# Can I Use Diodes to Convert AC to DC in an LC Circuit?

• Phantasm
In summary: I was wondering. I was thinking of a way to get an oscillator to output a square wave on its output, without having to use an amplifier or anything like that.If you add a rectifier and a couple of zener diodes to an LC circuit, you can create a crude dc square wave. But keep in mind that this is an approximation and the oscillator will produce the signal at a fundamental frequency and a couple of its harmonics (integer multiples), if that poses a problem the output is low-pass filtered.
Phantasm
Hello,

This is such a great community! I've got some great input from here in the past!

I took a bit of a break but I'm back to playing around and trying to learn about electronics and circuits and I wonder if I can pick your wonderful brains again :)

Ive learned that a circuit with a properly tuned capacitor connected to a coil will create an oscillator that will oscillate until the capacitor loses its charge.

Now, I'm experimenting with harmonic resonance

Id like to make an LC circuit... Normally the current in the coil is AC as the circuit oscillates... Correct?

Id like the circuit to drop its ac component but maintain the same frequency of dc pulses. I can use some diodes for this?

I am thinking of diverting the return oscillation to a second, smaller coil to generate a harmonic of the fundamental frequency and mix it back into the first coil in time to repolarize the cap so it can deliver its next DC Pulse (along with the harmonic)

If I wind the smaller coil on top of the larger one (fewer winds) I should be able to introduce the harmonic into the larger coil by induction correct?

My question is, how can I feed the end product back into the cap so it can deliver its next dc pulse at the self resonant frequency of the coil?

Thanks guys :)

Phantasm said:
Id like the circuit to drop its ac component but maintain the same frequency of dc pulses. I can use some diodes for this?

It doesn't work. A sequence of "DC pulses" has a pretty complicated frequency spectrum (with in theory an infinite number of harmonics), but at the same the you have an LC circuit which is quite literally a bandpassfilter (meaning everything except the resonance frequency is filtered).
So no, you can't make a passive oscillator that works with pulses.

You can create all sorts of strange effects if you add non-linear elements like diodes to resonanators, but not the effect you seem to be after.

couldn't he turn an ac wave into a crude dc square wave using a rectifier and a couple of zener diodes? it would have to be on the output of the LC circuit though, not interfering with the circuit itself.

especially:

You seem to be stuck on the idea of LC circuits oscillating by themselves.
In fact, LC circuits only oscillate briefly and under very special circuitry. "Ringing" on pulsed inputs is one.

Usually, tuned circuits just modify incoming waveforms like this:

This shows the harmonics of an incoming waveform. If you remove the harmonics the waveform looks more and more like a sinewave. If you remove all of them, it will be a sinewave.

A tuned circuit isn't perfect and it may remove all but the lower harmonics, producing a rounded version of the original but without the fast rise and fall times of the original.

You can tune the tuned circuit to just a harmonic if you like. This would give you a sinewave at the harmonic frequency.

LC won't oscillate by itself.

To utilize the resonator as an oscillator the circuit must amplify the oscillations of LC and feed them back to itself in correct phase - that way it will sustain stable oscillations, otherwise LC will die as soon as it was energized.

Also, because the amplifier is usually non-linear and components not ideal, the oscillator will output the signal at a fundamental frequency and a couple of its harmonics (integer multiples), if that poses a problem the output is low-pass filtered.

Once you have a stable oscillator, you can apply it to a schottky diode bridge rectifier which will produce sharp pulses, or apply it to some other non-linear diode scheme that produces combs.

Can you explain what you are trying to make. It sounds a bit like a complicated way of making an oscillator (which would require a power source such as a battery).

On the other hand, are you trying to make a system to keep working forever without power?

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It seems from others comments that the "perpetual motion machine" is in fact your aim.
Please try to understand that power losses in circuit components and wiring make this impossible.
Any such system will always run down as energy is lost, usually as heat.

People have been trying to do this in different ways (e.g. mechanical) for centuries - it never works.

vk6kro said:
You can tune the tuned circuit to just a harmonic if you like. This would give you a sinewave at the harmonic frequency.

Hi again! :)

Yes, this sounds good

earlofwessex said:
couldn't he turn an ac wave into a crude dc square wave using a rectifier and a couple of zener diodes? it would have to be on the output of the LC circuit though, not interfering with the circuit itself.

Hello

This is sort of what I was thinking. Though from what youre suggesting, it sounds like I need some kind of timing circuit to generate dc pulses in a second circuit.

waht said:
LC won't oscillate by itself.

To utilize the resonator as an oscillator the circuit must amplify the oscillations of LC and feed them back to itself in correct phase - that way it will sustain stable oscillations, otherwise LC will die as soon as it was energized.

It sounds like in order to get an LC circuit to emit pulsed DC, the first dc pulse through the coil would have to be routed around the coil on its return but somehow fed back to the cap with the right latency to be properly in phase.

waht said:
Also, because the amplifier is usually non-linear and components not ideal, the oscillator will output the signal at a fundamental frequency and a couple of its harmonics (integer multiples), if that poses a problem the output is low-pass filtered.

Once you have a stable oscillator, you can apply it to a schottky diode bridge rectifier which will produce sharp pulses, or apply it to some other non-linear diode scheme that produces combs.

