What's inside a cheap network cable tester?

Network cable testers from eBay. Buy cheap, buy twice!
Save your money and buy a different model, as it doesn't take much to kill these ones.

INDEX
0:00 Index
0:10 Introduction: Cheap network cable testers from eBay
0:25 What's fried them? Probably not PoE
1:17 How they're supposed to work
3:08 How they're NOT supposed to work
3:35 What's inside?
4:12 Receiver schematic
4:43 Emitter schematic
5:00 Wire mapper theory of operation
6:32 Tone generator theory of operation
7:41 Scoping the tone outputs
8:02 Design flaw: is the tone generator cooking the wire mapper?
8:40 A simple suggestion for improvement
9:43 Scoping the faulty tone generators
9:58 Hotspot spotting with a thermal camera
11:40 Power consumption testing
13:37 Testing with tone generator removed
14:50 Testing with tone generator re-added and wire mapper removed
15:56 Removing even more tone generator
16:15 Wire mapping into a network switch? No problem
16:52 Tone generating into a network switch? Magic smoke time!
17:43 Conclusions

[kevtris]

this is impossible to follow without being able to see the schematic. There's no schematic pdf link in the description. Anyways, it sounds like there's no protection at all of any kind on the tester though, so some protection (series resistors and TVS diodes and possibly polyswitches) would've been a long way helping prevent stuff like POE blowing up the sender or receiver.

[ratbag]

good ol cost cutting or incomplete designs

[iamdarkyoshi]

I'd love to see a homebrew version of one of these. Mine's packed up as well, and it didn't even have a tone generator on it. I'd rather build one from scratch.

[sparkyprojects]

Or you could carry a socket with a LED to test/discharge ;)
Ideally the units would have thi s built in permanently connected and maybe a push to test button

[icesoft]

I know they're money, but the Fluke Cable IQ kit has been worth every penny - ours is going on 15 yrs old at this point. Pretty durable too, have plugged it into PoE switches, phone lines, etc... Never sustained any damage, although the damn RJ-45 clips have broken off a couple of the wire-mapping terminators :-(

[j.f.christ]

Maybe I should RTFM more often, didn't know about voltage or continuity test.

[simontay]

The schematic is too small and too dark to see. I can't see a bloody thing, even on a large display.

[UberAlphaSirus]

They are dead test tools. Not sure what you would expect.