DC current clamps - UK Vintage Radio Repair and Restoration Discussion Forum

flyingtech55 · 3rd Feb 2016, 10:08 am

Hi all

Can any of you clever folk explain how a DC current clamp works. I'm still fiddling about with the DC to 220V AC inverters which were the subject of my previous thread. I have a DC current clamp to measure the primary current. I would like to understand how they work so that I can interpret the measurements (if I need to) rather just accept them prima facie without knowing the method by which they are derived.

Thanks in advance,

TimR

PS. I think this is the correct section for this query but if not would the mods do the necessary please and thanks in equal measure.

3rd Feb 2016, 11:10 am

Some (most, all?) us a hall effect device in the magnetic loop.

Radio Wrangler · 3rd Feb 2016, 11:32 am

There are two ways to sense a constant magnetic field. As MM says, most clamps use hall effect sensors ( Pass a current through a block of semiconductor and compare the voltage of points on opposite sides... transverse to the current flow, and you measure the distortion of the flow due to the field.

The other one is the 'Flux gate magnetometer' where a bit of ferrite is used to concentrate the field in a coil, then a second coil is driven hard to saturate the ferrite and the concentration effect disappears, do this repetitively and sense the AC voltage induced in the first coil. There's a trick that saturation happens twice per cycle of the hard driving current, whereas stray coupling will be at the drive frequency, so the sensing electronics looks for the doubled frequency voltage.

David

Andrew B · 3rd Feb 2016, 2:41 pm

I have only seen the clamps that use the "Hall" effect.

I used to see them all the time in car garages on "Sun" multimeters (100-0-100 Amps)and engine analysers (400-0-400Amps), where they are used to measure starter current, alternator current and heater plug current.

When the technicians got bored they seemed to enjoy opening the clamps and letting them "snap" shut

did the sensors and iron cores no

end of >good<, certainly kept me in business repairing and recalibrating them.

As I remember they had 4 or 5 wires onto the Hall device which was bonded to an aluminium plate, located at the open part of the jaws. Accurate to about 2% on the 400 Amp scale.

Andrew B · 3rd Feb 2016, 3:24 pm

Here's a simple diagram of a system I found that may be useful to you.

Herald1360 · 3rd Feb 2016, 4:11 pm

ISTR that most DC "current transformers" use a bucking coil servoed from the Hall Effect or Fluxgate to force the flux detected in the core to zero. At this point the current in the wire through the clamp will be N times the current in the bucking coil where N is the number of turns in the bucking coil. Depending on the speed of the bucking coil servo loop, the current transformer can be good up to some quite high frequencies. MHz or more in the case of oscilloscope current probes.

jimmc101 · 3rd Feb 2016, 6:19 pm

My final year project (back in 1970) was based on a Fluxgate to measure 20,000A DC and used a bucking coil as you describe.
I confess that I thought that Hall effect based sensors also used a bucking coil but when I investigated my LEM PR200 I found that it didn't use one.

As an aside, I rescued it after it had been binned at work for excessive drift, it wasn't until I used it with a 'scope that I found it was oscillating badly. (The output cable had been replaced at some time, possibly increasing the capacitive load on the output.)
After reverse engineering it, I found that there was an error in the design, the feedback capacitors (C1,2) were connected to the load side of the isolating resistors (R14, 15).
After moving the caps to the Op-Amp outputs, the oscillation stopped and the drift was tamed.

Jim

Nice demo of Hall effect on Sixty Symbols

kalee20 · 3rd Feb 2016, 8:24 pm

Hall effect devices are quite good, but have limited accuracy unless you do tricks like temperature compensation.

Accuracy can be increased in the method that Herald1360 describes, where you cancel out the measured current by an opposing current (or by 1/100th of the measured current in a 100-turn coil). That way, the Hall device does not need to be particularly accurate as it sits inside a feedback loop.

The same goes for a fluxgate sensor.

I have used both - most modern DC clamp meters use Hall effect devices but I did once use a Hewlett Packard HP428 DC clamp meter which used a fluxgate transducer. It was typical HP - compact, well-built. It had about a dozen valves in it and was mains operated. I knew it was something special when I saw it went to DC.

