# Non-contact AC blocking

#### electrophile

Joined Aug 30, 2013
167
I know that there are plenty of ways to measure AC voltage without breaking the circuit but is there a way to block AC voltage in the same way?

#### Kermit2

Joined Feb 5, 2010
4,162
With magnetic shielding.

The current flow and therfore the magnetic field around an AC conductor is always changing.
Varying magnetic fields induce current flow in conductive materials. Blocking the magnetic field will block the inductive effect of AC current.

#### gerty

Joined Aug 30, 2007
1,304
I know that there are plenty of ways to measure AC voltage without breaking the circuit but is there a way to block AC voltage in the same way?
Why do you have to break the circuit to measure voltage? Do you mean current?

#### electrophile

Joined Aug 30, 2013
167
With magnetic shielding.

The current flow and therfore the magnetic field around an AC conductor is always changing.
Varying magnetic fields induce current flow in conductive materials. Blocking the magnetic field will block the inductive effect of AC current.
Thanks. Is there a way to induce this magnetic field? For e.g. Is it possible to turn an appliance connected to main on and off using a contactless magnetic inducing device?

Why do you have to break the circuit to measure voltage? Do you mean current?
Exactly. You dont have to break the circuit to measure either. Most clamp meters measure current and quite a few good meters today have the NCV method to measure AC voltage.

#### gerty

Joined Aug 30, 2007
1,304
[QUOTE="
Exactly. You dont have to break the circuit to measure either. Most clamp meters measure current and quite a few good meters today have the NCV method to measure AC voltage.[/QUOTE]

You haven't told us what you are doing yet ! To measure current with a DMM you do have to break the circuit and insert the meter. You are not going to measure dc milliamps with a clamp on meter...

Last edited:

#### alfacliff

Joined Dec 13, 2013
2,458
my dvm reads dc current without breaking the circuit. it measures the magnetic field with a clamp on just like an ac clampon does, just a little bit different search for AC/DC CLAMPON METER. and there is no way to turn off the current to an apliance with a clampon device. the switching device must go in the line to break the circuit.

#### BR-549

Joined Sep 22, 2013
4,928
"Is it possible to turn an appliance connected to main on and off using a contactless magnetic inducing device?"

Yes, but is hasn't been used in many years. It's called reactance or reactor control.

It was used with magnetic amplifier control circuits. A long lost, but very elegant, simple and most reliable control system.

#### recklessrog

Joined May 23, 2013
985
Have a look at this.........

Joined Jul 18, 2013
27,620
To measure current with a DMM you do have to break the circuit and insert the meter. You are not going to measure dc milliamps with a clamp on meter...
My DMM also has a clamp-on ammeter that has a 300ma and a 10a scale AC or DC.
Max.

Last edited:

#### recklessrog

Joined May 23, 2013
985
In the '60's I was working on part of a missile guidance mechanism that used that principal for control and current stabilisation. A specially wound transformer and a rectifier that fed dc into a control winding. This controlled a 400 Hz synchro servo system, (a bit like an aerial rotator) and as the load on the motor increased, so the applied voltage was increased automatically.

#### BR-549

Joined Sep 22, 2013
4,928

#### Attachments

• 7.5 MB Views: 7

#### recklessrog

Joined May 23, 2013
985
BR-549's post is excellent, fig21 on page 9 with the servo control is similar to the control circuit I described in my post #11

#### GopherT

Joined Nov 23, 2012
8,009
Thanks. Is there a way to induce this magnetic field? For e.g. Is it possible to turn an appliance connected to main on and off using a contactless magnetic inducing device?

Exactly. You dont have to break the circuit to measure either. Most clamp meters measure current and quite a few good meters today have the NCV method to measure AC voltage.
You can also get an optoelectronic triac as a switch. I don't know what you consider non-contact but, the first unit (opto-isolated Triac) has a DC circuit with an IR LED emitter (a few milliamps) not connected to your AC voltage. The illumination of the IR LED will trigger the TRIAC. Triacs that can handle hundreds of AC volts are available - I don't know your current handling needs.

#### recklessrog

Joined May 23, 2013
985
What a lovely trip down memory lane! that's when electronics was an exciting hobby for me as a youngster! just browsing the adverts takes me back to such a wonderful time. Thanks for that BR-549, I shall be reading the whole magazine
There's an interesting article on page 73.

#### BR-549

Joined Sep 22, 2013
4,928
Yep, I spend more time on the ads than the articles.

#### gerty

Joined Aug 30, 2007
1,304
To measure current with a DMM you do have to break the circuit and insert the meter. You are not going to measure dc milliamps with a clamp on meter...
My DMM also has a clamp-on ammeter that has a 300ma and a 10a scale AC or DC.
Max.[/QUOTE]

His original question was about voltage not current..Just wanted to clarify his need to "break circuit" to measure voltage

I know that there are plenty of ways to measure AC voltage without breaking the circuit but is there a way to block AC voltage in the same way?

#### AnalogKid

Joined Aug 1, 2013
10,564
He clarified that he meant current. I think he wants to make a non-contact on/off switch by somehow inducing into an AC power line an out-of-phase current strong enough to offset the load current and bring the net current down to zero.

ak