Why does my LC circuit not oscillate its energy between Electric & Magnetic fields?

[davenn]

Don't you have access to some old scrap electronics... TV's, VCR's etc?
They are a great source for parts

 

[James2018]

Don't you have access to some old scrap electronics... TV's, VCR's etc?

I did fabricate some 16k resistors out of graphite and paper and they worked as indicated my multimeter measuring in resistance mode.
The graphite from a #2 pencil lead will give you somewhere between 5k ohms and 40k ohms of resistance per centimenter.
https://makecode.adafruit.com/learnsystem/pins-tutorial/devices/make-a-resistor

 

[davenn]

I did fabricate some 16k resistors out of graphite and paper and they worked as indicated my multimeter measuring in resistance mode

Not really suitable for what you are trying to achieve.
Surely you have some family, friends etc that have some old gear
you can strip for bits.
I have been doing that since I started in electronics as a kid 55 or so years ago. Am 65 now and still salvage parts from old gear

 

[Baluncore]

If there is somewhere that repairs electronics nearby, they may give you used items for parts.

If there is someone nearby who has many antennas on their house, they might be a member of Federaţia Română de Radioamatorism. They can help you find what you need.

Or contact FRR for an introduction to a local amateur or club.
https://www.hamradio.ro

 

[James2018]

If there is somewhere that repairs electronics nearby, they may give you used items for parts.

If there is someone nearby who has many antennas on their house, they might be a member of Federaţia Română de Radioamatorism. They can help you find what you need.

Or contact FRR for an introduction to a local amateur or club.
https://www.hamradio.ro

There is an electronics shop nearby but they have only resistances between 1 and 1000 ohms. Well, I'll just order them online sometimes.

 

[Merlin3189]

My guess is at about 120 MHz, in the VHF radio band, lasting for less than 1 us.
You might hear a momentary single click from a VHF AM radio receiver.

It would not be visible on a scanning spectrum analyser, because it is too rare, and so would probably not happen at the right time during one sweep.
You could see it on a storage or digital oscilloscope, but only with a scope bandwidth better than 1 GHz, and then only if it triggered and stored in one single sweep.


If you increase the number of turns on your coil from 8, by a factor of 10, to 80 turns, the frequency of resonance will come down by a factor of 10² = 100, into the MW AM broadcast band. You could then hear it in that band, and see it on a low-cost oscilloscope with 20 MHz bandwidth.

If you could arrange a circuit to repeat the resonance at an audio frequency, you could tune in to the resonance frequency and hear that audio tone on an AM radio.

How are you switching the current through the LC circuit ?

My guess is at about 120 MHz, in the VHF radio band, lasting for less than 1 us.
You might hear a momentary single click from a VHF AM radio receiver.

It would not be visible on a scanning spectrum analyser, because it is too rare, and so would probably not happen at the right time during one sweep.
You could see it on a storage or digital oscilloscope, but only with a scope bandwidth better than 1 GHz, and then only if it triggered and stored in one single sweep.


If you increase the number of turns on your coil from 8, by a factor of 10, to 80 turns, the frequency of resonance will come down by a factor of 10² = 100, into the MW AM broadcast band. You could then hear it in that band, and see it on a low-cost oscilloscope with 20 MHz bandwidth.

If you could arrange a circuit to repeat the resonance at an audio frequency, you could tune in to the resonance frequency and hear that audio tone on an AM radio.

How are you switching the current through the LC circuit ?

I think 122 kHz not MHz. The clue is uF. RF tends to come in at pF for VHF.

 

[Baluncore]

I think 122 kHz not MHz. The clue is uF. RF tends to come in at pF for VHF.

I agree, I slipped an engineer's order of magnitude in my estimate.
But why quote it twice?
For so few turns on the inductor, the capacitance was ridiculously large and resistive.

 

[James2018]

I agree, I slipped an engineer's order of magnitude in my estimate.
But why quote it twice?
For so few turns on the inductor, the capacitance was ridiculously large and resistive.

I assembled your circuit with the 33k and 15k resistors and the BC547B transistor, but I kept the values in the resonant LC to the original effective capacitance of 50 nF and effective inductance to 1.35 uH, I did not use 220 pF and 1 nF and 120 uH because I want to keep the original frequency of 612.6 KHz. I will test the radio AM reception soon, when I am not busy.

