Impossible to Turn DC into AC in Simple Circuits?

grahamed

Joined Jul 23, 2012
100
Hi

#1 was a little imprecise. I am taking it to mean that whilst it is easy (although it does in fact require at least one electronic rather than electrical component) to go from (continuous, smooth, etc) a.c to smooth d.c. the reverse process is not apparently possible.

Leaving aside the references to mechanical devices (including but limited to - motor-generators, choppers. spark-gap generators) and exotic negative resistance devices (again including but not limited to gas-tube - neon or fluorescent , tunnel, Impatt & Gunn diodes, quantum wells, etc) we end up up with why is not possible?

Generally electrical theory invokes reciprocity - generally processes are reversible,and in fact as described #39 it actually is possible. Given ideal components with zero losses (electrical or radiative) an oscillating current in a parallel LC would continue for eternity. How the condition started is not for us to worry about since such a current would continue for infinite time and so may be assumed to have run since infinite time.

All this is useless theory but understanding the limits set by theory tends to stop us wasting our time. The number of times I have seen people, good, practical electronicians all, trying to do that which cannot be done due to a lack of adequate theory, is, well, it's quite a few - and I have done it myself many times

So ZP start writing your notes now, if you carry on long enough noting anything you find useful you will end up with notes equal to the sum of all human electronic knowledge. Except you won't because you dismiss that which you do not understand as useless - my guess is that you are quite young and such arrogance is characteristic of the young.

Maybe you listen to people who dismiss the usefulness of Calculus, differential equations, stability analysis, Bode diagrams and Smith's charts, Laplace transforms, not to mention Ohm's, Thevenin's and Norton's theorums, and all the rest - "studied in college, never used again". But then how many original and/or optimised designs do they have to their name? It is truly a mark of genius to go forward without theory and then go beyond current theory, however any fool can blunder around with a circuit till he finds something that works, or he doesn't.

Some things are hard to understand. Some are too hard for most of us to understand. This is not to say they are without value.
 

BobaMosfet

Joined Jul 1, 2009
2,053
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.
No, actually you get a perfect sine-wave (meaning it is proportionate, not distorted). As for amplitude, yes, that slowly diminishes over time, unless you add energy back into the circuit. But that wasn't what the OP asked-- he asked if AC could be done with a DC circuit using passive components. The answer is - yes -.
 

The Electrician

Joined Oct 9, 2007
2,914
No, actually you get a perfect sine-wave (meaning it is proportionate, not distorted). As for amplitude, yes, that slowly diminishes over time, unless you add energy back into the circuit. But that wasn't what the OP asked-- he asked if AC could be done with a DC circuit using passive components. The answer is - yes -.
It can be argued that the switch is an active device, just like a vibrator, or a relay, or a transistor.

Also, if the wave is decaying over time, it's distorted.
 

BobaMosfet

Joined Jul 1, 2009
2,053
"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.
No, actually a momentary push-button switch will do the trick. This isn't imagination, the circuit was easy to build. All that's necessary is putting energy into the tank and watching it oscillate with a scope. Press it with the finger and let go. The OP didn't say it had to run indefinitely-- he just wanted to know if DC could be used to create AC with passive components-- the answer is - yes- unequivocally.
 

BobaMosfet

Joined Jul 1, 2009
2,053
It can be argued that the switch is an active device, just like a vibrator, or a relay, or a transistor.

Also, if the wave is decaying over time, it's distorted.
Not relevant. OP asked if DC could create AC with just passive components it can. The switch is tertiary to the part of the circuit that converts the energy from DC to AC, and is only there to charge the circuit to a starting point. If one claims the switch is an active part, then one could also argue that the power-supply is an active component as well. Silliness.
 

MrAl

Joined Jun 17, 2014
9,622
Hi again,

With ideal components you would not even need a switch because the parallel combination of an inductor and capacitor would resonate even with the smallest RF input which would come from just about anywhere on earth these days and picked up by the leads or any part of the two components.

These finer points are probably not what the OP was asking about but they are interesting anyway. The OP was probably asking about a practical circuit using R, L, and C alone that could actually convert say a 12v battery into a 120vac sine wave that could power maybe a small radio or something. Given those three components alone it can not be done without some trick such as a local high energy magnetic field that varies with time.
 

