it? \$\endgroup\$ – DohnJoe Apr 21 at 8:43
  • \$\begingroup\$ Transformerless supplies are basically never worth it except for extremely low margin mass produced goods. For a hobby project they are a totally unnecessary false economy and hazard. \$\endgroup\$ – replete Apr 21 at 8:52

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