Why electronics do not use AC.

[Virogen]

Hi,

Would I be correct in saying that we do not use AC current in electronics because in the circuits, the presence of current would be a "1" and no current would be a "0". Thus with DC on = "1", off = "0". With AC, the current becomes zero 60 times a second (for 60 Hz) and thus would not be useful in electronics.

 

[Drakkith]

That depends on what you mean by "electronics". There are many devices that use AC power. I believe that most devices which use computers or integrated circuits use DC because of things like Diodes, Inductance, and other factors that would be bad with AC power.

 

[Virogen]

I mean electronic devices (opposed to electric devices) as ones that use current in computations or storage.

 

[Femme_physics]

That depends on what you mean by "electronics". There are many devices that use AC power. I believe that most devices which use computers or integrated circuits use DC because of things like Diodes, Inductance, and other factors that would be bad with AC power.

Hmm, not to hijack the thread, but is AC power weaker than DC?

 

[cragar]

In DC you have a constant voltage and in AC the voltage goes from a max to zero and then to -max . So there will be times were the current is zero in an AC circuit.

 

[yster]

Hmm, not to hijack the thread, but is AC power weaker than DC?

No. Electricity is produced at power plants and supplied in AC form. Most heavy(power hungry) machinery uses an AC electricity supply and then DC is typically used in lower powered devices.

 

[Femme_physics]

No. Electricity is produced at power plants and supplied in AC form. Most heavy(power hungry) machinery uses an AC electricity supply and then DC is typically used in lower powered devices.

Really? So it's exactly opposite to what I thought? How curious!

 

[yster]

I mean electronic devices (opposed to electric devices) as ones that use current in computations or storage.

I don't know what the exact reason for it is. All I know is that all the electronic equipment that we design and use operates on dc voltage. I asked one of the electronic engineers and he said that's just the way it is. I'm waiting for the boss to come in, he's also an electronic engineer(30+years experience). Hopefully someone will give you an answer before then

 

[yster]

Apparently it's because dc gives a more stable current flow. I think that can somehow be linked to what you initially thought was the idea.

they carry on designing them in this was because a lot of them are battery power( I have never heard of an AC battery :) )

 

[I like Serena]

Actually power is power. :)

1 watt of DC power yields the same amount of power as 1 watt of AC power.

 

[yster]

You're right. I didn't say that 1 watt AC is more powerful than 1 watt dc. But when it comes to high power applications you see less and less DC machines.
I have heard of a 75kW dc motor but I have never seen 1MW DC power supply. I am not saying they don't exist but how often have you seen or heard of one? 10, 20, 50+ Mega watts is common with AC but not with DC.

 

[cragar]

AC is better for long transmission lines and AC power is easier to produce.
And also you need DC for a capacitor to work, If AC is used it will bet blocked.
And in DC you have have a steady current so there won't be any induced E fields.

 

[yster]

I think we have gone a bit off topic here. Can anyone add to my explanation to answer the OP or give better reasons?

 

[sophiecentaur]

AC is better for long transmission lines and AC power is easier to produce.
And also you need DC for a capacitor to work, If AC is used it will bet blocked.
And in DC you have have a steady current so there won't be any induced E fields.

Actually, AC is 'worse' for long transmissions lines. This is because of losses due to line capacity and also due to the fact that all the generators need to be operated in accurate sync with each other. However, in order to operate long lines at a high voltage (which is a very efficient form of transmission), transformers are used. These need AC to operate and they are the most economic way to step up and step down the voltage of power lines.

There are occasions when DC is used for power transmission. For instance, some very long trans continental links and the Electrical Power Link across the English Channel use a DC connection involving many MW of power. This is in order to let the two AC networks to operate non-synchronously. The rectifiers and inverters are pretty fearsome beasts in an application like this but it is worth it, in engineering terms.

 

[cragar]

Interesting, What do you mean losses due to line capacity?

 

[sophiecentaur]

When there is significant reactance in a circuit, the Current and Volts will be out of phase. The useful Power transferred will be VI cos(theta) (Where theta is this phase angle), which is less than VI. For a required power at the consumer's end, you need a higher current to be flowing in the transmission line. Cos(theta) is referred to as the Power Factor and it means that you need to be generating more power than the user actually gets and pays for.

