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Selection of glass fuse

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alok1982

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I want to use a glass fuse in my power supply circuit design. I want to know what parameters i consider before selecting the glass fuse?
 
Dimensions (length, diameter), current rating and either "slow" or "fast" blow.
 
Voltage rating is very important also. Using a 12v automotive 3AG fuse as a line fuse for 120 or 240v systems will be a disaster if it blows.
 
Come on guys. Give the OP a break. Remember that the transistor usually blows to protect the fuse. It's HARD to makeit happen the other way.

At work, they were replacing $25 a pop I^2t fuses until I put an end to that. Fuses that cost a $.25 were added. Selected right, the quarter fuse would blow first if their was a short and the $25 fuse would protect the SCR's.

When you don't understand, then a fuse is a fuse, which it isn't.
 
OP = Original Poster

Every fuse has a time = trip (% rated) curve. The glass fuse falls into the category of non-resetable. You have to replace them. They come in indicating and alarm switch actuating versions versions for telecom apps.

The BIG FEAR is that the user will replace them with a penny or higher rated fuse.

Fast and slow blow are common terms as well. Slow blow is also known as Time delay. It's provided for transformers and motors so they don't nucence trip.

A typical transformer based power supply can be protected using a rule of thumb: 1.5 X transformer rating and slow blow fuse.

Resetable is basically a circuit breaker. Sometimes used as a power switch. They come in magnetic and hydraulic. They can't be held into position. They are not easy for a consumer to change, They can be electrically turned off and their state can be sensed with proper options. Your house probably has Circuit breakers. Critical breakers can be designed so if a diffusion pump shorted, that breaker would pop and electrically the vacuum pump associated will turn off too.

NTC thermisters reset on their own by cooling. Battery packs are a good application for these and so is class II power supplies for fire alarms.

I will mention fast-acting instrument fuses and I^2t fuses for semiconductors. The former protect instruments from overloads and the latter protect semiconductors.
 
Fast blow fuses are used as line fuses in all Tektronix equipment, to cite an example. The initial turn-on current is not that great compared to the operating current, which should increase slowly on older tube-type equipment as the tubes warm up. On solid-state equipment, the initial turn-in current is a bit higher for a fraction of a second, then quickly settles down to a lower value. Don't get the idea that a fast-blow or instantaneous fuse is that. If you look a a fuse data sheet, you'll find that a 1a fuse will operate all it's life with a current of 1a through it. If a current of 1.1a flows continously, it may hold for several hours. If 1.2a flows continuously, it may blow after ten seconds. With 1.5a, it may blow after 1 second. The higher the current, the faster it will blow. So, it isn't truly instantaneous, blowing at the instant that the current crosses the fuse rating threshold.

Slow-blow (time delay) fuses work the same way, but on a much more extended scale. They may handle a 100% overload for 5 seconds; a 50% overload for 20 seconds; a 20% overload for a minute. These are hardly true figures for fuses, just typical representations of the idea that fuses don't blow quickly. For either a fast-blow or a slow-blow fuse, a 400% overload or a dead short will cause the fuse to blow instantaneously.
 
And that's how they work. The I²R power/heat dissipation of the fuse element is what causes it to melt once the dissipation exceeds a certain level. In fact, the simple application of I²R as you increase the current in steps from the fuse rating and on up helps explain why it takes longer for it to blow just a bit over the current rating and much faster as the current increases.
 
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