Be sure to click 'more' and select 'suggest as answer'!
If you're the thread creator, be sure to click 'more' then 'Verify as Answer'!
Dear friends,
I found it is impossible to online buy a cheap Non-contact Direct Current voltage tester pens.
They only sell AC pens.
Does anyone know where to buy cheap Non-contact Direct Current (DC) tester pens , that is not those very expensive Fluke pens?
Thanks for all your great help and May God bless you and your family 
Hi kmikemo, here are some thoughts that may explain your measurement with the ET30 magnetic field detector: Let's first remember that electric current flows in electric loops, so for any "forward" current flowing through a wire there is a "return" current flowing in opposite direction through a different wire back to the voltage supply. When you measure the magnetic field generated by a current through a wire you are actually measuring a superposition of the magnetic field lines generated by the "forward" current and "return" current. Here is a picture from my book "Noise Coupling in Integrated Circuits" that shows the magnetic field lines generated by an electric current:
The return current flows in opposite direction, so in the next figure Wire A is the forward current and Wire B is the return current. Imagine that wire B generates the same circular magnetic field lines but rotating in opposite direction:
So in between the wires the magnetic field lines of wire A and wire B have the same direction, so the magnetic field intensity doubles up, and outside the magnetic field lines of wire A and wire B have opposite direction and the magnetic intensity subtracts. If the two wires are next to each other the magnetic field intensity outside gets close to zero, so your magnetic probe will not measure anything. This is often the case of PCBs with ground planes where the spacing between the forward current and the return current is the thickness of the PCB dielectric. So if you try a measurement on a setup that allows you to measure the magnetic field in the space between the forward current and return current you will probably be able to get a valid result. Like for example, place your probe inside the current flow loop as shown in the diagram below:
Hope my explanation helps clarify your measurement issue. If anyone is interested in my book here is the link to the listing on Amazon.com:
Best Wishes,
Cosmin
Hi kmikemo, here are some thoughts that may explain your measurement with the ET30 magnetic field detector: Let's first remember that electric current flows in electric loops, so for any "forward" current flowing through a wire there is a "return" current flowing in opposite direction through a different wire back to the voltage supply. When you measure the magnetic field generated by a current through a wire you are actually measuring a superposition of the magnetic field lines generated by the "forward" current and "return" current. Here is a picture from my book "Noise Coupling in Integrated Circuits" that shows the magnetic field lines generated by an electric current:
The return current flows in opposite direction, so in the next figure Wire A is the forward current and Wire B is the return current. Imagine that wire B generates the same circular magnetic field lines but rotating in opposite direction:
So in between the wires the magnetic field lines of wire A and wire B have the same direction, so the magnetic field intensity doubles up, and outside the magnetic field lines of wire A and wire B have opposite direction and the magnetic intensity subtracts. If the two wires are next to each other the magnetic field intensity outside gets close to zero, so your magnetic probe will not measure anything. This is often the case of PCBs with ground planes where the spacing between the forward current and the return current is the thickness of the PCB dielectric. So if you try a measurement on a setup that allows you to measure the magnetic field in the space between the forward current and return current you will probably be able to get a valid result. Like for example, place your probe inside the current flow loop as shown in the diagram below:
Hope my explanation helps clarify your measurement issue. If anyone is interested in my book here is the link to the listing on Amazon.com:
Best Wishes,
Cosmin
