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From: boB <boB@K7IQ.com>
Newsgroups: sci.electronics.design
Subject: Re: nice polyfuse
Date: Thu, 21 Mar 2024 16:27:13 -0700
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On Wed, 20 Mar 2024 19:38:38 -0700, John Larkin <jl@997PotHill.com>
wrote:

>I've been designing relay-matrix switch modules (how the mighty have
>fallen) and I don't want the customers igniting my PC boards or
>welding my relays by ignoring our 2 amp max current spec.
>
>Polyfuses are usually terrible, but this Yageo part is pretty nice.
>
>https://www.dropbox.com/scl/fi/w7x8rvqgrdua8boqmxg7y/BK60_1-1955033.pdf?rlkey=xpavzb8b8movr2xd4o5amkvx9&dl=0
>
>https://www.dropbox.com/scl/fi/czk9ids5bj68ytcimcrb5/BK60.jpg?rlkey=77lrxc915it7y083quld9ectb&dl=0
>
>It (slowly) trips at 2.5 amps in still air at room temp, 3.2 amps with
>some air flow. It survived 120 volts DC, which is all I had available
>on my bench, pulling about 25 mA. 
>
>The good part is that its cold resistance is only about 0.07 ohms.
>
>The next question is, if I put it in series with a 1 ohm 5 watt WW
>shunt resistor, does the poly protect it from, say, a stiff 60 or so
>volt source?
>
>And does it absolutely protect an inner-layer 50 mil wide 2 oz PCB
>trace? I need to do a multilayer board that's 1 oz on the outsides,
>for BGAs and stuff, but the board houses don't mind making all the
>inner layers 2 oz copper.
>
>

The series power resistor idea is interesting.  Are you wanting to put
the PTC in a circuit with voltage above its rating ?

If  1 Ohm  5 watts  limits the voltage across the PTC then it's
probably good.

0.07 Ohms, cold, is good.

We use a 250V 1/2 amp PTC as well as the 1206 size ones.

One problem we had with a 1206 15V PTC was that sometimes (rarely) it
would burn through and short to the next layer down which was 5V.  A
thicker PCB should fix that as well as getting rid of copper just
below the PTC on the next layer down.

boB