For pure AC, of course, there is the traditional current transformer.

3rd Feb 2016, 10:08 am	#1
flyingtech55 Octode Join Date: Nov 2005 Location: Ayrshire, UK. Posts: 1,096	DC current clamps Hi all Can any of you clever folk explain how a DC current clamp works. I'm still fiddling about with the DC to 220V AC inverters which were the subject of my previous thread. I have a DC current clamp to measure the primary current. I would like to understand how they work so that I can interpret the measurements (if I need to) rather just accept them prima facie without knowing the method by which they are derived. Thanks in advance, TimR PS. I think this is the correct section for this query but if not would the mods do the necessary please and thanks in equal measure. __________________ All those moments will be lost in time, like tears in rain.

3rd Feb 2016, 11:10 am	#2
Guest Guest Posts: n/a	Re: DC current clamps Some (most, all?) us a hall effect device in the magnetic loop.

3rd Feb 2016, 11:32 am	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,902	Re: DC current clamps There are two ways to sense a constant magnetic field. As MM says, most clamps use hall effect sensors ( Pass a current through a block of semiconductor and compare the voltage of points on opposite sides... transverse to the current flow, and you measure the distortion of the flow due to the field. The other one is the 'Flux gate magnetometer' where a bit of ferrite is used to concentrate the field in a coil, then a second coil is driven hard to saturate the ferrite and the concentration effect disappears, do this repetitively and sense the AC voltage induced in the first coil. There's a trick that saturation happens twice per cycle of the hard driving current, whereas stray coupling will be at the drive frequency, so the sensing electronics looks for the doubled frequency voltage. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

3rd Feb 2016, 2:41 pm	#4
Andrew B Heptode Join Date: May 2015 Location: Bradford, West Yorkshire, UK. Posts: 512	Re: DC current clamps I have only seen the clamps that use the "Hall" effect. I used to see them all the time in car garages on "Sun" multimeters (100-0-100 Amps)and engine analysers (400-0-400Amps), where they are used to measure starter current, alternator current and heater plug current. When the technicians got bored they seemed to enjoy opening the clamps and letting them "snap" shut did the sensors and iron cores no end of >good<, certainly kept me in business repairing and recalibrating them. As I remember they had 4 or 5 wires onto the Hall device which was bonded to an aluminium plate, located at the open part of the jaws. Accurate to about 2% on the 400 Amp scale. Attached Thumbnails __________________ worried about the electrons entering the circuit and the smoke leaving Andrew Last edited by Andrew B; 3rd Feb 2016 at 2:57 pm.

3rd Feb 2016, 3:24 pm	#5
Andrew B Heptode Join Date: May 2015 Location: Bradford, West Yorkshire, UK. Posts: 512	Re: DC current clamps Here's a simple diagram of a system I found that may be useful to you. Attached Thumbnails __________________ worried about the electrons entering the circuit and the smoke leaving Andrew

3rd Feb 2016, 4:11 pm	#6
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: DC current clamps ISTR that most DC "current transformers" use a bucking coil servoed from the Hall Effect or Fluxgate to force the flux detected in the core to zero. At this point the current in the wire through the clamp will be N times the current in the bucking coil where N is the number of turns in the bucking coil. Depending on the speed of the bucking coil servo loop, the current transformer can be good up to some quite high frequencies. MHz or more in the case of oscilloscope current probes. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

3rd Feb 2016, 8:24 pm	#8
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: DC current clamps Hall effect devices are quite good, but have limited accuracy unless you do tricks like temperature compensation. Accuracy can be increased in the method that Herald1360 describes, where you cancel out the measured current by an opposing current (or by 1/100th of the measured current in a 100-turn coil). That way, the Hall device does not need to be particularly accurate as it sits inside a feedback loop. The same goes for a fluxgate sensor. I have used both - most modern DC clamp meters use Hall effect devices but I did once use a Hewlett Packard HP428 DC clamp meter which used a fluxgate transducer. It was typical HP - compact, well-built. It had about a dozen valves in it and was mains operated. I knew it was something special when I saw it went to DC. For pure AC, of course, there is the traditional current transformer.