Non Contact Voltage Detecting Relay

[R44K]

I am looking for an inexpensive relay that will be triggered by detecting a voltage in a cable. I am looking for something that will use the same tech as these very cheap voltage detecting pens,

 

[Buk]

For a specific cable? If so, what voltage and current are flowing there? And could you coil a few turns around a small former?

 

[R44K]

12v DC. I could tape something to the wire but doubt I will be able coil cable around it.

 

[Pommie]

What are you trying to do?

Mike.

 

[Buk]

12v DC.

Current drawn?

 

[rjenkinsgb]

I am looking for an inexpensive relay that will be triggered by detecting a voltage in a cable. I am looking for something that will use the same tech as these very cheap voltage detecting pens,

Voltstick type devices work on capacitance from an AC voltage in a cable.

You cannot use that technique for DC.

DC current detection is a possibility, but for that you must be able to attach a sensor to just one of the power wires so there is a defined current flow through the sensor.
eg. if it's a two core cable with both wires in it, the current is equal and opposite in each wire and a current sensor reads zero.

You can get or make clip-on sensor, the wire or insulation does not need cutting, as long as you can get to just one of the wires.

 

[Buk]

If the cable is a 2 core flat cable something like this

or thisthen the resultant magnetic field around the cable looks like this in cross section:
**broken link removed**

Whilst the overall magnetic field is neutral (shown in blue), the fields either side of the cable are strongly magnetic (bright pink). (Assuming a reasonable level of current flowing.)

A simple reed switch positioned on either side with the appropriate orientation will be activated, and that can then switch whatever relay or SCR you need.

 

[rjenkinsgb]

A reed switch needs a field strength of anything from typically 10 to 100 Ampere-Turns to operate.

Also, that magnetic diagram appears to show current flowing the same way in each core rather than opposite, if the arrows are indications of field polarity??

 

[crutschow]

The suggest techniques require current in the wire.
If you need to detect the voltage with no (or little) current flow, that will require a different technique.

 

[Danwvw]

What Kind of Voltages? AC, DC, 125 VAC 220 or what 5 volts DC? Oh, I see You want to detect 12 volts? What limits? anything over say 11 volts? Mabe get one of the "Auto Car Truck Voltage Circuit Tester 6V/12V/24V DC Hook Probe Test Light Pencil" on ebay.

 

[Buk]

Also, that magnetic diagram appears to show current flowing the same way in each core rather than opposite, if the arrows are indications of field polarity??

You were right, now corrected. (That's what comes from rushing to post having been called for dinner.)

A reed switch needs a field strength of anything from typically 10 to 100 Ampere-Turns to operate.

Close differential reed switches go lower; but that's why I kept asking about the current draw.
The application where I've used this was an e-bike where the initial current draw of the inductive load is much higher than the steady state draw.