 

[Baluncore]

... because I want to keep the original frequency of 612.6 KHz.

I don't think the frequency is 612.6 kHz, because if it was, you would hear the quiet spot there with the AM radio.

You need to wind a coil with the correct inductance to oscillate at 1 MHz, that is the mid-point of the BC band. You can then have 60% errors, and it will still be in the AM BC band.

 

[James2018]

no problem I will increase the coil length of the variable inductor so it can have 0.506 uH instead of 1.35 uH to get a resonant frequency of 1.001 MHz.

 

[James2018]

I don't think the frequency is 612.6 kHz, because if it was, you would hear the quiet spot there with the AM radio.

You need to wind a coil with the correct inductance to oscillate at 1 MHz, that is the mid-point of the BC band. You can then have 60% errors, and it will still be in the AM BC band.

Problem, this time no detectable voltage is reaching the inductor (yellow tube covered in tape) at all. May be under 0.1 milliAmperes, the transistor is correctly connected as I know and the resistances are all the right values and connected like in your diagram.

 

[Baluncore]

Problem, this time no detectable voltage is reaching the inductor (yellow tube covered in tape) at all.

The coil is grounded at one end, and DC isolated at the other with two capacitors, C1 & C2, so it should have zero DC voltage.

This circuit oscillates with a high signal voltage, about half the battery voltage, so that might make voltage measurements with a DMM difficult.
Check the base voltage is half the supply.
Check the emitter voltage is about 0.6 volts below the base voltage.

Build your circuit over a sheet of paper, so you can write clear values against components, and C B E against the transistor.

Compute the inductance. I cannot see the coil detail. What is the number of turns on the coil? What diameter, and how long is the coil?

 

[James2018]

The coil is grounded at one end, and DC isolated at the other with two capacitors, C1 & C2, so it should have zero DC voltage.

This circuit oscillates with a high signal voltage, about half the battery voltage, so that might make voltage measurements with a DMM difficult.
Check the base voltage is half the supply.
Check the emitter voltage is about 0.6 volts below the base voltage.

Build your circuit over a sheet of paper, so you can write clear values against components, and C B E against the transistor.

Compute the inductance. I cannot see the coil detail. What is the number of turns on the coil? What diameter, and how long is the coil?

37 turns (not counting the half turns at the legs of the coil), 6.4 cm long, 0.8 cm diameter. I made one bad connection in the circuit, the wires are NOT supposed to touch where the red circle is, right?

The simulation shows that if the wires touch there, a "capacitor loop with no resistance" forms and no current flows. If the wires don't touch there, everything is working.

 

[Averagesupernova]

I made one bad connection in the circuit, the wires are NOT supposed to touch where the red circle is, right?

Of course not. What would be the point of this? Think about how the transistor works and why that connection would be wrong. At this point I would have hoped you would understand this.

 

[James2018]

Of course not. What would be the point of this? Think about how the transistor works and why that connection would be wrong. At this point I would have hoped you would understand this.

Don't worry, I fixed the circuit.

 

[Averagesupernova]

Don't worry, I fixed the circuit.

I wasn't.

 

[Baluncore]

If the wires don't touch there, everything is working.

So, do you now hear a quiet frequency in the AM BC band that gets noisy again when you turn off the oscillator?

 

[James2018]

So, do you now hear a quiet frequency in the AM BC band that gets noisy again when you turn off the oscillator?

C1 and C2 do not allow AC to pass. On one terminal they have full 5 V DC voltage and on the other they have 0 mV. Maybe they are faulty. Only C3 does block 5V of DC and pass 8 mV of AC. Strangely, because they are newly bought just a week ago and by the same manufacturer as C3. I used them in the previous circuit too. Every other component works as intended.

When I said everything was working fine, I meant a circuit identical to the real one, but in the simulation. In reality, the two capacitors don't work.

 

[Baluncore]

Check with ohmmeter that all grounds are connected to battery negative.

Disconnect L1 at the ground end. It should stop the oscillator.
Then measure the base and emitter voltages as in post #117.

Measure the disconnected L1 voltage. That voltage will be Vb or Ve, and tell you if C1 or C2 is shorting.

Meanwhile, I will go for a drive for a few hours.

