Thursday, 25 July 2013

Polyswitch Current Limiting

Polyswitch Testing

Just a quick little video I took while testing my Raspberry-Pi protection circuits.

To recap, I was testing the current drawn when a 5V1 zener clamp is protecting an input from an over volt condition.

Note that the polyswitch is rated to 100mA.  Beware of that as it will *not* limit current to that value!

Here's a snapshot of the test setup.

When the input is raised above 5V1 (up to 6.4V in this case, in the net labeled 5V in the pic above) the ployswitch starts to limit current.  They don't rapidly transition to a high-z state (i.e. act like a regular fuse).  You can see in the video below that after almost a minute the current is 120mA (after a high of nearly 200mA).

What you need to realise when working with polyswitches is that they are thermal, not electrical devices.  Rather than explain what I mean, this video below gives a bloody good lesson in how they work!

Thanks Thomas!

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