Parralelifying two polyswitches...

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In a universal board we're making there will be a selectable fuse maximal current. I wanna use polyswithces.

Now. If I want to have, say, 5A, but the biggest ones I can get are 2.5A, can I connect them in two parralel and therefore get a fuse current of 5A?

Thanks,

David

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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I have read a lot of polyswitch app notes and designed a few circuits.

I have never seen that suggested. I suspect it is that they have a resistance that is not very small and its also not that consistent (unit to unit). So, if you put two in parallel, the current is not shared equally. Just a hunch.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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http://circuitprotection.com/aut...

According to this table, they have a small resistance. I did mean ofcourse using two equal polyswitches (like 2x 2.5A, or 2x 1.35A ... )

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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It doesn't really matter if you use two "equal" polyswitches. Manufacturing variances in the switches will guarantee that the resistance will be slightly different. One switch will inevitably get more current than the other, if only by a little bit. Since current through that resistance creates heat, my guess is the resistance will change more in one than the other, leading most likely to more imbalance (Mother Nature rarely works for you in these cases! :wink: )

While you probably will not see the paired polyswitches to trip at 2.5A, my guess is that they will trip well below 5A, perhaps as low as 4A. The other gotcha is that the trip level will vary from pair to pair and from time to time.

This, as Jim noted above, is hunch. I would definitely test this in a "safe" environment before using it - be sure to test with more than one pair. I would not use it in a production device.

Stu

Engineering seems to boil down to: Cheap. Fast. Good. Choose two. Sometimes choose only one.

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