how do clamp meters work?

[John Smith]

Hello,
I was wondering if there is a way to create a simple cheap version of a
clamp meter for detecting if there is current flowing through a
powercord. I tried wrapping small wire around the powercord and then
connecting a volt meter, but I didn't read any voltage. I guess maybe
I am missunderstanding how a clampmeter works?

thanks,
-Me

 

[Joerg]

Hello John,

I was wondering if there is a way to create a simple cheap version of a
clamp meter for detecting if there is current flowing through a
powercord. I tried wrapping small wire around the powercord and then
connecting a volt meter, but I didn't read any voltage. I guess maybe
I am missunderstanding how a clampmeter works?

Use Google to search "current transformer". Then you'll know ;-)

A power cord carries current back and forth so if you run the whole cord
through a clamp meter it should not read anything. The field cancels
because of opposite phases of current flow between line and neutral. If
it does read something make sure that the equipment at the other end of
this power cord is taken out of service and repaired. Because that would
mean some of the current returns outside the power cord which is dangerous.

BTW, this is also how GFCI switches work. They rely on the fact that all
current phases cancel because all wires that can carry current go
through the core. PE doesn't so if a leaky device lets enough milliamps
slip onto PE the balance is upset and the GFCI is supposed to trip.

 

[[email protected]]

Hello,
I was wondering if there is a way to create a simple cheap version of a
clamp meter for detecting if there is current flowing through a
powercord. I tried wrapping small wire around the powercord and then
connecting a volt meter, but I didn't read any voltage. I guess maybe
I am missunderstanding how a clampmeter works?

thanks,
-Me

You won't see much if you enclose both wires in your clamp... You have
to isolate one single current carrying conductor to be enclosed by the
clamp.
Then you have to design the thing.

 

[Paul E. Schoen]

Hello,
I was wondering if there is a way to create a simple cheap version of a
clamp meter for detecting if there is current flowing through a
powercord. I tried wrapping small wire around the powercord and then
connecting a volt meter, but I didn't read any voltage. I guess maybe
I am missunderstanding how a clampmeter works?

thanks,
-Me

Most of the magnetic field will be between the conductors, but there will
probably be a detectable field close to one of the wires. An air core
inductor with many turns of fine wire can be used as a magnetic field
sensor, and you can probably detect some voltage if you connect it to a
sensitive voltmeter or scope, and position the coil properly. There will
also be some component from other EM radiation, which may also be
directional.

Paul

 

[Don Lancaster]

Hello,
I was wondering if there is a way to create a simple cheap version of a
clamp meter for detecting if there is current flowing through a
powercord. I tried wrapping small wire around the powercord and then
connecting a volt meter, but I didn't read any voltage. I guess maybe
I am missunderstanding how a clampmeter works?

thanks,
-Me

Why not use an electrostatic sensor instead?

As in the $7 Radio Shack units?

You can easily build your own from a CMOS hex inverter.


I've got a good schematic somewhere on my website, but I can't seem to
find it.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Paul Hovnanian P.E.]

Why not use an electrostatic sensor instead?

As in the $7 Radio Shack units?

That'll tell you if the circuit is energized, not carrying some level of
current.

 

[John Smith]

A power cord carries current back and forth so if you run the whole cord

through a clamp meter it should not read anything. The field cancels
because of opposite phases of current flow between line and neutral.

Oh crap... I see. I've never actually used a clamp meter before so I
didn't realize that it worked like that. That makes sense.

Thanks!
-Me

 

[John Smith]

An air core

inductor with many turns of fine wire can be used as a magnetic field
sensor, and you can probably detect some voltage if you connect it to a
sensitive voltmeter or scope, and position the coil properly.

So I should try placing that next to the cord instead of wrapping it
around the cord?

-Me

 

[John Smith]

Why not use an electrostatic sensor instead?

That'll tell you if the circuit is energized, not carrying some level of
current.

I noticed that if I wrapped the wire around the powercord and then
connected only one probe from the voltmeter, I would get a reading when
the powercord was pluged in, but it didn't seem to matter how much
current was being used. That must be the same type of principle.

-Me

 

[Paul Hovnanian P.E.]

I noticed that if I wrapped the wire around the powercord and then
connected only one probe from the voltmeter, I would get a reading when
the powercord was pluged in, but it didn't seem to matter how much
current was being used. That must be the same type of principle.

Yes. Your 'winding' acts as a capacitor and couples the AC voltage from
the cord into your voltmeter. Most DVMs have very high impedances and
will pick up ambient E fields and indicate a volt or two.

 

[John Larkin]

Oh crap... I see. I've never actually used a clamp meter before so I
didn't realize that it worked like that. That makes sense.

Thanks!
-Me

But if you wrap a sense wire around one of the conductors, you still
won't get a usable signal; there's no magnetic coupling in that
direction.

