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Following the tracks on the PCB there is a MR851 diode across the +&- and the TDK thing is in line of the + side, It's an old fluke 620 cable tester that runs off to 1.5V batteries, now knowing a little more what it is its there method of protecting against if the batteries where inserted wrong just need to find the blow current or if it limits the current
I've sort of copied the way they step up the voltage from 2x1.5v batteries. To start with I never thought about adding protection against inserting the batteries wrong way round. I know there others ways if stepping up the voltage but this was a simple way. I've removed the parts from the meter and built it up and tested it works perfect in my application but still need to find the correct part as I will need to build another 30 or more units. Looking at the website there are ptc/ntc current limiters and ptc over current limiters. The diode is rated at 3amps so my guess is that it limits the current at very small amount of amps trolling there data sheets/products
"looks like" doesn't mean it is. wizpic did already find out that the device is series connected to the power supply. You can only say it's a varistor if you ignore the previous posts.
According to the circuit function it must be a PTC fuse. If you don't locate a specification by type number, you should measure the trip current with a lab supply. Or select a suitable PTC or regular fuse according to maximum instrument current demand.
1) its leads obviously are too large for surge protectors or capactors.
2) Tracing PC board copper traces would have made it obvious what it was or what its function is. Those thicker leads suggest #3 probably identified it.
3) TDK is a manufacturer of inrush current limiters. That is similar in appearance to current limiters. It exists because electronics better prefer low voltages when starting (despite so many ill edicuated techs who say otherwise). But to say more means one must identy the circuit - what connects to what.
The board is a 4layer pcb but manged to trace the track so it is in line of the plus I will be taking it out to carry that test out with pus as the diode is preventing anything happing with it will let you know the out comes
Thanks for all the replies