testing coil inductance and srf with neodynium magnet?

[dbanner]

I saw this video and wondered if this coil was connected to an oscilloscope, would ringing be visible and hence the ringing frequency could be measured and the srf and capacitance computed?
waving the magnet across the coil' shows an induced voltage. It may yet be easiest way to quickly check out coil characteristics.
https://m.youtube.com/watch?v=SGhfdXy8eCc#

[6666]

Interesting, I have used a fridge magnet to test for EFE on a PI detector.

[surfdetector]

I saw this video and wondered if this coil was connected to an oscilloscope, would ringing be visible and hence the ringing frequency could be measured and the srf and capacitance computed?
waving the magnet across the coil' shows an induced voltage. It may yet be easiest way to quickly check out coil characteristics.
https://m.youtube.com/watch?v=SGhfdXy8eCc#

That was an interesting video. I shall have to acquire a larger magnet and try this approach although I think that simply loose coupling of the coil requiring measurement to that of a working PI is a quick method as well.
Cheers

[Carl-NC]

I saw this video and wondered if this coil was connected to an oscilloscope, would ringing be visible and hence the ringing frequency could be measured and the srf and capacitance computed?

Probably not. Ringing occurs on a step response, waving a magnet is relatively very slow, no matter how fast you wave it.

[Andi68]

It is not clear to me what should be tested there

[dbanner]

Probably not. Ringing occurs on a step response, waving a magnet is relatively very slow, no matter how fast you wave it.

True.
I'd rather have 400 bad ideas and a few good ones, rather than having zero ideas.

What about a resistor in series with the coil, maybe there will be some measurable slope or transient which could reveal some useful information about the inductor.

Just seems like such a nice little trick that is otherwise useless.

[Melbeta]

True.
I'd rather have 400 bad ideas and a few good ones, rather than having zero ideas.

What about a resistor in series with the coil, maybe there will be some measurable slope or transient which could reveal some useful information about the inductor.

Just seems like such a nice little trick that is otherwise useless.

Well, here is my two cents worth of information. From what I was told, technicians hooked a coil up to an oscilloscope, then watched for a particular waveform, then adjusted to that wanted waveform. But an noise generator must have been used to inject noise or something to create the waveform. So CARL, can you comment and explain what I just said??? Melbeta

[bbsailor]

The easiest way to test a coil or multiple coils, designed differently, is to use the coil self resonant frequency (SRF) which will reveal its total capacitance. Higher SRF means lower capacitance and lower SRF means higher capacitance.

Here is how to do it. Attach a signal generator with a maximum frequency above 1MHZ to the coil though a 1 to 2 meg ohm resistor. This resistor isolates the coil from the output load of the signal generator and allows the coil's full resonance to occur unloaded. Attach the scope probe set on 1X to the coil but through a 1 pf capacitor. You can make a "gimmick" capacitor by twisting about 1 to 1.5 inch of insulated copper wire together, using the capacitance between the strands. This capacitance is in series with the probe capacitance and eliminates it from the SRF measurement. Make sure that you are observing a good low level on the oscilloscope and a good output level on the signal generator. As you sweep the signal generator and get near the 1 MHZ range, you will start to see the output level on the oscilloscope begin to peak, keep rising and then peak and then decline past the SRF point. Note the peak SRF point and that will be useful in calculating the capacitance if you know the coil inductance or compare various winding techniques to see which designs have the least capacitance. If you really want to see the effect on the circuit performance, measure the coil though the coax to include that in the SRF which will always be there.

This method is a very reliable way to compare different coil styles, wire types, insulation thicknesses and insulation dielectric constants.

I hope this helps.

Joseph J. Rogowski

[Carl-NC]

Well, here is my two cents worth of information. From what I was told, technicians hooked a coil up to an oscilloscope, then watched for a particular waveform, then adjusted to that wanted waveform. But an noise generator must have been used to inject noise or something to create the waveform. So CARL, can you comment and explain what I just said??? Melbeta

Probably you're referring to the method of nulling an IB coil. That's done with a sig gen on the TX. What we're talking about here is how to measure the SRF of a PI coil, which is also normally done with a sig gen. But it's looking at something entirely different.

[dbanner]

I am wondering if it might be possible to measure inductance by means of defining xL using a series pot with a fixed frequency sinus drive. Adjust the pot until there is equal voltage across both pot and inductor. This means that the pot value will now be equal to xL. Given the frequency, you coul calculate L.

If you wanted to calculate inductance in the range of say100-1000uH, an appropriate pot and sinus frequency could be selected for such measurements.

[Carl-NC]

Sure, that'll work.

[Altra]

If you wanted to calculate inductance in the range of say100-1000uH, an appropriate pot and sinus frequency could be selected for such measurements.

My BK 875B uses 1khz and the Extech 1khz, switchable to 120hz. The 120hz must be for large inductors? I use the 1khz for coils.

See attached schematic of a simple L meter that uses a dvm as a readout. This was a kit that I built as I recall it worked ok?

[dbanner]

My BK 875B uses 1khz and the Extech 1khz, switchable to 120hz. The 120hz must be for large inductors? I use the 1khz for coils.

See attached schematic of a simple L meter that uses a dvm as a readout. This was a kit that I built as I recall it worked ok?

I have 74132 chip, but not 74HC132. Do you think it will work for the circuit you posted?

[surfdetector]

Here is a link to a slightly cleaner copy of the Rainbow kit schematic and BOM
http://www.rainbowkits.com/kits/instructions/IA-1.pdf

Cheers

[dbanner]

Here is a link to a slightly cleaner copy of the Rainbow kit schematic and BOM
http://www.rainbowkits.com/kits/instructions/IA-1.pdf

Cheers

Thanks.