Fuse for 12V DC? - Page 1
Forum section: electronics / beginners

author: tester43
author_rank: Regular Contributor
timestamp: May 11, 2020, 06:40:47 pm
content: Hi,So I did a prototype. It's atmega 328 + screen + some buttons + stepper driver etc. and other. Powered by small 12V 500mA power supply.I was designing a PCB for manufacturing and then I noticed: I did not put the fuse.Would you consider fuse at 12V as rational? I wanted to put 0,5A fuse... because why not. But here is the question. Which kind of fuse is the one to go? I spent around 1h of random googling and did not find any reasonable answer. Seems like all fuses are designed to work in AC 100-400VAC applications.What is your opinion?

author: ArthurDent
author_rank: Super Contributor
timestamp: May 11, 2020, 06:44:29 pm
content: As long as the AC fuse voltage rating is much higher than your DC supply voltage, you are okay. There is one problem is using DC and AC fuses of the same current rating. A DC fuse may have a wider gap inside when blown than an AC fuse of the same voltage rating so a higher voltage AC fuse would work in a low voltage DC circuit but an AC fuse of the same low rating might arc over internally if used on DC.

author: ogden
author_rank: Super Contributor
timestamp: May 11, 2020, 06:46:31 pm
content: If whatever number of hours is not enough for you to lear then try more.You better dont use low voltage fuse for high voltage application, yet you can do opposite. What matters for fuse is current. Voltage is just isolation concern.

author: kallek
author_rank: Regular Contributor
timestamp: May 11, 2020, 07:52:52 pm
content: Quote from: ArthurDent on May 11, 2020, 06:44:29 pm+1One thing to also know about fuses is breaking capacity. When short circuit current is high, ceramic HRC fuse is better than glass fuse, because it is filled with sand to prevent arcing.

author: ogden
author_rank: Super Contributor
timestamp: May 11, 2020, 07:58:29 pm
content: Quote from: kallek on May 11, 2020, 07:52:52 pmBS. 12V 0.5A can't create any arcing. Any fuse will do the job.

author: tester43
author_rank: Regular Contributor
timestamp: May 11, 2020, 07:59:40 pm
content: my question was more of a type: is adding a fuse to 12V device consuming 0,2A, powered by external 12V 0,5A power supply an option or must have ?

author: ogden
author_rank: Super Contributor
timestamp: May 11, 2020, 08:02:14 pm
content: Quote from: tester43 on May 11, 2020, 07:59:40 pmIf you are experimenter then fuse is good idea - to save you from financial loss if you do any mistake. Otherwise you don't need fuse because 12V 0.5A is hardly any fire hazard.

author: kallek
author_rank: Regular Contributor
timestamp: May 11, 2020, 08:09:07 pm
content: Quote from: ogden on May 11, 2020, 07:58:29 pmWhat you have said is correct. But it is good to know when selecting primary fuse.Quote from: tester43 on May 11, 2020, 07:59:40 pmMost likely power supply has already some kind of protection, but extra protection doesn't hurt and is good idea.

author: tester43
author_rank: Regular Contributor
timestamp: May 11, 2020, 08:14:43 pm
content: thank you all for the help.

author: Gregg
author_rank: Super Contributor
timestamp: May 11, 2020, 08:32:16 pm
content: Fuses are not current limiters; they are safety devices that take some time to open the circuit. They merely prevent things that go wrong from going terribly wrong; (full credit to Big Clive for the term ‘Terribly Wrong’.)So if you trust your power supply to current limit at 0.5A and your device operates at 0.2A you probably don’t need a fuse for a prototype; this is subject to how expensive the device may be and a host of other parameters.Fuses specifically designed for DC have better means of quenching an arc when the fuse opens. For instance, high amperage DC fuses have multiple parallel fuse elements surrounded by powered gypsum. For 12VDC, any quality automotive fuse should be good for low amperage.

author: ArthurDent
author_rank: Super Contributor
timestamp: May 11, 2020, 08:36:51 pm
content: Breaking any energized circuit can create arcing, especially DC inductive circuits. That is why well designed inductive circuit has some form of arc suppression. Here's an article that explains it.http://www.artisancontrols.com/UtilityImages/ProductSupportPdf/Switching%20Inductive%20Loads.pdf

author: Siwastaja
author_rank: Super Contributor
timestamp: May 11, 2020, 09:07:00 pm
content: Not must-have, but often a good idea; fuses are cheap.All normally available fuses are rated to work at 12VDC, no issues with reduced breaking capability.But, if you need to protect against someone connecting:1) A massive battery pack, and/or2) a considerably higher voltage than 12V (say, mains)instead of your small 12V wallwart, and want the fuse to protect against fire / plasma damage, THEN you need to rate the fuse's1) voltage rating,2) ac/dc capability rating,3) breaking current specificationaccording to that worst-case input you want to protect against. Particularly for #3, you should know the maximum short-circuit current capability of the source.This is why proper multimeters have HUGE, fairly expensive, sand-filled HRC fuses; they are designed to be able to interrupt very large currents a mains distribution box is able to deliver (upstream to the normal 10A/16A/25A fuses), even if they are notintendedto blow in normal operation, or even connected to such a place. This is because if the multimeter itself fails, it will quickly carbonize as a conductive chunk. You want the fuse to be of a type which have the physical properties to not maintain conductive plasma, carrying the almost unlimited current.But I guess your application does not need to protect against someone forcing a direct connection from mains to the 12V input! Such protection is rarely implemented, and not required except for very special cases; generally, such idiot deserves what they get.So any cheap type of fuse does fine.

author: Jwillis
author_rank: Super Contributor
timestamp: May 11, 2020, 09:21:54 pm
content: I would be more inclined to rate a fuse to the devices maximum current draw and not to the supplied current . The working amperage of the device should be less than the absolute maximum the device will handle before failure. So you would choose a fuse rated between the maximum Working current draw and the maximum current rating of the device. This insures that the device it self doesn't become the fuse before the supplied current fuse fails.If the device has a maximum current rating higher than the supplied current then by all means go with a fuse rated for the maximum supply rating.