# Impossible to Turn DC into AC in Simple Circuits?

#### SLK001

Joined Nov 29, 2011
1,548
That's the point. It's not so hard to make AC from DC. Admittedly, there's an active component, but only one, plus one inductor. That's all.
It's not a single active component, but an active system to do the conversion. This is not possible without an active component(s).

#### gerty

Joined Aug 30, 2007
1,302
I'm writing up my own electronics notes so I can discard the useless stuff and home in on what works. I have a question.

Is it accurate to say that in a simple circuit, while you can rectify AC pretty easily and get rid of the time variation, you can't turn DC into AC or change the frequency of AC?

I know you can get fancy gizmos to do these things. I'm talking about your basic R, L, C, voltage source, and current source circuits.
If you're willing to add one IC you can have AC sine wave with variable frequency.

#### wayneh

Joined Sep 9, 2010
17,189
[reponding to #22]
True, but so what? I can make AC from DC using two parts with 6 pins between them, one of which would not be used if you aren't switching a solar light.

#### SLK001

Joined Nov 29, 2011
1,548
[reponding to #22]
True, but so what? I can make AC from DC using two parts with 6 pins between them, one of which would not be used if you aren't switching a solar light.
I'm not arguing that. The OP's wants to be able to do the transformation of DC to AC with only passive parts and batteries. He even says that since you can do AC to DC with only passive parts (no, you can't), why can't you do DC to AC the same way? (Answer: You can't do that either.)

#### Zero Potential

Joined Mar 25, 2015
84
Well, I'm already using the "ignore" feature. That's disappointing.

I received the information I wanted, so thanks.

#### WBahn

Joined Mar 31, 2012
27,875
I should probably write a more detailed reply.

I am trying to learn this stuff, and I am not that far along, so if I start making lists of what I consider useful and useless, there will be a lot of errors and argument.

I will go this far: some of the methods for solving systems of equations are needlessly tedious and truly stupid.
Since you believe you are already in a position to decide what is needlessly tedious and truly stupid, then write a text book that leaves all of that out and only has the stuff you consider useful. You will become rich and famous -- unless, of course, if turns out that what you consider needlessly tedious and truly stupid is actually useful down the road where you haven't ventured yet.

#### WBahn

Joined Mar 31, 2012
27,875
I'm not arguing that. The OP's wants to be able to do the transformation of DC to AC with only passive parts and batteries. He even says that since you can do AC to DC with only passive parts (no, you can't), why can't you do DC to AC the same way? (Answer: You can't do that either.)
The active vs passive is only part of the issue. Even if you have a bunch of active components you can't turn DC into AC is those active components are all linear.

A linear system cannot create signals in the output signal at frequencies that are not already present in the input signal.

#### BobaMosfet

Joined Jul 1, 2009
2,053
Really? Please educate us on this technique.
It's called an LC tank circuit. Basic stuff. 1mH inductor. 1mF capacitor. 5VDC power supply. Voltage across the tank circuit will be a perfect sine wave centered on zero volts at about 159Hz. Will go +/-450mV or so, with just a momentary touch-switch. Hold switch longer to get full +/-2.5V swing.

I'm sure you can find the circuit in any electronics text book of quality.

#### SLK001

Joined Nov 29, 2011
1,548
It's called an LC tank circuit. Basic stuff. 1mH inductor. 1mF capacitor. 5VDC power supply. Voltage across the tank circuit will be a perfect sine wave centered on zero volts at about 159Hz. Will go +/-450mV or so, with just a momentary touch-switch. Hold switch longer to get full +/-2.5V swing.

I'm sure you can find the circuit in any electronics text book of quality.
You might get a damped sinusoidal response when the switch opens or closes, but you won't get a "perfect sine wave" - not even close.

#### Zero Potential

Joined Mar 25, 2015
84
Since you believe you are already in a position to decide what is needlessly tedious and truly stupid, then write a text book that leaves all of that out and only has the stuff you consider useful. You will become rich and famous -- unless, of course, if turns out that what you consider needlessly tedious and truly stupid is actually useful down the road where you haven't ventured yet.
Exactly the opposite of what I said, but thanks.