A current transformer, even a clamp-on, has a ferrous core that both
the current-carrying wire and the sensing wire pass through. The power
wire makes one pass through the core, "one turn", and the sense wire
usually makes many.

You could make a crude CT by winding a bunch of turns of fine wire
through a steel washer and also running the current-carrying conductor
through the center. It's not a very good core at 60 Hz, but you will
get a signal that depends on current.

You can also place an air-core coil near a current-carrying conductor
in the proper orientation and induce a small signal into it,
proportional to current.

Google "current transformer" and "current transformer burden resistor"


John

 

[Don Lancaster]

So I should try placing that next to the cord instead of wrapping it
around the cord?

-Me

To sense the current in a line cord, you have to SPLIT the cord so you
can sense only ONE wire.

Or else go to some Hall Effect scheme where the sensitivity to ONE wire
is MUCH GREATER than that of the other.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[BobG]

While we are talking about clamp on ammeters, lets not forget the DC
clamp on ammeters from HP with a hall sensor in them. Way cool.

 

[Joerg]

Hello John,

You could make a crude CT by winding a bunch of turns of fine wire
through a steel washer and also running the current-carrying conductor
through the center. It's not a very good core at 60 Hz, but you will
get a signal that depends on current.

Golly, I've got to try that. That would have to be the cheapest CT there
is. Should have kept the rusty washers from out old basketball hoop ;-)

You can also place an air-core coil near a current-carrying conductor
in the proper orientation and induce a small signal into it,
proportional to current.

Maybe there is a pre-calculated setup but if not that would probably
require a serious tour through Maxwell's equations. Or just calibrate it
if nobody is looking...

 

[Homer J Simpson]

Golly, I've got to try that. That would have to be the cheapest CT there
is. Should have kept the rusty washers from out old basketball hoop ;-)

You could stack 1" washers 1/2" high. But I'd buy a cheap audio transformer
that I could still run a wire through.

 

[Joerg]

Hello Homer,

You could stack 1" washers 1/2" high. But I'd buy a cheap audio transformer
that I could still run a wire through.

I used to do that with audio transformers. But they are neither cheap
nor widely available anymore because new designs don't use any.
Sometimes luck has it that there is a common mode choke with enough
clearance to squeeze through.

 

[John Larkin]

Hello John,


Golly, I've got to try that. That would have to be the cheapest CT there
is. Should have kept the rusty washers from out old basketball hoop ;-)



Maybe there is a pre-calculated setup but if not that would probably
require a serious tour through Maxwell's equations. Or just calibrate it
if nobody is looking...

Google "Rogowski coil". The idea here is to make an air-core toroidal
coil that encompasses almost all of the flux around a wire. The output
is the time derivative of the current. The good ones, the ones that do
capture most of the flux, can be beautiful.

John

 

[Joerg]

Hello John,

Google "Rogowski coil". The idea here is to make an air-core toroidal
coil that encompasses almost all of the flux around a wire. The output
is the time derivative of the current. The good ones, the ones that do
capture most of the flux, can be beautiful.

This is a nice write-up about them:

http://www.coilws.com/Current Sense/App1_rogowski.pdf

My very first lesson at my alma mater happened to be at their Rogowski
Institute.

 

[Don Lancaster]

To sense the current in a line cord, you have to SPLIT the cord so you
can sense only ONE wire.

Or else go to some Hall Effect scheme where the sensitivity to ONE wire
is MUCH GREATER than that of the other.

Found it.

Schematic of the Radio Shack e field sensor is in
http://www.tinaja.com/glib/muse89.pdf


--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[ehsjr]

But if you wrap a sense wire around one of the conductors, you still
won't get a usable signal; there's no magnetic coupling in that
direction.

A current transformer, even a clamp-on, has a ferrous core that both
the current-carrying wire and the sensing wire pass through. The power
wire makes one pass through the core, "one turn", and the sense wire
usually makes many.

You could make a crude CT by winding a bunch of turns of fine wire
through a steel washer and also running the current-carrying conductor
through the center. It's not a very good core at 60 Hz, but you will
get a signal that depends on current.

Or you could just tap off the CT in a GFCI receptacle
with a second regular receptacle providing connection
to whatever appliance whose current draw you want to
measure:

---------- ---------
| | | | | |
Hot-----0|Line Load|0---------0| o |
| | | |
| GFCI | | x |
| | | |
0|Line Load|0 +----0| | | |
| | | | o |
---------- | ---------
|
Neutral-------------------+


Mount both inside the same junction box and connect
it up with a heavy duty line cord.

Since the GFCI is not connected to the neutral, it
can't trip, and will always induce a current in the
CT when the connected device draws current. You need
to open the GFCI receptacle and solder on a couple
of wires to bring the CT secondary to the outside.

Ed