# Impossible to Turn DC into AC in Simple Circuits?

Discussion in 'General Electronics Chat' started by Zero Potential, Apr 7, 2016.

1. ### Zero Potential Thread Starter Member

Mar 25, 2015
84
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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.

2. ### paulktreg Distinguished Member

Jun 2, 2008
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Whilst what you say is basically true electronics would be pretty useless if all you had in your parts drawer are resistors, capacitors and inductors!

I don't really understand what you are trying to say here because without "active" components there's not a lot you can do?

3. ### profbuxton Member

Feb 21, 2014
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You can DC into AC without any R. L or C components at all. Use d to have a valve car radio which used a mechanical "chopper" to switch DC at a high rate and convert battery DC to Ac for the High voltage on the valves (tubes, for you guys).
The points ignition system also converts DC to AC (although pulsed). So you see there are a few ways to convert DC to AC without electronics.

4. ### paulktreg Distinguished Member

Jun 2, 2008
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Are we talking true AC or a DC waveform here?

5. ### Zero Potential Thread Starter Member

Mar 25, 2015
84
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I know DC can be turned into AC. I am just talking about basic passive components. Thanks.

6. ### Tonyr1084 Active Member

Sep 24, 2015
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It's not possible using just basic passive components. But the most basic form of conversion from DC to AC that I can think of (not using elaborate circuitry) using a dynamotor to change DC to AC would work.

I'm not highly familiar with the process, so somebody correct me if I'm wrong, but applying a DC voltage to a stator or rotor will induce an alternating voltage in the opposing rotor or stator of a spinning motor.. Of course the motor needs to be spinning at 60 Hertz to produce a pure 60 Hz sine waveform. Given that basically you've created an alternator - that sine wave would be a pure sine wave. But then you're limited to the amount of current the alternator could produce. In fact, your automotive alternator is basically a dynamotor. The only difference is instead of using an electric motor to spin the alternator you're using a gasoline engine to spin it. Then it gets rectified back to DC to charge the battery and power the electronics.

Zero Potential likes this.
7. ### AnalogKid Distinguished Member

Aug 1, 2013
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1,297
If by passive components you mean wire, then yes. Before the vacuum tube was invented, high frequency radio carriers were created with a motor-generator set, a DC motor with a mechanical governor driving a mult-multi-multi-pole AC generator. This is what made the first voice radio broadcast in 1906. Technically, all passive components.

But to answer the question you're really asking - no. A circuit with sustained oscillation requires a circuit gain of at least 1.000, and all passive components have loss. In terms of energy going through the circuit, there is no combination of 0.9 this and 0.999 that that is greater than 1. It's that whole first law of thermodynamics thing.

8. ### Fluke289 New Member

Aug 8, 2013
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I’m not sure if I’m missing something, but you could use some diodes to make a bridge rectifier. You can throw in a capacitor to smooth out the ripple a little. Diodes are pretty basic components.

Edit:
My bad, I see that you want to go from DC to AC.

To change DC into AC, you could use some transistors to make a transistor inverter. It’s not exactly LRC, but transistors are simple enough.

Last edited: Apr 7, 2016
9. ### Tonyr1084 Active Member

Sep 24, 2015
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111
Technically, a dynamotor is not a passive component. It's active by the nature of being driven by a motor. You're converting DC into mechanical energy then using that mechanical energy to turn an alternator, producing AC. At whatever frequency you produce, it is still AC.

Back in high school our teacher demonstrated a dynamotor for us. Allowed us to play with it. Boy did I get bit! I don't remember the voltages being generated, but it's anything BUT passive. True, if you look at it from the viewpoint of just being a series of coils - spinning and stationary, that is pretty passive. But the act of using mechanical force to generate an alternating signal is "Active".

10. ### #12 Expert

Nov 30, 2010
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OK. Back to the question at hand. AC to DC can be as simple as a diode and a capacitor. DC to AC can be a sine wave oscillator, a 555 timer, an inverter, or a chopper, but none of them are as simple as a couple of diodes and a capacitor. You can call a dynamotor active or passive, but it's still a lot more complicated than a couple of diodes and a capacitor. There's your bottom line. DC to AC is always more complicated than 2 or 3 passive components.

11. ### Zero Potential Thread Starter Member

Mar 25, 2015
84
7
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.

12. ### SLK001 Well-Known Member

Nov 29, 2011
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By this reasoning, you can't even make DC out of AC using on R, L and Cs, unless you throw in some "fancy gizmos".

And what, pray tell, is the "useless stuff"?

13. ### Zero Potential Thread Starter Member

Mar 25, 2015
84
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There isn't time to write all the useless stuff here, and besides, it's useless.

14. ### nsaspook AAC Fanatic!

Aug 27, 2009
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BR-549, Tonyr1084 and #12 like this.
15. ### BobaMosfet Active Member

Jul 1, 2009
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Turning DC into AC is as simple as offsetting DC voltage, and can be done with nothing more than passive components. Any oscillating wave-form (even a square-wave) becomes AC when DC voltage is offset negatively for a portion of the wave-forms amplitude.

16. ### SLK001 Well-Known Member

Nov 29, 2011
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Really? Please educate us on this technique.

17. ### ErnieM AAC Fanatic!

Apr 24, 2011
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I have a bunch of solar powered garden lights in my backyard I bought for about a buck each that all have a DC to AC inverter to power a 3v LED off a 1.5v battery.

18. ### Zero Potential Thread Starter Member

Mar 25, 2015
84
7
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.

19. ### SLK001 Well-Known Member

Nov 29, 2011
849
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...and they are made of only sticks and twigs!

20. ### wayneh Expert

Sep 9, 2010
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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.