Slow Blow Fuses

[DRD]

Are these things "standard" please ? Or do you have to be careful what you buy ? I am inclined to stick with brands that I have heard of, but do you need to watch out what the maximum surge current is ?

Many thanks

 

[Calimori]

There is also a debate about slow blow vs time delayed fuses which I never really worked out.

 

[DRD]

Glad I am not the only one !

I don't mind a fuse popping now and again, I just don't want to fry more important things by purchasing the wrong one when they are so cheap

 

[Spandangler]

You can't go wrong with a good old fashioned sturdy nail

 

[Fintan Stack]

There is also a debate about slow blow vs time delayed fuses which I never really worked out.

That is weird! Just yesterday I was trying to find out if there is a difference between the two or if it's just two different terms for the same thing. Seems to be conflicting answers on the web. I couldn't find a conclusive answer. Does anybody here know the difference? Is there even a difference?

 

[Paul]

There ARE differences... All but minimal nowadays tho...

A slow blow fuse is different from a fast acting fuse in its capability to withstand transient pulse currents, i.e., it can withstand the surge current upon power-on/off, thus ensuring the equipment works normally. Therefore, slow blow fuses are often called time-delay fuses. Technically, a slow blow fuse features a higher I2t value, and it requires more energy to blow, so it is more capable of withstanding pulses compared with a fast acting fuse of same rated current.

When an overcurrent occurs in a circuit, the breaking time of a slow blow fuse takes longer than that of a fast acting fuse because of the larger I2t. Is it less protected this way as some people are worried? The answer is no. Once the circuit fails, the overcurrent will last and corresponding energy released will go beyond the I2t of the fuse until it blows out. The timing difference of slow blowing and fast acting is not significant to their protection. Slow blowing will affect the protection performance only when sensitive components existing in the protected circuit need to be protected.

Due to the previous difference, slow blow and fast acting fuses are applied to different circuits. Fast acting fuses must be used in purely resistive circuits (no or fewer surges) or the circuits where IC and other sensitive components need to be protected, while slow blow fuses are preferably used in capacitive or sensitive circuits where surges occur upon power-on/off and power input/output. Apart from circuits for IC protection, most applications with fast acting fuses can be replaced with slow blow ones to enhance anti-surge capability. Contrarily, replacement of applications with slow blow fuses to fast acting ones may cause the fuse to break as soon as the equipment is switched on and fails to work.

Furthermore, economic consideration is also an indirect factor for selection because a slow blow fuse is much expensive than a fast acting one.

realistically though, as far as I'm aware, either could be used

 

[lukewells]

Buy a decent brand where possible, some of the Chinese ones are way out of spec. I am currently performing some scientific testing of some of the lessor known brands in order to find a cheaper fuse that performs properly.

Time delay / slow blow pretty much interchangeable.

You often find slow blow in pins, because the initial current spike when the coil is first energised is significantly higher than the normal on current.

In a Bally for example, you have the 1 amp (slow blow) playfield fuse. You will find that many of the coils will draw 750-900ma when on, but they will spike at 1.5 -2amp when firing.

The Slow blow fuse suppress that initial spike and doesn't blow.

They have to do this (have the current rating of the coil close to that of the fuse) to stop failing coils going up in smoke. If they just fitted a 3-4 amp fuse to get around the initial spike, then a coil that is shorting out might draw 2-3 amps, get red hot and go up in smoke, without blowing the fuse

 

[Nedreud]

Interested to know how you perform your "scientific testing" of fuses? I bought a different unknown brand of 8A fast-blow fuses (they were tenth the price of Bussman, so I got 100 for the same price as 10) and I'm curious to test them before put them in a pin. Do you just wire them up via your digital multimeter on the 10A circuit, enable the MAX reading setting and then crank some juice through it with a bench PSU?