PolySwitch (and other PPTC) advice needed
PolySwitch (and other PPTC) advice needed
(OP)
I am considering using some type of PPTC (specifically the PolySwitch by RayChem) as a "fusing" solution for a board I am designing. I am looking to provide protection on 5 and 12 volt lines.
I'd like some input from others who have used this product before. After reviewing the data sheets, it seems to me that the dependance on ambient temp could be problematic. I am designing for a large operating temp range, and the current to trip the device varies greatly with ambient.
Does anyone have a better explanation than the data sheets as to what exactly Ihold and Itrip currents are? It seems there is a large gap between the two for which operation is undefined.
Thanks for any input,
JohnD





RE: PolySwitch (and other PPTC) advice needed
Assuming they are outputs from your board, then you'd need to design the board (its actual failure limits) accordingly.
It might be easier to use a more active current limiting scheme (foldback).
RE: PolySwitch (and other PPTC) advice needed
RE: PolySwitch (and other PPTC) advice needed
Most protective devices would be located at or in the PS to protect the PS and the wiring.
If this is a high power system, then you may need to worry about wiring topology. You might not want dasiy-chain high current wiring going along to dozens of mid-power boards.
By way of comparison, have a look at the specs for typical fuses and circuit breakers. Those typically take a verrryy looonnnng time to blow if the overload is 'only' something like 150% of the current rating. If you want them to blow/open 'instantly', then you need to provide something approaching a dead short.
RE: PolySwitch (and other PPTC) advice needed
RE: PolySwitch (and other PPTC) advice needed
It depends on how the PS is built. High current power supplies would obviously tend to have a higher peak fault current than lower power supplies. For example - a car battery can source 1kA or more, while an alkaline D cell might be good for about 2A (YMMV). Modern power supplies often provide very nice current foldback circuits - these might be set just above the rating.
A more interesting, complicated and important question is when things go wrong, what exactly happens and is it safe?
I've noticed that the household wiring standards (North American electrical codes) are pretty good with respect to making sure that the protective devices (circuit breakers) are in the right place and connected in a manner that provides very good protection. Basically, they make sure that the circuit (wiring and everything downstream, at least as far as the outlet) can handle all the available current.
RE: PolySwitch (and other PPTC) advice needed
RE: PolySwitch (and other PPTC) advice needed
Like most said, the polyswitches are slow to respond if the overcurrent that you want to monitor is within the same numbers as the normal board consumption. If the failure modes involve much larger currents, then the polyswitch may be a good device. And yes be very careful with the temperature deratings.
If you use the SMT versions, be careful when you solder them. They really do "fuse" and can get melted down and destroyed by the soldering heat.
RE: PolySwitch (and other PPTC) advice needed