#### BR-549

Joined Sep 22, 2013
4,928
"It's called an LC tank circuit. Basic stuff. 1mH inductor. 1mF capacitor. 5VDC power supply. Voltage across the tank circuit will be a perfect sine wave centered on zero volts at about 159Hz. Will go +/-450mV or so, with just a momentary touch-switch. Hold switch longer to get full +/-2.5V swing."

The switch will have to be accurately timed or rotated. A DC source has neither one.

#### Tonyr1084

Joined Sep 24, 2015
7,198
OH MY GAUD! The flashing NEON. Haven't built one of those in a hundred years. Well, maybe more like 40.

Funny thing I learned about those things: The bulb flash rate will increase if you approach the bulb with something statically charged. I first noticed it many years ago when I put my finger near the bulb. I noticed the flash rate increased visibly. Later when I learned about ESD I figured out that by approaching the bulb with a charged body part (finger) the trigger point of the bulb would be enhanced with the static from my body. Hence effectively lowering the flash point of the bulb.

Gaush, I think I'm going to build one just for the heck of it. Maybe even incorporate it into an ESD presentation.

TOTALLY FORGOT ABOUT THAT TRICK. Thanks for the memory.

#### Wendy

Joined Mar 24, 2008
23,073
Given a neon bulb can be considered a type of tube, there is no way to convert DC to AC passivily. You can use a DC motor to drive an AC motor, or you can toggle a switch on and off (in which case you are the active component). You may not like it, but there is a reason active components exist, and AC oscillators are just the beginning.

A LC circuit will ring, but it will damp out. It requires an active component to keep it ringing, just like it required an external action to start it ringing to begin with.

Not sure why you are fighting the concept of active components, they are the most interesting components. The resistors, capacitors, and inductors exist to support their activities.

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#### nsaspook

Joined Aug 27, 2009
10,679
Given a neon bulb can be considered a type of tube, there is no way to convert DC to AC passivily.
I think the small neon bulbs we all use are technically called gas-discharge lamps, the 'tube is what's the lamps in.
The thing that's 'special' about the neon bulb circuit is passive negative differential resistance.

#### Wendy

Joined Mar 24, 2008
23,073
Much like a tunnel diode, which is not very passive. You can also use neon bulbs as a crude voltage regulator, and vary the break over over voltage with slight additions of radioactive gases. It may be semantics, but I consider a neon bulb as an active component.

#### The Electrician

Joined Oct 9, 2007
2,914
I will go this far: some of the methods for solving systems of equations are needlessly tedious and truly stupid.
Saying this is bound to arouse people's curiosity. It suggests that you know less tedious methods. Perhaps you could start a thread in the Math forum and explain them. Anyone having homework in circuit analysis would find them useful.

#### pebe

Joined Oct 11, 2004
626
Before tubes, there were spark gap transmitters. Just passives.

#### MrAl

Joined Jun 17, 2014
9,633
That's all I wanted to know. I am not trying to make AC. Just getting a better understanding of the basics of circuits, instead of just doing it the monkey-see, monkey-do way.
Hi,

Then you will want to take note of the following.

The answer is yes in theory, with a few catches.

Catch #1: Inject at least a small amount of energy.
You need to inject at least a little energy into the system. This can come from RF too. There has to be some way to start the oscillations.

Catch #2: Components must be ideal.
Using just one inductor and one capacitor, you can make a generator but only if the components are ideal. This means no resiistance, no leakage, etc. Once even the tiniest energy is injected (almost anything would start it) it will oscillator sinusoidally forever.

Catch #3: Can not extract any energy at all.
There can be no energy take off. Even a little energy take off will eventually cause the oscillations to cease unless you want to get really down to the nitty gritty amounts of energy in the Classical sense only, in which case it would take forever to loose the oscillations. In the Quantum world however the oscillations would in fact cease with even the tiniest load, although it could take a long time with an extremely light load.

Why mention this theoretical scenario?
It's part of basic electrical theory. An oscillator can be made with one ideal inductor and one ideal capacitor.
We then start to impose constraints on how good the components really are, but that's the starting point. It also brings to light a basic difference between quantum theory and classical theory.