I've had a read up on them, it's rather interesting, they use a crystal of an exotic material that drops it's resistance with increasing temperature. That temperature is caused by the current through the device.
Very clever. Thanks for the suggestion, I'll get some ordered in the morning.
Hi all,
I've got a problem which is so daft it hurts.
I'm using lithium batteries, and I'm well aware of the dangers, so despite being fine with working on an un-protected cell, I changed to protected ones, and then issues appeared. The current is drawn from a pair of parallel 18650's...