Calculate fuse voltage drop

[xmen_xwk]

I have a fuse coming from 5V power supply whose resistance is 15 OHM max, now I'm wondering how much voltage the load will have? Im having some confusion about calculating it as I do not know the resistance of load and how much current it will take(exactly) but somehow I have a feeling it's not important.

 

[KlausST]

Hi,

Just use Ohm's law.

Voltage(drop) = R x I. (Corrected)

Klaus

Btw: I doubt the fuse resistance is 15 Ohms.

 

[xmen_xwk]

So if load is 100mA, it will get 4.5V?

 

[betwixt]

Probably more useful to use V=I*R in this instance as 'R' and 'I' are the known quantities so the drop would be 0.1*15 = 1.5V leaving 3.5V at the load.
However, a 5A fuse with a resistance of 15 Ohms is unreasonable, a practical fuse would be less than 1 Ohm.

Brian.

 

[xmen_xwk]

I might be wrong but datasheet says something like 15ohm max

mc36204

 

[KlausST]

Hi,

Your picture in post#1 shows a 5A fuse.

Then you talked about 100mA load current.

Klaus

 

[xmen_xwk]

Oh thats my fault, I drew that quickly on phone.

 

[betwixt]

The MC36204 is a PTC fuse and rated at 0.3A not 5A, that's 17 times less than you told us!
In any case, the 15 Ohms figure is AFTER the fuse has tripped, not while it is conducting normally. Under normal conditions it would be about 1.6 Ohms and the voltage drop at 100mA would be 0.1*1.6 = 0.16V so you have 4.84V remaining.

Brian.

 

[FvM]

In fact, MC36204 has 300 mA trip current with typically 10 s time-to-trip. Continuous load current shouldn't be above 100 mA.

Using this fuse with 100 mA load current at 5V supply makes little sense though.

The high voltage drop and loosely defined trip characteristic is a general problem of resettable fuses, you need to double check if they are suited for a specific application.

Referring to your initial post

I do not know the resistance of load and how much current it will take(exactly) but somehow I have a feeling it's not important.

I believe, you should know the load current when choosing a fuse.

 

[xmen_xwk]

The MC36204 is a PTC fuse and rated at 0.3A not 5A, that's 17 times less than you told us!
In any case, the 15 Ohms figure is AFTER the fuse has tripped, not while it is conducting normally. Under normal conditions it would be about 1.6 Ohms and the voltage drop at 100mA would be 0.1*1.6 = 0.16V so you have 4.84V remaining.

Brian.

That's more confusing now, a fuse should have very high resistance when tripped so almost no current passes through, 15OHM is very low. Also if go further down in the table of datasheet. It goes way lower than 1 OHM for high current fuses. I think it's not trip resistance?

- - - Updated - - -

I believe, you should know the load current when choosing a fuse.

Very true but this is for general purpose bench power supply for many things, I can't exactly know the load but since it's for low consumption circuits I guess 100mA would be enough.

 

[FvM]

That's more confusing now, a fuse should have very high resistance when tripped so almost no current passes through, 15OHM is very low.

Involves a misunderstanding. 15 ohm isn't in tripped state, it's state after tripping. The datasheet doesn't tell clearly if the fuse will ever recover to the initial resistance of <1 ohm.

I don't know any low voltage bench power supply with only 100 mA rating. But if I make it, I won't use this fuse. If using a resettable polyfuse at all, I would refer to a better specified major vendor product like Littelfuse.

 

[KlausST]

Hi,

A PTC fuse has an initial resistance. (Here 1.6 Ohms)
When current flows caused by th load, then there is a voltage drop across the fuse .. causing power dissipation .. causing temperature rise.
The temperature rise increases resistance.

At as certain load current this causes an avalance and the fuse "trips".
But the resistance doesn´t become infinite.

There is a certain fuse resistance where a balance exists:
* increasing resistance cuses the current to drop and thus causing the power dissipation to drop
* decreasing resistance causes the voltage to drop and thus causing the power dissipation to drop
When the power dissipation drops below "tripped power dissipation" (here 0.8W) then the avalanche stops.

I made a chart, I hope it helps to understand the working principle of a resettable PTC fuse.
5V power supply, 4 Ohms load, PTC fuse with 0.8W tripped_power_dissipation.

Please see:
* The cyan line. It is the current of a 4 Ohms load related to the voltage drop of the fuse.
(With 0V fuse_drop the full 5V are at the 4 Ohms load causing 1.25A to flow. With 5V across the fuse there is no voltage remaining for the load. 0V --> 0A)
* The blue line. This shows the (fuse) current related to the voltage drop of the fuse, where the fuse dissipates 0.8W.
(Some points: 1V x 0.8A = 0.8W; 2V x 0.4A = 0.8W; 4V x 0.2A = 0.8W)
* The pink line: This shows the power dissipation of the fuse in series with the load (related to the voltage drop of the fuse)

Now if you follow the pink line (beginning from left) you see with increasing current the power dissipation increases. Somewhere above 0.3W (Depending on fuse characteristic, ambient temperature, fuse thermal resistance...) the avalance starts.
Expect a voltage drop of about 0.5V (Means only 4.5V at your load !!) when the fuse starts to trip.
Then follow the dotted line "avalanche". During the avalanche the power dissipation in the fuse is higher than the 0.8W making the time during the trip to become short. (Partially overheat).
Then there is the point where the avalanche stops. This is where the power dissipation becomes exactely 0.8W and the fuse comes into a balance.

What current (fuse resistance) this means depends mainly on the supply voltage and the load resistance.
In my example you see it somewhere at 185mA. Not close to zero like a standard fuse. (Remember: fuse hold current is 0.1A!!)
The voltage at the 4 Ohms load still is somewhere at about 0.75V.
The fuse_tripped_resistance in my 5V/4Ohms example is somewhere at 23 Ohms.

So you see the behaviour of a PTC fuse is very different to a standard fuse.

Klaus

 

[xmen_xwk]

PTC seems a bit complicated, a regular fuse would do better job but then again needs to be replaced.

 

[asdf44]

Yeah I think it's worth reiterating that PTC's are not very ideal. Personally I think it's a problem that they're marketed as 'fuses' at all.

There are many 'load switch' ICs and for 5V you have the benefit of overlapping with USB which means there are hundreds of devices that approach the cost and size of a PTC. For example this has a 200mA limit for ~40 cents:
https://www.fairchildsemi.com/datasheets/FP/FPF2100.pdf

Or 100mA for ~15 cents:
https://ww1.microchip.com/downloads/en/DeviceDoc/MIC2090_1.pdf

I personally tested the FPF2700 under hard short circuit conditions and it performed well. The rest of the system saw minimal disturbance and kept running even when a sub-system had a hard short. It would be hard to guarantee that with a PTC.


To be clear this doesn't provide the safety of a fuse. But a fuse combined with a load switch may achieve the desired 'reset' functionality under most conditions.