 

[James2018]

Your circuit works at 0.1 mV AC and the power dissipates quickly without having the chance to be radiated. Perhaps my battery is low quality, perhaps what the engineers use is an AC-to-DC converter instead of a battery.
Whether I use a professional inductor with ferrite core or a homemade inductor, the result is the same. I cannot get even 1 mV of AC.

This is different from my first, imperfect circuit, which worked at 10 mV AC, although not producing a sine wave, but an irregular noise.

 

[Baluncore]

A digital multimeter will not measure a 1 MHz voltage or current. That is why you get silly results like 0.1 mV AC.

The circuit I gave you runs on about 200 uA total. That is almost nothing.
The RF output comes from the magnetic field of the inductor, where there is a 1 MHz current oscillating between ±2.6 mA, plotted below in yellow.
The base voltage is plotted in red, with the emitter voltage in green.

If you disconnect the inductor at the ground end, the oscillator will stop.
Then measure the base voltage, Vb, which will be about 2.5 V.
Also measure the emitter voltage, Ve, which will be about 1.6 V.

 

[James2018]

A digital multimeter will not measure a 1 MHz voltage or current. That is why you get silly results like 0.1 mV AC.

The circuit I gave you runs on about 200 uA total. That is almost nothing.
The RF output comes from the magnetic field of the inductor, where there is a 1 MHz current oscillating between ±2.6 mA, plotted below in yellow.
The base voltage is plotted in red, with the emitter voltage in green.

If you disconnect the inductor at the ground end, the oscillator will stop.
Then measure the base voltage, Vb, which will be about 2.5 V.
Also measure the emitter voltage, Ve, which will be about 1.6 V.

I connected my multimeter in series between the capacitor and the inductor and it detects 2 uA maximum, not 200 uA of AC. I suppose this is why the radio AM antenna is not perturbed.

 

[Baluncore]

I connected my multimeter in series between the capacitor and the inductor and it detects 2 uA maximum, not 200 uA of AC. I suppose this is why the radio AM antenna is not perturbed.

There are two capacitors and an inductor that connect to that node. Which one did you try? It is your multimeter that is not perturbed. It is not designed to measure AC current at RF frequencies.

I believe your coil is too long for its diameter, and the capacitors are too high a value. For an air cored coil, the length should be similar to, or less than the diameter.
What wire have you used to wind the coil? Is it insulated, have you got a shorted turn?

no problem I will increase the coil length of the variable inductor so it can have 0.506 uH instead of 1.35 uH to get a resonant frequency of 1.001 MHz.

37 turns, 6.4 cm long = 64 mm long. 0.8 cm = 8 mm diameter.
I calculate 1.7255 µH.
If your series capacitors make 50 nF. Then the frequency = 541.8 kHz.
Being at the very edge of the AM BC band, there is a 50% chance the oscillator frequency will not fall in the band.
That is why I suggest 1 MHz as the target.

 

[James2018]

There are two capacitors and an inductor that connect to that node. Which one did you try? It is your multimeter that is not perturbed. It is not designed to measure AC current at RF frequencies.

I believe your coil is too long for its diameter, and the capacitors are too high a value. For an air cored coil, the length should be similar to, or less than the diameter.
What wire have you used to wind the coil? Is it insulated, have you got a shorted turn?


37 turns, 6.4 cm long = 64 mm long. 0.8 cm = 8 mm diameter.
I calculate 1.7255 µH.
If your series capacitors make 50 nF. Then the frequency = 541.8 kHz.
Being at the very edge of the AM BC band, there is a 50% chance the oscillator frequency will not fall in the band.
That is why I suggest 1 MHz as the target.

The wire used to wind the coil is thin steel wire. There is no shorted turn because this wire is elastic and the turns do not touch each other. The entire circuit is made of elastic steel wire except the very thin copper wire bits that connect to the base, collector and emitter of the transistor.

I connected in series means I connected the inductor to one terminal of the ammeter and the wire that connects the tank circuit to the inductor to the other terminal of the ammeter. I registered no more than 2 microamperes. And the AM radio is not perturbed either. Even so, the radio AM has the 530 KHz frequency.

 

[James2018]

I bought 4 meters thin insulated multifilar copper wire and I will rebuild the circuit. Let's see if it works this time.

 

[sophiecentaur]

C1 and C2 do not allow AC to pass.

You don't mean that. Capacitors do not pass DC (infinite impedance).

 

[James2018]

You don't mean that. Capacitors do not pass DC (infinite impedance).

