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Can somebody please help me in understanding the fuse specifications. Actually i went through many fuse parameters. I am getting cofused between the hold current and the cut-off current since there is a large gap between the two.

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IamDp
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1 Answers1

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Little fuse has this app note. It's very good and explains many of the terms. This one is also good.

TE connectivity has another good app note.

Your terms might be the same thing because it will cut off below hold, so perhaps there's confusion there. Usually the terms you're looking for is the difference between the trip current and the hold current, which is a hysteresis effect. The fuse will trigger at the trip current (lets say 0.5A) but once triggered, will stay triggered until the current drops to reache the hold or cut off current limit. This has obvious implications for the design to where you need to select those parameters to match what you expect will be short circuit or over current conditions. In PTCs the hysteresis has to do with the thermal design. Once hot (which is the way they work), it requires the temperature to be much less than the temperature caused by the trip current in order to reset itself.

Gustavo Litovsky
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  • So basically what I understood from your answer is that, "If I want to cut-off the supply at 500mA I should choose a fuse which has its trip current parameter as 500mA". Correct me if I am wrong. – IamDp Mar 19 '13 at 15:20
  • Yes, but 500mA should not be near the operational level of your circuit. Your normal circuit operating current should be less than the hold current of the fuse. Remember that fuses are not current limiters. They're used for events that are completely out of spec. – Gustavo Litovsky Mar 19 '13 at 15:38
  • Ok. Now if I choose a fuse with trip current parameter as 500mA. What is level of current at which it trips I mean what could be the range? and also what is highest possible accuracy we get in the market? – IamDp Mar 19 '13 at 15:46
  • @Durgaprasad: I have not seen it specified anywhere. Perhaps contacting the manufacturer is required, but it's important for you to realize that it's likely not to be 1% since it's not intended to be accurate. Also, it depends on temperature. – Gustavo Litovsky Mar 19 '13 at 15:56
  • Ok Gustavo.. Thanks for your useful information. I will try to talk with manufacturers to know more about the accuracy issue. – IamDp Mar 19 '13 at 16:18
  • Exact trip current depends on ambient temperature - I'd be suprised if you could get more accuracy than 10%. If you need accuracy you need a current limiter. – pjc50 Mar 19 '13 at 16:30
  • That would be great for my application if i get an accuracy more than 10%. That keeps my tripping current from 450mA to 550mA which is good enough to protect components from being over-heated. – IamDp Mar 19 '13 at 17:48
  • @Durgaprasad: Then you need to choose a 600mA tripping point. Never rely on the PTC to be accurate. It won't be. Temperature is a big effect. You'll end up having a circuit that will stop working in some situations otherwise. Remember that PTC is also relatively slow. – Gustavo Litovsky Mar 19 '13 at 19:38
  • @GustavoLitovsky : But we can have a fuse with shorter response time ryt? – IamDp Mar 20 '13 at 03:35
  • @Durgaprasad: PTCs have relatively long response compared to the non resettable types. It's part of the tradeoff because it takes time for the heat to build up and cut the connection. – Gustavo Litovsky Mar 20 '13 at 05:53