Voltstick puzzle

[Tim Streater]

Perhaps the wire is kinky.

[Tim+]

I was trying to diagnose a dead under-cupboard LED light for a friend.

Using my analogue multi-meter I couldn't detect any power at the light. My volt stick beeped and flashed merrily though anywhere near the Q12V supply lead.

Going "upstream" to the power supply showed 240v in but nothing out.

So why does my volt stick get so excited about a "dead" wire?

Tim

[Brian Gaff]

Electrostatic charge. I imagine these things are very high impedance so they can detect a field behind a wall or something. It could of course be still connected to one end of the mains so don't grab it and the metal water pipe at the same time. Brian

[Martin Brown]

Checked as AC?

The wire is floating and there is some capacitive coupling to mains along its length (or a parallel mains cable that you can't see).

[Tim+]

I did check the 12V lead for ac voltages just in case something had gone wrong with the power supply. Nothing on AC or DC on my meter.

No parallel cable. Had to take the cupboard off the wall to trace all the wiring so I can be certain of this.

I guess it must be capacitative coupling unless anyone has a better explanation.

Tim

[ARW]

Volt sticks are only a guide. And not every volt stick works as well as others.

However the fact that the stick said live on the 12V side suggests a duff LED driver. There is not 230V on the 12V side just, some blown capacitor or something in the driver that tricks the stick into lighting up.

[Tim+]

That was my guess. We?ve ordered a replacement.

Tim

[Archibald Tarquin Blenkinsopp]

Whatever is wrong with your PSU, what you are left with is a couple of pins connecting into a live socket. They will ultimately connect to a fully wound transformer, it will be physically small, but will nevertheless couple the mains through to the LV side. This coupling is magnetic.

The transformer also has capacitance from primary to secondary, this would be in the order of nanofarads. It would be boosted by the capacitive coupling in the rest of the ass'y.

the 1/2pixfc, or impedance of the circuit certainly isn't a lot, but inside the Volt stick is a dirty great op amp with more than enough gain to boost the "mains hum" to a sufficient level to drive an LED.

Almost any "two pin" power supply will light your stick, whether the supply works or not.

I had an Electrician call me once, he was using one of those things to check the mains in a rarely used cabin. He couldn't work out why the power went off when he turned the kettle on. The damp air provided enough leakage across the blown fuse to light his Volt stick. Of course the Volts available to the kettle element dropped "slightly" when the boil button was pressed :-)

AB

[PeterC]

Necrofilia