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August 13, 2009
Creating Time Delay

I’ve talked about the need for having time-delay in fuses in past Blogs (mainly to prevent circuit in-rush currents from opening the fuse)...

So that’s the "why"...today I want to talk about the "how"...
Fuses are constructed by connecting a piece of wire or metal strip to two terminals or end-caps.  When too much current tries to pass through the fuse...the wire or metal strip is designed to melt (very quickly for short-circuits but much slower for overload conditions). 
Current causes heat...and heat melts the wire or fuse link...so the one of the ways to get time-delay in a fuse is to find a way to temporarily remove the heat before it melts the fuse. 
One secret to removing the heat is to create more surface area for the heat to radiate from.  So how do you get more surface area?...by simply lengthening the wire.
But wait you say...the fuse length is set (maybe in a 1 ¼" or 20 mm glass tube)...I can’t get a 6" wire in a 1 ¼" tube...well...the answer is yes you can if you coil the wire like a spring around a piece of nylon string...
Spiral Wound
Another very popular way of getting time delay is to use a larger diameter wire which allows more current to pass through the fuse...
But wait you say...how can I possibly get my 4 amp protection (for example) when the fuse has a larger 6 amp wire?
Well...the secret of this trick is to add a "doping agent" to the wire.  This creates "weak spots" on the wire.  The "doping agent" is typically a metal alloy that leaches into the wire when an over-current occurs BUT only if the over-current exists for a period of time (so temporary in-rush currents don’t effect the fuse link during "normal operation")
Solder Spot 
Now the methods I discussed here today are only two of the most popular ways of getting "time-delay" out of a fuse...there are other ways (especially in the larger industrial fuses)...but this is a good start for your knowledge base...
So first I talked about the "why"...now you know a little more about the "how"...I truly believe that knowledge is king when trying to add value to what you sell or make...
Thank you very much for your unwavering support of OptiFuse where you’ll find no delay in helping you find answers...

Jim Kalb

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