Hi everyone! I am working on a project that uses a 5v 2a usb wall plug for a power supply. I wanted to have a fuse in case something broke and it could be fixed after avoiding a fire. If I am using a 5v 2a usb supply, will I have to use a 2 amp fuse? I have a 250v 2a fuse and I assume that is what I would use because I am using a supply that can deliver up to 2 amps, but I am unsure. Here is the basic drawing of how I would wire it up: (And sorry that it is a file and not a picture. I dont know how to do that either, but I will look it up.)
And for larryd and whoever else was with me on my previous thread, I am using a different motor that has one axle instead of two because I came up with a different way to move the door. I measured the resistance across it and it is 14.3 ohms, so based off of the math you showed last time, it uses nearly half the current! Sorry to everyone else that doesnt have context, but it was basically about a password lock that runs an H bridge that runs a motor to lift a door. Thanks!
Rapid80:
And sorry that it is a file and not a picture. I dont know how to do that either, but I will look it up.
Instructions for embedding an image in your forum post are here: http://forum.arduino.cc/index.php?topic=364156.0
Doing that will allow people to easily view the image rather than having to download it, which makes it more likely you will get an answer to your questions, so definitely worth the effort.
Rapid80:
Sorry to everyone else that doesnt have context, but it was basically about a password lock that runs an H bridge that runs a motor to lift a door.
Rapid80:
Hi everyone! I am working on a project that uses a 5v 2a usb wall plug for a power supply. I wanted to have a fuse in case something broke and it could be fixed after avoiding a fire
Many people do not understand the purpose of a fuse. In your case, all you are protecting is the Wall Wart and even if the wart is the cheapest junk you can buy, no fuse in the DC output is going to protect your house.
The purpose of a fuse or circuit breaker is to isolate the failure from upstream circuits.
Thanks pert! Also, yes thats the one. And Steve, so what you mean is that if I wired it like I showed, it wouldnt do anything? I thought that when there was a short circuit, the fuse would get more current due to its non-resistive properties, heat up, snap, and it would prevent the circuit from closing. So even if it didnt protect the usb supply, wouldnt it atleast protect the stuff in my project?
Also I originally meant the name to be "Fuse Protection" so I changed it.
Yay! I made the image not a file! Thanks!
And also what is a "Wall Wart" - K I looked it up and you are referring to the ac to dc usb adapter that goes into the wall. If the supply were to get short circuited it could potentially cause a fire, and that is why I want to use a fuse. Even if it doesnt directly protect the whole house (Not its purpose) it still does provide some protection by not starting a fire.
The "wall wart" is the little black thing you plug in to get your USB power for your project.
IF the wall wart can only supply 2 amps, then a 2 amp fuse will never open because the voltage in the wall wart will begin to drop when drawing 2 amps. You will have to draw more than 2 amps to pop the fuse. And the fuse may never open at 2 amps because that is it's rated current. It can carry 2 amps for ever.
IF your project has a fault of some kind, your project will be dead long before any fuse will open up.
By the way, the voltage rating of your fuse is the maximum voltage that it can carry when the actual fuse opens up. There will be an arc/flame and the fuse cartridge will contain the arc/flame so it won't start a fire.
You can't tell by looking at them, but many wallwarts are output short-circuit protected as well - the output just turns off, and remains off until the short is removed.
Thanks for the replys. I also have a 1.5 amp fuse, but if I have the usb wall wart like crossroads described, then thats awesome! Is there any way to tell if I have one like that? And I would much rather my project be dead then start a fire, so I am just trying to cover myself.
The purpose of a fuse or circuit breaker is to isolate the failure from upstream circuits.
The main purpose of most fuses is to prevent a wiring or transformer fire in the down-stream device.
Fuses basically prevent overheating damage by sacrificing themselves first. They don't protect anything
upstream(*), and they don't fully isolate in a mains circuit, because you have to open live and neutral to do that.
However fuses take time to fail, they aren't very effective at protecting semiconductors.
(*) I've seen a hard mains short take out 3 fuses simultaneously of various values, because 100's of amps
causes the smaller fuses to arc heavily and stay conducting long enough to vaporize the big fuses.
Fuses are not simple things as there is a lot of physics and chemistry that goes into designing them.
If I remember correctly, when testing a fuse, its holder is not allowed to heat up. The fuse will carry
120% of its rated current. It will open at 125% of rated current within 120 seconds. It will open at
200% of rated current within 5 seconds. This is for a simple normal lag fuse, one with a single element,
not a slow-blow type.
That is very helpful information, so I guess a fuse isnt the best option. In that case is there any other way I could protect against a short circuit? Again, I would rather the whole project go to crap than start a fire if something went wrong. I wouldnt expect something to go wrong, as I have been very careful, but you never know.
Is 10W (maximum power delivered by your source) enough you cause any fire in a reasonable setup? I mean no highly flammable material directly touching a part of naked circuit.
AFAIK fuse is the right device to prevent fire caused by a short. If you are lucky it also prevents damage to other parts of the circuit but preventing fire is the most important job.
Of course it is possible to get a lot of heat in normal operation of a poorly designed circuit - no fuse prevents this.
That is very helpful information, so I guess a fuse isn't the best option. In that case is there any other way I could protect against a short circuit?
I don't think you need to worry about it. You have a Samsung power supply, I would expect Samsung to have included short circuit protection already, I would expect that if the output is shorted then it will limit the current to a low level until the short is removed, which is all the protection you need.
Rapid80:
Thanks for the replys. I also have a 1.5 amp fuse, but if I have the usb wall wart like crossroads described, then thats awesome! Is there any way to tell if I have one like that? And I would much rather my project be dead then start a fire, so I am just trying to cover myself.
Yes. Short it out. If you have a super-cheap Wall Wart, it will start to smoke after a few minutes. Throw it away as it's crap anyway. If you have a protected or current-limited wall-wart, nothing will happen. Come back an hour later and remove the short and the wall-wart voltage should be unchanged from before the short.
Thanks guys! I really appreciate everything! I am a lot less worried about something happening now. All of the wires and traces I use can always handle atleast two times the current they need to handle and are always good quality heat resistant wires because thin wires and traces just bug me. I will definitely test the supply. Thanks again!
