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Disclaimer: I'm computer scientist

I want to test a system described in an old (~1990) french patent and experiment with it if it works. The system requires to generate an "intense" varying magnetic field in the audio frequency range (<20kHz). The inventor used special signals, but I plan to use only a sin wave.

To produce the "intense" varying magnetic field, the inventor used a conventional 60W RMS audio amplifier and a simple 4 Ohms coil. The coil was described to be made of 300 turns of a 0.5mm wire, a height of 80mm and an internal diameter of 50mm. The inventor specified that he see 10Vefficient ("tension efficace") at the coil terminations. I'm not sure what it exactly means. Is it 14.1Vtt, 28.2 Vtt ? What power could that be ? Is it roughly 10*10/4 = 25W ?

A friend feared that the 0.5mm wire might be too thin and that the coil might heat up. Is this true ? I assumed that this should not be a problem if the inventor had no problem, but I'm not 100% sure that the system described by the inventor is valid.

I then made my own coil using 0.5mm wire. I used this service to determine the number of turns required to get a 4 Ohms coil with a height of 25mm and internal diameter of 16mm. This is what I got.

enter image description here

I then tried to measure the inductance and ESR with the method described here that uses a triangular signal. According to the method I should see this

enter image description here.

I already tried this method with a small coil I made but with much less turns. I did indeed see the expected image. I noticed that I started to see an distortion (overdrive?) at a voltage around 2V if I remember well. Is this distortion due to the interaction of the coil with the generator or is it only a property of the coil ?

Now trying the method with my newly made coil of 220 turns, what I see is this

enter image description here.

It doesn't match at all the expected look. It seem that the signal is shifted. I'm no electronic engineer, but I suspect that it could be due to a non-negligible capacitance. Also, above 4V the signal becomes distorted. I didn't try with the 160W amp yet. I don't want to break anything.

Do I see this because I made a compact coil instead of a big and stretched coil as did the inventor ?

This is what I see with a sinusoidal wave of 3kHz and 10Vtt

enter image description here.

The sin wave is distorted and significantly attenuated.

These results are unexpected for me since coils of speakers are usually compact and not stretched. I assumed that the coil of the inventor is stretched because he puts big thing in the core. Could it be to reduce inductance ? I need only to put small liquid containers (Eppendorf) in the core.

Will I see such distortion too if I connect the coil as is to the audio amplifier ? Before testing this I prefer to ask the experts if what I do is OK.

What should/could I do to produce a powerful not distorted sinusoidal signal ?

Last question is how to determine the approximate output power of the audio amplifier if all I have is an oscilloscope. I don't have a good ampere meter.

Edit: I now remember that the support service of the oscilloscope told me that a coil connected to the output of the generator should have an impedance of 50 Ohms, otherwise the signal would be attenuated. What I see could thus be normal and result from the interaction with the generator.

Edit 2: I finally manage to measure the inductance using the method described here. The inductance measured with this method is 0.5 mH which is not that far from the 0.44 I was expecting. I need a good multimeter. I could then measure DC resistance of the coil to determine impedance. I'll need it also to measure current to determine the input power of the coil. Next tool to buy.

Edit 3: I tested the coil by connecting it to the audio amplifier output. I checked the voltage with the oscilloscope. The voltage is <200mV and every 5 to 10s I see an 8V short peak. The coil becomes hot. I checked that the amp is OK by connecting a speaker. It works witch also proves that the generator connected to the input line works as expected. It's just the coil which is bogus. Speaker coils have a magnet as core and when current flows through the coil a force is produced that I assume generates a resistance to the current flow. My coil doesn't have that. The inventor's coil doesn't have it too, so I assumed it was OK. The geometry of my coil and the coil of the inventor are different. Could this be the explanation ? It seam that my coil has a non negligible capacitance because of the compactness of the wiring. Is this the problem ?

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    \$\begingroup\$ "Tension efficace" would be "RMS voltage" in English. That's the "effective DC" equivalent of an AC signal. \$\endgroup\$ – JRE Jan 13 at 11:47
  • \$\begingroup\$ your sine seems to be about 3.1kHz, if the coil is really 0.44mH that should be an impedance of about 8.7R. The amp should handle this with no issues. So I'm doubting if your inductor is really that value. My suspicion is that for some reason it's a lot less, and the amp is seeing a load of < 4R. \$\endgroup\$ – danmcb Jan 13 at 12:23
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    \$\begingroup\$ @danmcb thanks for the help. I calculated myself that 0.44mH it should be 4Ohms at 1kHz. 0.44*2*3.14157+1.29= 4.05 Ohms. I didn't expect the impedance would change that much with the frequency. \$\endgroup\$ – chmike Jan 13 at 12:35
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    \$\begingroup\$ and just as background info, note that real loudspeakers present a quite complex load impedance which (generally) has a minimum of 4 or 8R but varies a lot with f. en.wikipedia.org/wiki/… \$\endgroup\$ – danmcb Jan 13 at 12:45
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    \$\begingroup\$ @chmike: If you have a PC with line in and line out, you could use the software described here to measure the impedance over the audio range and the inductance over the audio range. \$\endgroup\$ – JRE Jan 13 at 12:49
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What you have to keep in mind is that the impedance of a coil varies strongly with the frequency.

