fusesparallel
Let's say I need a 45A fuse.
However, I have only a 40A and a 5A fuse in my pocket.
If I put them together in parallel, will it act as single 45A fuse ? (so it will blow up if current is superior to 45A).
My guess is the 5A fuse might blow up before reaching 45A, because the resistance ratio of each fuse might not be the same as the ratio of their current rating (5/40). Because of that, there might be more than 5A going trought the 5A fuse before reaching 45A.
Why don't you just design (or buy) a small voltage regulator that feeds your bank of LEDs. Then there are no complex issues like fuses blowing and notifications etc. Think simple. Design a simple small buck regulator that can supply (say) 12 volts from the 24 volt power option.
It looks like this would be OK: -
But there are many other options from Linear technology and Texas Instruments.
if I or anyone else mistakenly sets the outputs at 24V instead of 12V the ringlight is protected
With this idea you set it at 24 volts and it regulates to 12 volts. If you set the input to be 12 volts then the LEDs will be a bit dimmer because the LT3970 won't be able to supply 12 volts from a 12 volt input (it'll be more like 11.75 volts as per graph on pg 6 of data sheet). But it won't fuse!
As an aside, I also want my circuit to be very small, with the fuse fitting in a 150 series inline fuse holder from Littelfuse, catalog # 150274, And the zener diode fitting on this tiny PCB, which I can fit inside the case with existing electronics
I expect the regulator circuit will be smaller.
So if 12V is applied the Zener acts like an open circuit
Of course if you do decide to go down the zener diode route and pick a 5% tolerance one (about as good as they come) it may only be regulating at 11.4 volts and this may blow the fuse under normal circumstances. Go for a 15 volt zener is my advice then there is never any possibility of it blowing the fuse at 12 volts.
Like all electronic components, fuse:
For instance, littlefuse recommends a 25% derating:
- NORMAL OPERATING CURRENT: The current rating of a fuse is typically derated 25% for operation at 25ºC to avoid nuisance blowing. For example, a fuse with a current rating of 10A is not usually recommended for operation at more than 7.5A in a 25ºC ambient. For additional details, see RERATING in the previous section and AMBIENT TEMPERATURE below.
Operating temperature is also an important factor. The higher the ambient temperature, the hotter the fuse will be, the sooner it will melt.
Repeated surge also will determine the fuse lifetime, as reported in this article (J. McLinn, "The Simple Fuse", IEEE Reliability Society 2008 Annual Technology Report.).
The higher the current spike, the smaller the number of surges the fuse will withstand.
This is the case of your pump: despite it's rated at 8A continuous load, it will probably have a larger inrush current.
By the way, remember that if the current of your load has large high-frequency components, the equivalent fuse series resistance will be larger, due to skin effect (i.e. it will likely blow earlier at the same RMS component). This might not be your case though. (It's rather a problem if you PWM a load, like an heater).
Best Answer
Fuses are safety devices and they are specified and characterized to be used "standalone".
Although in theory you could characterize a fuse so that you could determine what happens when you put two in parallel, no vendor will do that, because it is something no sane designer would want to do.
Therefore, putting two fuses in parallel will go against the very purpose of a fuse, i.e. having predictable behavior under the specified conditions.
This means that, by doing that, you put at risk the equipment (and possibly its user) the fuse is intended to protect.
Bottom line: DON'T DO THAT!
BTW: a fuse is not simply characterized by its current rating, but by lots of other parameters, which vary among manufacturers. Think only of the "reaction speed". What do you expect if putting a 5A(T) (slow) fuse in parallel with a 40A(FF) (ultrafast) fuse? Which will blow first? Very messy and unreliable behavior is ensured!