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. >
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>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