You measured the inductance of the coil to be 440 microhenries.

440 microhenries has an impedance of 4 ohms only at about 1500 Hz. If you get much above that, the impedance goes up. If you get much below that, the impedance drops.

You need to keep the frequency in the vicinity of 1000 to 2000 Hz with that coil.


You mentioned in another post that you are using an SMSL SA 98E amplifier. It uses a TDA7498E amplifier IC.

That amplifier uses a class D bridge output. Clipping the ground wire of the scope to one of the output wires may cause a short in one side of the bridge. That would cause your coil to be exposed to DC - and your coil is basically a short circuit at DC.

A large capacitor in series with the coil would help in that case.

The best thing to do would be to try it without connecting the scope for now. If the coil stays cool when the scope is disconnected, then there's ways to make the measurements.

If the coil alone still gets hot, you might put a large capacitor in series with the coil. I'd try about 50 microfarads. That'll give you about 3 ohms impedance at 1000 Hz added to the impedance of the coil. That should prevent overheating in the coil from any DC that may be present.


You've confirmed it was a problem with the oscilloscope.

Connect a 100 nanofarad capacitor to the probe and another to the ground lead of the probe (that's two capacitors.)

Connect the free ends of the capacitors to the coil.

Your scope should show the driving signal with the correct shape and voltage.

For a start, you can estimate the power from the voltage and the impedance of the coil.

You measured inductance of the coil and got 440 microhenries. Use that and the driving frequency to calculate the impedance (\$Z = 2 \pi fL\$ where Z is in ohms, f is in hertz, and L is in henries.)

Measure the voltage with the scope. Calculate RMS voltage from the peak to peak voltage, or use the RMS function if your scope has it.

Effective power is \$P = \frac{V_{RMS}^2}{Z} \$, in watts, where V is in volts and Z is in ohms.

That won't be perfectly accurate, but it'll give you numbers that are at least somewhat realistic.

That'll be a bit inconvenient because of the math - if you change the frequency you have to start all over again with calculating the impedance and then the power.

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  • \$\begingroup\$ Awesome. It looks like you found the problem. It works now. I generate 1kHz sinusoidal signal and amplify it with the audio amp. The problem is that I couldn't tell like that if it worked. I used another coil I have (2000 turns, 300 Ohms) and put it in front. I then measure the voltage at second coil with the oscilloscope. I do see the 1kHz inducted tension. I checked with the FFT. It's impressive. However, the coil is hot when the volume is set to 50% and above. Since the purpose of the coil is to induce a varying magnetic field, will the capacitor still allow that ? \$\endgroup\$ – chmike Jan 14 at 14:54
  • \$\begingroup\$ I'll add that there is no visible distortion of the signal seen on the oscilloscope. Thank you very much. You saved my experiment.How can I measure the output power ? Can I measure the voltage and current with a multimeter or would this be also problematic ? \$\endgroup\$ – chmike Jan 14 at 15:06
  • \$\begingroup\$ Which part actually fixed the problem? \$\endgroup\$ – JRE Jan 14 at 15:12
  • \$\begingroup\$ Disconnecting the oscilloscope from the 4 Ohms coil. It could be that it worked already since the coil heated, but the oscilloscope output showed 0V (<200mV) and every 5 to 10s an 8V signal. After removing the oscilloscope, the coil still heat, especially with high volume. I checked with induction that the 4 Ohms coil properly worked. I can't measure the power. \$\endgroup\$ – chmike Jan 14 at 15:17
  • \$\begingroup\$ It would take two multimeters to measure the power, and both would have to be capable of measuring at audio frequencies. Most inexpensive meters can't. You'd have to check the specifications of the meter. I expect a meter capable of what you need would cost a couple of hundred euros. \$\endgroup\$ – JRE Jan 14 at 15:45
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with a core and multi layer, it prob won't be ideal inductor. don't know if that matters for you, but here's a possible impedance measurement setup without anything fancy

enter image description here

DUT is your coil. measure voltages. small (1-10ohm ? depends on range of L) sense resistor gives current, math function on scope gives voltage across coil.

use low ish amplitude sine signal, that way actual output of amp is not so important since you measure it anyway. test frequencies at 10,100,1k,10k etc. fill in if curious. pay attention to phase. try different DC offsets, and think of them in terms of current. try different amplitude too.

