Hi

I am working on a small design where my PCB will connect the external sensor, device, and PCBs. These external devices range from 3.3V to 12V, from 10mA to 1A.

I would like to add some protection, such as a fuse, to ensure that excessive current does not damage or disable my PCB.

I was initially thinking about using
TR5 Fuses - Littelfuse

But I have also seen the resettable fuses such as:
PolySwitch Resettable PTCs Devices - Littelfuse
and the SMD version, such as:
PolySwitch Resettable PTCs Devices - Littelfuse

So am I correct for resettable fuse? You simply remove what you are connected to, and that fuse will be workable again.

 

So am I correct for resettable fuse? You simply remove what you are connected to, and that fuse will be workable again.

Yes, that is the way they operate.
With the TR5 fuses, the fuse blows and requires the source of the problem to be solved before replacing.

 

I've used various PolySwitch devices for years, and cannot remember a "real" fuse designed onto any of my boards. One problem I had was with SMD versions, I think low temp solder could be an idea as a number of them were damaged during soldering when I just used the standard solder. Maybe I ran my iron too hot?
But thumbs up for the PolySwitch! A great device.

 

PolySwitches are great, but really don't ever exceed the rated voltage. If you do they catch fire.

 

Polyfuses are indeed a wonderful option, but without exception they are not "FAST". So for many applications a polyfuse is OK. But they might not save some semiconductor devices.
I had one application where several sensors were powered from a single supply, and a situation where the connections to one sensor might become short circuited. (Crash Testing Data Acquisition). Loss of power to the remaining sensors was not acceptable. So I was able to purchase FAST fuses that would clear in less than one millisecond. THAT was the solution.
So the choice and rating of circuit protection devices depends very much on the specific application. How long can it last for an over-current incident??

 

Well i need protection from a short circuit, so that it does not draw excessive power from the source.

 

OK short circuit protection is useful,but here is a caution, which is that the polyfuse that quickly heats up to limit the current does not open the circuit. So in the event of a short circuit the current is limited but a reduced current will continue to flow. So if the source is a battery it will be drained, but not at the short circuit current. So in those applications a standard, one time fuse will be a better choice, if battery drain would be a problem.

 

Ok, so best to get fuse with a holder from the little fuse range

 

There is the standard glass fuse, they come in various blow rating, slo-blo up to fast sweep-through acting.

 

Thanks

But are they suitable for DC, or do they need to be AC?

 

Most fuses are rated for current and VOLTAGE, and often the DC voltage rating is less than the AC voltage rating, if both voltages are listed. The reason is that under some conditions a DC circuit will establish an arc inside the fuse, after the fuse element opens. So for a 12 volt DC circuit the fuse rating should be at least 50 volts. MOST low current glass fuses are rated for 250 volts AC but that will be quite adequate for 12 volts DC. (This is based on my guess that the rated current will be less than one amp.)

 

But are they suitable for DC, or do they need to be AC?

For med, to high DC current, you can get what is known as rectifier fuses, they have very fast sweep through to prevent arcing..

 

If the application is still only "up to one amp" and the voltage is not more that 12 volts then for those fuses a 250 volt rated fuse gives you a large safety margin. So it will not matter because the operating conditions are far from where problems happen.

 

Fuses are to protect two things :
the device,
wiring to a device.

The device normally has two types of amps:
initial,
stable.
However, when the initial value is higher a slow blow fuse may be used at the stable amps value.
If the slow blow fuse does hold, then other current limiting methods will have to be applied.
Each device should be Fused separately!
Ganging devices with one fuse will cause all devices to be without power if a short circuit occurs on one.

Wiring
If the Max wire amp capacity is less than either the initial or stable current then increase the wire size to be greater than highest
of the two.
The wire is then protected by the fuse sizing of the device.

 

Hi

I am working on a small design where my PCB will connect the external sensor, device, and PCBs. These external devices range from 3.3V to 12V, from 10mA to 1A.

I would like to add some protection, such as a fuse, to ensure that excessive current does not damage or disable my PCB.

I was initially thinking about using
TR5 Fuses - Littelfuse

But I have also seen the resettable fuses such as:
PolySwitch Resettable PTCs Devices - Littelfuse
and the SMD version, such as:
PolySwitch Resettable PTCs Devices - Littelfuse

So am I correct for resettable fuse? You simply remove what you are connected to, and that fuse will be workable again.

I have used them in my V/Ohm meters for a long time now. They saved replacing a lot of fuses!

 

Primarily, fuses are to protect the UNDAMAGED portion of wiring when a short circuit causes excess current. They may also protect devices and systems when an overload leads to excessive current,
The challenge with fusing DC circuits is the tendency for arcs to not extinguish when the fuse element melts. The arc tends to continue because there are no periodic zero current moments like in AC circuits, and so the metal vapor never has a chance to cool.