which is better overcurrent protection, polyfuse or resistor?

[tehtehteh]

I have a string of LEDs in parallel, each has it's own resistor, 100 or 200 ohms depending on the colour, the total draw is around 600ma, and the power supply is rated 5v 1a

I would like to add some protection in case of a short circuit, but can't decide.....

a polyfuse, if I choose one say 800ma, I am not sure would blow fast enough to protect the power supply, but if I choose one too close to the 600ma the data sheet graph suggests it would blow anyway after a couple of minutes

or another resistor, to be small enough to fit inside the connector I might only be able to handle 0.5w, so it would need to be say 50ohms at least, the problem here is that it would dim the LEDs, but at least it would be reliable

thoughts?

thanks

[Jeroen3]

You have several options:
- A fuse. Real fuse, either cartridge or SMD. Make sure the power supply is able to deliver the fusing current (often >2x Inominal).
- A polyfuse, which does not blow, but gets hot and increases it's resistance, limiting current.
- A current limit IC, often called eFuse. They might come in a combination of a high side driver with current limiting. Like the  ITS4140N.
A resistor is not a suitable fuse, the fusing characteristics of resistors are undefined unless you buy special fusible resistors.

There are more things to consider. If the fuse is there to prevent damage to the circuit, you will need an eFuse or current limiting circuit.
But, most of the times the fuse prevent fire, thus a normal fuse or polyfuse is used. Cheap, but circuit damage may occur.

[Zero999]

A polyfuse is a resistor. It has a resistance with a sharp positive temperature coefficient. When too much current flows the PTC resistor heats up to the point its resistance increases further, causing increased power dissipation, resulting in more heat, until the resistance is so high, only a small current flows.

Yes, a PTC resistor is too slow to protect the LEDs, in fact I doubt any fuse will be fast enough to do that, unless you're talking about a current limiting IC, which shouldn't be relied on as a safety device anyway.

You could make the individual resistors in series with the LEDs a low value, so they only drop 10% to 20% of the nominal forward voltage and put most of the voltage drop into one resistor but that will probably need to be too big to fit in the connector.

Does the power supply have short circuit protection? If so, that should be more than enough, otherwise use a 1A fast blow fuse for protection against fire.

[tehtehteh]

the way it is constructed I can't realistically see the LEDs being a problem, so I'm not bothered about protecting them specifically

the power supply does have protection, but it's a fuse resistor so it will destroy itself in the process and I won't be able to fix it, also in doing this it is unlikely to reach the current limit of the wiring, so really everything is quite safe as is, but if I can save the power supply it would be nice

also I didn't make this clear, but the fuse would need to fit in the small amount of free space inside the USB A connector I'm using, and once constructed I would not be able to get at it, so a proper fuse is not really an option

so it comes down to if a polyfuse will blow fast enough to protect the power supply basically, I understand that the higher above the fuse rating the current is the faster it will operate

[T3sl4co1l]

What is the PSU?

If it's a dumb linear PSU (i.e., iron transformer, diodes and cap), or battery, it is capable of delivering fault current long enough to clear a fuse.  A fuse is a reasonable choice.

If it's a switching supply, or the load is switched with solid state devices (as opposed to mechanical switches, relays, or hard-wired), then the current is probably limited by one of those elements, and a fuse will not clear (or not reliably).

Most SMPS are designed to fail gracefully into momentary or continuous shorts.  (If you have one that is not, please dispose of it and get one that's worth your safety!)  No fuse is necessary here.

If the load is solid state switched, you must use a current limited switch, such as the aforementioned protected switch types, or with current limiting circuitry added.  A fuse WILL NOT protect a transistor, unless the transistor is obnoxiously oversized for the application (a 1A fuse will easily carry >50A of fault current, so you'd need a 50A transistor to reliably switch and protect that 1A branch).

Power distribution rules still apply.  Example: if you have a 20A PSU, whether it's SMPS or not, it's capable of plenty of current.  If you then wire up a couple dozen of these strings, each branch only needs to handle the ~1A it consumes, and therefore can use smaller wire.  Where that wire connects to the main supply, it must be fused accordingly.

Tim

[tehtehteh]

What is the PSU?

it's an apple 5w charger, official, I only knew about the fuse resistor protection from a teardown someone did, other than that I know nothing else about it other than they're supposed to be very well built

[T3sl4co1l]

That will be one of the current limited SMPS.  It won't even be capable of clearing a fuse, let alone causing problems with wiring.

Tim

[Zero999]

What is the PSU?

it's an apple 5w charger, official, I only knew about the fuse resistor protection from a teardown someone did, other than that I know nothing else about it other than they're supposed to be very well built

Are you sure the fusible resistor isn't on the primary side?

I've never encountered a fusible resistor on the secondary side of an SMPS before. Normally it's on the primary side to protect the wiring if the switching transistor fails short circuit. Current limiting on the secondary is usually implemented by reducing the duty cycle of PWM to the switching transistors and by its nature is fully resettable.

[tehtehteh]

could well be, but if it's current limited then I'm happy

I'll include an appropriate polyfuse anyway, in case of use with other power supplies

[KL27x]

You could use a polyfuse and a regular fuse with a much higher rating? Dispose of product if it blows?

[Zero999]

Generally an ordinary fuse will blow faster than a resettable fuse.

I believe the USB specification requires that the device supplying the power is adequately protected against short circuit so there should be no need for additional short circuit protection.