Why are polyfuses so big? I was looking at a 500mA polyfuse, it is in an 0805 package, but there was only one of them - the rest were 1206. I have two questions.
- Why so big?
- Does anyone make 0603 polyfuses?
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As I can see from wikipedia, a polyfuse works as that it is a thermistor where the resistance changes non-linear with it's temperature. It's designed to heat up much faster when the current goes after a certain point. That's why there is a tripping time, because the fuse has to heat up.
I can see why they aren't in 0603 because you would blow them up too quickly. Polyfuses aren't indestructible , for sure they will go bang if you trip them with too much current. I think to make them a lot more durable, because they need some room to have enough material in place.
Also I can imagine that the series resistance can't be very low (it needs to heat up), and it might be difficult to produce a polyfuse with very low series resistance for it's size and current.
Hans
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1The series resistance is very low, usually on the order of 1 ohm. It's the thermal runaway that makes the thing work. Also, you said it has to heat up, but remember that it also needs to cool down fast once the overcurrent causing condition is removed. It's all about heat transfer! – Kevin Vermeer Oct 24 '10 at 21:17
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1Surface-mount parts that dissipate heat have another failure mechanism: I've seen resistors unsolder themselves and slide off the board. – markrages Jan 03 '11 at 16:36
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Because they have to absorb a (relatively) large amount of energy in a short time. This 0805, 500mA Poly-Fuse has a minimum resistance of 150m\$\Omega\$, and at 8A trips in 0.1s. That's 10W in 0.1s, the same energy as 1W during a second. Note that 0805 resistors are only rated at 125mW.
And yes, they also exist in 0603.
stevenvh
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