 

[cragar]

So i could put inductors in at my house and rip the power company.

 

[yster]

I don't think an inductor will do because most power factor correction is done with capacitors. I am mechanical engineer so I only learned electricity to pass the course. I didn't work with it much after I graduated but that lecture I remember quite well. I think he said the phase angle is negative so only a capacitor will make it smaller(magnitude) and bring it closer to being in phase.

Come to think of it i think(remember) power transmission lines have got some inductance which also contributes to the change in phase angle so you can only do the opposite with a capacitor.

 

[Neandethal00]

Hi,

Would I be correct in saying that we do not use AC current in electronics because in the circuits, the presence of current would be a "1" and no current would be a "0". Thus with DC on = "1", off = "0". With AC, the current becomes zero 60 times a second (for 60 Hz) and thus would not be useful in electronics.

One problem I see of using AC voltage in everyday electronics is most circuits are used for 'time varying' signals (amplifiers..etc). AC voltage will interfere with time varying signal.

Second problem is we have to make all electronic parts compatible with AC voltage, which may not be possible because some electronic parts require 'thresholds' to operate but AC crosses 0V and changes.

There may be other problems too escaping my mind at this moment.

Note: the rms (root mean square) value of an AC voltage can be considered as a fixed constant DC voltage.

 

[vinniewryan]

Have you ever seen an AC battery? It's much easier and cost-effective to produce a simple AC/DC converter in a wall wart than it would be to convert a DC battery to AC. Since the days of the transistor, DC has been the desired form of electrical power because of the constant current required to make a transistor hold a 0 or 1 state. What would the world of electronics be like if the transistor was created as an AC device? This is the kind of thinking that leads to innovation in electronics.

 

[256bits]

Would I be correct in saying that we do not use AC current in electronics because in the circuits, the presence of current would be a "1" and no current would be a "0". Thus with DC on = "1", off = "0". With AC, the current becomes zero 60 times a second (for 60 Hz) and thus would not be useful in electronics.

Well , one circuit that makes use of the 60 Hz AC current is your light dimmer. Look up triac or scr ( silicon controlled rectifier ) .

And all circuits are not composed of bits of ones and zeroes, as you mention.
That is usually reserved for circuits that are modeled to work with boolean algebra such as that in your computer - digital circuitry.

Google transistor and see how a transistor can be used to amplify a signal entered into the base of the transistor - bipolar, NPN, PNP, FET, Mosfet for example.. The signal can be thought of as being a type of AC. But then again we need a DC voltage - a DC voltage is at the bipolar collector and emitter for that type of signal amplification.

Early transistorized computer circuitry used bipolar transistors arranged so that they could be led to saturation, producing the on/off , 1/0, high/low, that everyone knows about. A saturated transistor can be in only one of 2 states - conducting current or not conducting current. There was and still are different logic families - RTL, TTl, CMOS are just a few are dated.

One cannot consider evisioning transistor digital circuitry being based on AC.

 

[sophiecentaur]

Hold it hold it hold it, chaps.
What about all the Analogue circuits? And what about all the multistate digital systems that have been used and which may well be used extensively in the future? We're not only concerned with 0s and 1s, which 'could' actually be handled quite tolerably with an AC supply (you'd just need a slightly more intelligent form of gate input).

The reason that systems use DC goes way back before digital systems. It is very hard to remove a hideous level of 50 or 60Hz modulation which would turn up on any analogue system which used AC power. Hi Fi with even -80dB of Hum is 'rubbish' to some people's ears.
And, of course, nearly every circuit element used, these days, is going to be included in some battery operated piece of kit - so DC makes (and has always made) sense.

 

[256bits]

Hold it hold it hold it, chaps.
Ha. I like that.


I have here an "old" vacuum tube radio which plugs into 117v AC power and works like a charm. It still pulls in a station - no semiconductors - just a bunch of resistors, capacitors ( condensors was the terminology in that era ), inductors ( chokes ), wires and of course the well renowned vacuum tube. But even though it is of a bygone era, the 117v AC was rectified with a vacuum tube diode into DC and passed along to the other pentodes and triodes to do their amplification magic so the machine would operate without the hideous level of 50 or 60Hz modulation as you say.