PS - in addition to impedance, to get a good mental model of the coil/core, look up the B-H behavior, hysteresis, saturation. if you're winding your own, I think a sense winding would be helpful for that, this measurement is not a great way to get insight into that in my limited knowledge. If you're curious they have cheap magnetic flux sensors.

PPS - original question was about audio amplifier. it might do bad things with improper load on the output, or too much DC on input, suggest to double check

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  • \$\begingroup\$ Sorry, this explanation is not clear enough. How can I determine if the coil is safe to use with my amp expecting a 4 Ohms load ? Same question for input. \$\endgroup\$ – chmike Jan 13 at 16:32
  • \$\begingroup\$ Look on a Q&A website such as StackExchange =) ... I think I know, but I don't want to cause you to blow anything up, so would encourage you to get a second opinion. For DC input, the amplifier specs may say. Almost certainly AC-coupled so unlikely to be a big deal. Unfortunately it would prevent you from making any DC offset. The amp output, definitely look up. \$\endgroup\$ – Pete W Jan 13 at 16:35
  • \$\begingroup\$ I asked the support of the company that build the generator and they say it's OK to connect the generator to the input line of the amplifier as long as the voltage doesn't exceed the input specifications (0.4 - 2V). I thus assume input is fine. It's the coil that I'm not sure. It should not be too different from 4 Ohms (1kHz). Unfortunately I don't have an easy way to check that. What worries me is the distortion I see when connected to my generator. Where does this distortion come from ? The generator ? The coil ? What will happen with the amp ? Will it also be distorted ? \$\endgroup\$ – chmike Jan 13 at 16:59
  • \$\begingroup\$ After doing this you would learn the answer :-P ... you could use a big enough sense resistor to ensure a minumum impedance, but some amps also don't like too weak of a load, though I think this is less common in modern times. Search for that on StackExchange, I'm sure it has been discussed to death on some other post \$\endgroup\$ – Pete W Jan 13 at 17:09
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    \$\begingroup\$ I managed to measure inductance of my coil. It was simple and fast. See edit 2 at end of my question. \$\endgroup\$ – chmike Jan 13 at 17:49
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The impedance of your coil is too high for 20kHz.

You have 2 choices raise the voltage significantly (440/30) = x15 (unwise with parasitic capacitance unless SRF >> 20kHz with winding method)

... or reduce the inductance /15 as shown below to 30 uH and heavier wire.

enter image description here

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  • \$\begingroup\$ I won't need 20kHz. I'll only generate at most up to 5kHz. I find it counter intuitive that the impedance is too high because the impedance of speakers are supposed to be in the range 4 to 8 Ohms (what frequency?). To reduce impedance I would have to reduce the number of turns, and this is also counter intuitive because the goal is to generate an "intense" varying magnetic field. With less turns, the induced magnetic field would be weaker. I'm thus a little bit reluctant to reduce impedance. Is my reasoning correct ? You're the expert. \$\endgroup\$ – chmike Jan 14 at 16:03
  • \$\begingroup\$ Impedence is not uniform unless you have a very strong magnet like a speaker. Thus you need lower L to raise current. Thus the accepted other answer will not work. \$\endgroup\$ – Tony Stewart EE75 Jan 14 at 17:36
  • \$\begingroup\$ Thank you for your help. So you suggest a wire with bigger diameter and less turns ? Won’t this create a shortcut and harm the amplifier ? The frequency range I plan to use if the experiment works is between 100Hz and at most 10kHz. Using your graphic, L should then be around 80uH. I targetted 440uH and measured 500uH. Will the coil heat less if I use a thicker wire ? Will I have a stronger induced magnetic field ? I thought that the induced magnetic field was somehow proportional to L. \$\endgroup\$ – chmike Jan 14 at 19:11
  • \$\begingroup\$ For an air coil use the chart for impedance and V to compute current. But can you link the patent? Something is missing. \$\endgroup\$ – Tony Stewart EE75 Jan 14 at 19:18
  • \$\begingroup\$ I’m sorry, I don’t understand what you mean by « raise the voltage » in your answer. What voltage are you referring to and how could I change it ? As far as I know the only controls I have are the volume and input voltage. Unfortunately, I can’t share the patent because I don’t want to disclose the application. It is in french anyway. I gave all the details related to the coil and amplifier that are given in the patent. My goal is just to reproduce the system and experiment with it if it works. Sorry \$\endgroup\$ – chmike Jan 14 at 19:35

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