Non-contact AC blocking

Thread Starter

electrophile

Joined Aug 30, 2013
126
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
4,163
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.
 

Thread Starter

electrophile

Joined Aug 30, 2013
126
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
1,267
[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...
 
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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
4,938
"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.
 

MaxHeadRoom

Joined Jul 18, 2013
19,466
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.
 
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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.
 

GopherT

Joined Nov 23, 2012
8,012
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.
 

gerty

Joined Aug 30, 2007
1,267
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
8,231
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
 

Thread Starter

electrophile

Joined Aug 30, 2013
126
@AnalogKid Bingo! Here is what I'm thinking of building - a device that can turn on or off AC connected appliances but without breaking the path to AC mains, i.e. without a physical or electronic switch in the circuit.

@BR-549, @recklessrog and @MaxHeadRoom : Thanks that is useful. I'll dig into it.

@GopherT Thanks for pointing me to these. I'll still need to break the circuit to put these in.
 
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