One of my capacitors in my circuit passes AC and blocks DC while the other two block both AC and DC. Or so the multimeter tells me. Maybe the ones not passing AC are flawed or maybe the multimeter has a slow response.

 

[James2018]

OK I built a new circuit with copper wire and I used an inductor made of steel wire with 5.5 cm length, 21 turns and 8 mm diameter. That would be 0.506 uH. Which coupled with effective capacitance of 50 nF because of the two 100 nF capacitors in series in the tank circuit, would be a resonant frequency of 1001 KHz AM.

 

[sophiecentaur]

C1 and C2 do not allow AC to pass.

You don't mean that. Capacitors do not pass DC (infinite impedance).

A capacitor (value C) has a reactance Xc = 1/2πfC. That will tell you how much current will pass for 1V PD. A few sums can be very useful. What current would the 220pF capacitor pass for 1V at 1MHz? Would your meter detect that - bearing in mind its likely frequency response? (Ignore the phase.)

 

[James2018]

 

[Baluncore]

There are big problems with your tank circuit. The L and C values you have chosen make it difficult for the oscillator to start. You need to wind a higher inductance coil. The way you have it now, the circulating current in the tank would need to be ±250 mA to keep it oscillating, that might be possible if your wire was thick enough, and the capacitors were perfect, but they are not.

There are good reasons to build the tank circuit for 1 MHz using;
C2 = 1 nF ; C3 = 220 pF ; L1 = 120 uH .

MW BC band coils can be wound on toilet roll tubes. For 120 uH you should wind 75 turns, on a 38 mm diameter tube, and spread the coil over a length of 50 mm. That will require 9 metres of wire, but it will work well.

 

[James2018]

There are big problems with your tank circuit. The L and C values you have chosen make it difficult for the oscillator to start. You need to wind a higher inductance coil. The way you have it now, the circulating current in the tank would need to be ±250 mA to keep it oscillating, that might be possible if your wire was thick enough, and the capacitors were perfect, but they are not.

There are good reasons to build the tank circuit for 1 MHz using;
C2 = 1 nF ; C3 = 220 pF ; L1 = 120 uH .

MW BC band coils can be wound on toilet roll tubes. For 120 uH you should wind 75 turns, on a 38 mm diameter tube, and spread the coil over a length of 50 mm. That will require 9 metres of wire, but it will work well.

But with 6.3 cm length, 6 turns, 3 cm diameter of insulated copper wire coil I get a 508 nanoHenries inductor which with 50 nF effective capacitance give a resonant frequency of 0.9986 MHz. In this new circuit, the two 100 nF capacitors in series give an effective capacitance of 50 nF.

 

[James2018]

There are big problems with your tank circuit. The L and C values you have chosen make it difficult for the oscillator to start. You need to wind a higher inductance coil. The way you have it now, the circulating current in the tank would need to be ±250 mA to keep it oscillating, that might be possible if your wire was thick enough, and the capacitors were perfect, but they are not.

There are good reasons to build the tank circuit for 1 MHz using;
C2 = 1 nF ; C3 = 220 pF ; L1 = 120 uH .

MW BC band coils can be wound on toilet roll tubes. For 120 uH you should wind 75 turns, on a 38 mm diameter tube, and spread the coil over a length of 50 mm. That will require 9 metres of wire, but it will work well.

If I use a 470 uH inductor with ferrite core, what capacitance value would I need? By the way they do not really have a 1 nF capacitor anywhere for sale, but I did find a 220 pF capacitor and a 15 nF capacitor.

 

[Baluncore]

If I use a 470 uH inductor with ferrite core, what capacitance value would I need?

For 1 MHz ;
f = 1 / ( 2 * Pi * Sqrt( L * C ) ) hertz.
C = 1 / ( L * ( 2 * Pi * f )^2 ) farad.
C = 53.9 pF.

 

[James2018]

For 1 MHz ;
f = 1 / ( 2 * Pi * Sqrt( L * C ) ) hertz.
C = 1 / ( L * ( 2 * Pi * f )^2 ) farad.
C = 53.9 pF.

But for 700 KHz I would need two 220 pF connected in series. That I can do. Hopefully if they still have 220 pF capacitors in stock.
You know when this 470 uH inductor with ferrite core is directly connected to a battery, it turns the compass by 100 degrees, from South to North-West.