I'll have to play around with what kind of signals are the most interesting - This is great info though, thanks very much! :D

Can you explain what you are trying to make. It sounds a bit like a complicated way of making an oscillator (which would require a power source such as a battery).

I thought about it, and youre right, there must be an easier way to generate dc pulses.. Not sure how you'd do that though :\ Can you point me in the right direction?

Like I said, I've got myself interested in harmonic resonance. My previous posts arnt really related to what I'm looking at here. I'm just interested in the properties of harmonic resonance and the interaction effects of harmonic frequencies.

On the other hand, are you trying to make a system to keep working forever without power?

0_o

That sounds a lot more interesting than what I'm doing but I don't really think you can actually do that... Can you explain what that has to do with harmonic resonance and LC circuits?

Last edited:
It seems from others comments that the "perpetual motion machine" is in fact your aim.
Please try to understand that power losses in circuit components and wiring make this impossible.
Any such system will always run down as energy is lost, usually as heat.

People have been trying to do this in different ways (e.g. mechanical) for centuries - it never works.

Hello again

I'm not sure what gave you the impression that I'm trying to defy the laws of physics.. I'm aware that energy in a system is finite unless new energy enters the system. In either case, I said I'm interested in the interaction of harmonic frequencies and harmonic resonance. What gave you the impression that this relates to perpetual motion? Is there something that I'm missing?

Hello Phantasm,

Well, I think that I probably misunderstood your earlier posts - sorry. I had thought that perhaps you were trying to rectify the output from a resonator, so as to recover dc which would in turn be used to power the oscillator driving the resonator in the first place...

I was just trying to make sure that you were not wasting your time going down that particular false trail. Since evidently you are not, that's all I have to say about it.

Hello Phantasm,

Well, I think that I probably misunderstood your earlier posts - sorry. I had thought that perhaps you were trying to rectify the output from a resonator, so as to recover dc which would in turn be used to power the oscillator driving the resonator in the first place...

I was just trying to make sure that you were not wasting your time going down that particular false trail. Since evidently you are not, that's all I have to say about it.

Hey,

Sorry I didnt mean to come off harsh... and I can see what you mean about how I said I wanted to recycle half of the AC phase as implying that I'm trying to build a perpetual motion machine =p You must get a lot of those kinds of people around here =p

I was just trying to kill 2 birds with one stone by using half the AC phase to somehow generate a second frequency while maintaining the same fundamental frequency of dc pulses. Since I only have 1 signal generator and I'd need more than one to study harmonic interactions I thought I could just generate my signals with coils with their number of turns in the same ratio as the harmonic number I'm trying to generate.

Am I on the right track here at least?

----

On another note,

Someone suggested I could speed up my test setups by using circuitry simulation software to design the desired test setup so I can build it with minimal tweaking and re-designing (since I am new at all this)

OrCad was suggested but I'm told there are easier to use solutions that are free and still do much of the same thing.

I'd like to be able to just plug in the circuit components and then configure their properties and see the resulting current at various points in the circuit so I know how the circuit is behaving. Any tips in this regard?

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Phantasm said:
It sounds like in order to get an LC circuit to emit pulsed DC, the first dc pulse through the coil would have to be routed around the coil on its return but somehow fed back to the cap with the right latency to be properly in phase.

LC doesn't generate DC pulses, it generates AC sinewaves.

I'd like to be able to just plug in the circuit components and then configure their properties and see the resulting current at various points in the circuit so I know how the circuit is behaving. Any tips in this regard?

It is basically a free SPICE implementation with a reasonably easy-to-use user interface.

hi friends!
i have a doubt too in LC parallel circuits..how is the impedance related to quality factor and inductive reactance..Z=(Q,XL)...i would like to have an expression in this form...thank u in advance

## 1. How do diodes convert AC to DC in an LC circuit?

Diodes act as one-way valves in an LC circuit, allowing current to flow in only one direction. When the AC current flows through the circuit, the diode blocks the negative half cycles, effectively converting the alternating current to direct current.

## 2. Can any type of diode be used to convert AC to DC in an LC circuit?

No, not all diodes are suitable for converting AC to DC in an LC circuit. The diode must have a high enough voltage rating to handle the voltage of the AC source and a fast enough switching speed to effectively block the negative half cycles.

## 3. What is the role of the inductor in an LC circuit when converting AC to DC?

The inductor helps smooth out the output of the diode by storing energy during the positive half cycles and releasing it during the negative half cycles. This helps to reduce any ripples or fluctuations in the DC output.

## 4. Can you use multiple diodes in an LC circuit to improve the conversion of AC to DC?

Yes, using multiple diodes in a bridge rectifier configuration can improve the efficiency of converting AC to DC in an LC circuit. This configuration uses four diodes arranged in a diamond shape, allowing for a more consistent and efficient conversion.

## 5. What are the possible applications of using diodes to convert AC to DC in an LC circuit?

Diodes are commonly used in power supplies to convert AC to DC for various electronic devices. They can also be used in renewable energy systems, such as solar panels, to convert the alternating current generated by the panels into direct current for use in homes and buildings.

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