It all has to do with signal processing and no matter which way you look at it a DC reference is a lot lot easier to work with than if at all possible, a time varying AC reference.

From that two-electrode tube - the diode and its Dc voltage - came its offspring, the triode, the pentode, the tv tube and subsequent analysis, mathematics and circuits for radio, radar, computers, broadcast television to name a few of the well known.
Of course, semiconductors have now replaced all that power hungry gizmo stuff.
Sounds like an short electronic history.

 

[turbo]

I have here an "old" vacuum tube radio which plugs into 117v AC power and works like a charm. It still pulls in a station - no semiconductors - just a bunch of resistors, capacitors ( condensors was the terminology in that era ), inductors ( chokes ), wires and of course the well renowned vacuum tube. But even though it is of a bygone era, the 117v AC was rectified with a vacuum tube diode into DC and passed along to the other pentodes and triodes to do their amplification magic so the machine would operate without the hideous level of 50 or 60Hz modulation as you say.

If you want to move back in time consider the (pretty decent, for the era) Delco vacuum-tube radios available in GM vehicle's WAY back, and battery-operated radios that far predated them.

For decades, I have restored, repaired, and tuned up vacuum-tube guitar amps. When I started, I had to wrap my head around the need to rectify the AC and supply a ripple-free (or at least minimized) DC supply to exploit against chassis ground. After a couple of amps, it became second nature. The circuits on the DC side of that boundary in those old amps weren't all that different from those in solid-state amps, since solid-state components co-opted the functions of those tubes, resistors, capacitors, etc. Still, solid-state amps can't capture the vitality and response of the old tube amps. Pick up a guitar-oriented magazine, and then visit the nearest large library, and pick a volume from each year (same month) to see what's happening. Every year, the same ads breathlessly touting the "tube-like" sound of the most recent solid-state amp. It's pretty funny, as long as you don't take it seriously.

 

[sophiecentaur]

". Still, solid-state amps can't capture the vitality and response of the old tube amps."

Yes- and a 2011 Ford Focus isn't the same driving experience as a 1959 Morris Minor.

You may like the sound of a valve amp but it is only a characteristic distortion with which the enthusiasts are familiar and in love. What actually does "vitality and response" mean except that you recognise and have grown to like it? Do you still watch super-8 movies and listen to shellac records? Perhaps it may be time to move on - at least to acknowledge that things of the past were different, valid but not necessarily better.

 

[turbo]

No they can't. Think of the old Fender tweed amps. The rectifier circuits were unable to respond in real-time When you hit them with a large input signal. This results in "sag", in which the amplitude of the output moderates, and then the output signal blooms (and changes tonal characteristics of the amp) as the rectifier catches up. When you bang an old Fender with a large input, the voltage of the B+ rail drops and the tonal characteristics of the whole amp changes.

 

[DragonPetter]

OP, the answer is a bit more to do with the fundamentals of digital circuits.
In volatile memory, we use transistors that hold a charge or voltage value to represent a bit. If this bit is changing with the AC power to it, that would assign a phasor value to the bit and it would be a time dependent value. This is more complicated and requires a new clock/timing scheme for how to read, send, and write data. A digital machine could be created with AC power, but it would generally be more complex.

We also often transmit digital signals on AC power in the form of modulation techniques. Also, understand that a sequence of 1's and 0's certainly is not a DC signal. It is an AC signal, just not a single sine wave. Also, the clocks we use in synchronous digital circuits are AC power in a sense, although we still power them with DC power. AC power is used very much in digital circuits, just not in the sense of using the 60Hz power you get out of the wall.

 

[sophiecentaur]

Hold it hold it hold it, chaps.
Ha. I like that.


I have here an "old" vacuum tube radio which plugs into 117v AC power and works like a charm. It still pulls in a station - no semiconductors - just a bunch of resistors, capacitors ( condensors was the terminology in that era ), inductors ( chokes ), wires and of course the well renowned vacuum tube. But even though it is of a bygone era, the 117v AC was rectified with a vacuum tube diode into DC and passed along to the other pentodes and triodes to do their amplification magic so the machine would operate without the hideous level of 50 or 60Hz modulation as you say.

.

You are clearly bipolar, electronically, with a moniker like 256 bits but liking the old tube stuff. The young lads don't know they're borne.