SLK001

Joined Nov 29, 2011
1,548
No, actually you get a perfect sine-wave (meaning it is proportionate, not distorted). As for amplitude, yes, that slowly diminishes over time, unless you add energy back into the circuit. But that wasn't what the OP asked-- he asked if AC could be done with a DC circuit using passive components. The answer is - yes -.
As been said earlier, if you are actively switching the switch, you have become the "active" part.
 

hobbyist

Joined Aug 10, 2008
892
Not relevant. OP asked if DC could create AC with just passive components it can. The switch is tertiary to the part of the circuit that converts the energy from DC to AC, and is only there to charge the circuit to a starting point. If one claims the switch is an active part, then one could also argue that the power-supply is an active component as well. Silliness.
Very good point made.
 

BR-549

Joined Sep 22, 2013
4,928
I don't think a steady DC source used to create a very temporary AC signal is what the TS had in mind, but that's just me.

My taught definitions of active, passive, and electromechanical devices doesn't seem to fit any longer.

Whatever the real definitions turn out to be, you can't beat the neon solution.
 
Electrons effort a while similar a line of ants, which is a good and sufficient analogy for rather like a basic flashlight. In bigger domestic uses, electricity works a dissimilar way. The current source supply that comes from the pipe in your wall that is based on AC, wherever the electricity shifts way round fifty to sixty times each second. For example, the wire running amongst the spot and the wall filled full of electrons. Whenever you browse on switch all electrons filling the wire vibration back and that full scuffling near convert’s electrical energy into temperature and make the bulb glow. The electrons do not essentially have to running in loop to passage energy in AC.
 

jjustin

Joined Mar 1, 2011
2
You need a circuit with a loop gain greater than one in order to get a continuous oscillation - which can only be accomplished with an active component. Otherwise, you need an external mechanical repetitive action interacting with the circuit.
 

PhilTilson

Joined Nov 29, 2009
117
Electrons effort a while similar a line of ants, which is a good and sufficient analogy for rather like a basic flashlight. In bigger domestic uses, electricity works a dissimilar way. The current source supply that comes from the pipe in your wall that is based on AC, wherever the electricity shifts way round fifty to sixty times each second. For example, the wire running amongst the spot and the wall filled full of electrons. Whenever you browse on switch all electrons filling the wire vibration back and that full scuffling near convert’s electrical energy into temperature and make the bulb glow. The electrons do not essentially have to running in loop to passage energy in AC.
This is fascinating. Have you ever heard of the late Professor Stanley Unwin?
 

Tonyr1084

Joined Sep 24, 2015
7,190
@highvoltpower: Um, whAT?

First, I take it English is not your first language. Second, you have a great opportunity to learn much more about electricity. But in order to do that you'll need to take some basic classes first. THEN you can start thinking about ants bumping into things and making heat in whole new ways.
 

Plamen

Joined Mar 29, 2015
101
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.
Petkan:
In the old days when they had no better options they used dual wound electrical machines (umformers). On the same shaft you have the windings of a DC motor and of an AC motor. You can regard it as mechanically coupled generator and motor. The thing is reversable i.e. you can power it from DC and get 3 phase AC or other way around. Frequency (when converting DC to AC) is proportional to speed i.e. applied DC voltage. Later they came with transformer based self oscillating transistor.circuits, typically class C amplifiers tuned to a specific frequency by transformer inductance and associated capacitors. That was the way tape recorders generated AC for erasure and biasing. Although the transistors operate in class C, very clean sine could be achieved by the transformer inductance with associated capacitor.
Today sometimes we need to convert variable frequency (say 300 to 800 Hz as produced by aircraft engines doing flying) to fixed 400Hz (aircraft power systems). This could achieved for instance by variable gearboxes (to regulate generator speed, regardless of engine speed) but also by rectifying to DC and then using inverters to make it AC. Lately with proliferation of DC/DC ready made modules, it became possible to modulate a common DC to DC converter with a half sine of frequency of your liking. The half sine output is manipulated by H bridge or synchronous rectifier to alternate polarity i.e. recreate a sine. Microchip has an ap note illustrating this with one of their micro controllers.
Because switching frequency today could be high, modulating frequency of up to 400 Hz could be easily supported.
The benefit of this approach is that the isolation transformer is sized for the switching frequency (say 40kHz), not the desired frequency (say 400Hz). This makes the transformer smaller. If symmetrical square wave is acceptable (rather than sine), some DC to DC topologies can produce it on a platter without any added complexity i.e. provide both DC (rectified) and AC (before rectification). Study LM5041 (cascaded topology) to see how transformer core could be operated at constant 50% duty cycle and produce symmetrical square wave.
 

Plamen

Joined Mar 29, 2015
101
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.
Petkan: The notion of "simple" is debatable:
Is two transistors, transformer, couple of resistors and capacitors too complex for you?
All tape recorders used it.
 
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