I am working on a 12 volt DC project that will use about 6/10ths amp at 12 volts. I have a 12 volt DC power supply that can power a few amps at 12 volt DC. I want to protect the project with a glass fuse, 6mm x 30mm fast blow. I purchased some glass fuses that were listed as 12V 1amp, but printed on the side is 1 Amp 250 volts. Is a 12V DC 1 Amp fuse the same as a 250V 1 amp fuse? If not, what do I need to protect my project? I can go to a 5mm x 20mm fuse or different size if needed. Also, does it matter if the fuse is on the positive or negative side?



Thanks

 

For that current and application it should be no problem.
3AG is a good all around size.
Max.

 

By convention a fuse is normally in the positive line.

With regards the voltage rating of fuses – as long as the voltage rating is equal to or greater than the voltage of the circuit it is protecting – then the voltage rating is acceptable.

Bear in mind that as a rule of thumb, a fuse can pass 1.6 times current rating for a considerable time, but will blow fairly quickly at a current 2.1 times rating.

So you could probably get away with a 630mA fuse protecting your circuit, without nuisance operation of the fuse.

 

The only thing that "counts" with a fuse is the RMS current (steady DC current is RMS current) - RMS because a fuse is essentially a resistor that will, with overcurrent, heat itself to the point that it melts and it is the RMS value of the current that determines the power dissipated and the heat generated. But that is just up to the point where it first goes open circuit.

Once a fuse has opened other things come into play. If the voltage is sufficiently high there can be arcing between the ends of the melted fuse element. DC is actually more difficult with regard to arcing than AC since the AC current "naturally" drops to zero at the end/start of each half cycle which usually helps to extinguish the arc. It also gets complicated by the amount of current that can be delivered by the power supply. AC mains can generally deliver hundreds of amperes, at least briefly until some other circuit interrupting device also tries to open the circuit. A small DC power supply usually can't deliver much beyond its rating for more than a very brief time. This usually means that a fuse rated for AC line voltage will be entirely satisfactory for lower voltage, either AC or DC, but not always. If you were using a fuse in a circuit powered by a big lead-acid battery that could deliver hundreds to thousands of amperes, you might need to look at spec's carefully. With your power supply, there will be no issues and the AC rated fuse will be just fine. Using a fuse rated for 32 VDC (common rating for 3AG fuses) for AC mains is a bad thing to do. Generally fuses rated for low DC voltage are a bit cheaper than equally-rated fuses for higher AC voltage use.

It can actually be pretty complicated to specify a fuse, depending on your objective. With electronic circuitry, it is often said that the circuit will blow to protect the fuse. The grim fact is that there is a lot of truth to that, and trying to "dimension" a fuse to protect the circuit is often next to impossible. The remaining objectives that are very important are to protect against things melting so they can't be repaired and preventing fire.

Usually the fuse goes in the side that isn't "ground" or "common", which means that with most modern DC circuits it goes in the positive supply line. In lots of cases it doesn't really matter, but sometimes, especially where one device connects to others or where different parts of the same device have separate fuses, there can be paths that would allow current to completely bypass the fuse if the fuse were put in the "common" side.

Most of the major fuse manufacturers will have decently detailed datasheets available, along with applications information. Bussmann (now an Eaton company) and Littelfuse are popular brands in North America. Schurter is more global.