 

[0xDEADBEEF]

I believe that AC logic is inferior to DC logic. All components have bandwidth limits. For the logic high and low to work the switching frequencies must be far below the carrier frequency, and the carrier frequency should be close to the bandwidth limit, while the logic speed is only limited by the bandwidth of the components if DC logic is used.

The only way how AC beats DC is in signal transmission. But building a logic that works on multiplexed signals without demultiplexing would be a terrible mess.

Long message short: AC logic would be much slower than DC logic

 

[sophiecentaur]

Is anyone seriously suggesting building circuits with an AC power supply? What would be the point when DC is so readily obtainable and convenient to use? It's not a beauty contest with votes for the prettiest. It's a matter of Engineering judgement.

 

[ttmark]

Hmm, not to hijack the thread, but is AC power weaker than DC?

No AC power is much more than DC for the same voltage rating because with AC power you are doing work with both positive and negative electricity. DC currents must use a commutator or diode to block the reverse flow.

 

[ttmark]

Is anyone seriously suggesting building circuits with an AC power supply? What would be the point when DC is so readily obtainable and convenient to use? It's not a beauty contest with votes for the prettiest. It's a matter of Engineering judgement.

The reasons to use AC in power circuits is for power efficiency. Another option is to utilize both a positive and negative rail.

 

[sophiecentaur]

No AC power is much more than DC for the same voltage rating because with AC power you are doing work with both positive and negative electricity. DC currents must use a commutator or diode to block the reverse flow.

You would have to look really hard on this forum to find as much rubbish in one post. Didn't you ever read anything about elementary electrical theory before you launched into all that?

 

[sophiecentaur]

You can produce AC from DC and vice versa with equal efficiency. There is nothing inherently better about either forms. The Power is described in exactly the same way.
The fact that ancient dynamos used commutators is not relevant.

 

[ttmark]

You can produce AC from DC and vice versa with equal efficiency. There is nothing inherently better about either forms. The Power is described in exactly the same way.
The fact that ancient dynamos used commutators is not relevant.

Nope, when you convert AC to DC you have voltage drop across the diodes, current leakage, switching loses, and capacitive loss to ground. You can not convert back and forth with no energy loss. The question of which is inherently better has to relate to specific applications.

 

[sophiecentaur]

Did I say 100% efficiency? I said the same efficiency. A very common way of producing electrical Power is to use batteries. Would it be sensible to convert DC from a battery to AC, unless there were a specific need?
The whole idea of saying that one is better than the other is futile. It has to depend on the circumstance.

 

[Drakkith]

Yes, "equal efficiency", agree it depends on the application, both have their advantages. But since electricity is both generated and transmitted in AC form in many instances it is more efficient.

You are talking about the generation of AC power, transmission, conversion to DC, and then used right? Directly generating and transmitting DC power does have its advantages, but due to the more widespread use of AC power generation and transmission most items still use AC power via a converter.

I think the question you answered originally was: "Is AC power weaker than DC?"
In the sense of JUST the power in a circuit, AC is NOT stronger than DC. I believe you were thinking about it in terms of converting AC to DC, not simply the power in the circuit. Is that correct?

 

[singhvi]

Really? So it's exactly opposite to what I thought? How curious!

No. Electricity is produced at power plants and supplied in AC form. Most heavy(power hungry) machinery uses an AC electricity supply and then DC is typically used in lower powered devices.

There's no such thing as stronger between AC or DC, power is generally produced in AC form (except solar power and maybe few others which do not involve generators). Instantaneous power is voltage(t) X current (t), doesn't matter what the functions in time are. But AC current does lead to losses due to oscillating magnetic fields and hence eddy currents and radiation.

Now why electronics use DC power, well the diodes used in electronics cannot sustain currents in reverse direction as they would breakdown, diodes do not have the same resistance in forward and reverse direction, and therefore they are useful only in one direction from the point of view of electronics.
1s and 0s are represented by voltages, a certain range (eg 0 - 4 V) is a zero signal, and (5 +) is a one signal. Current in diodes increases exponentially with voltage and hence they are useful in such an application, as the difference in current through diode at <4 V, and >5 V would be huge and easily detectable. Also the reason why AC voltage is not suitable or necessary.

Also even if we had diodes for AC voltage (though I don't see how), one would have to measure the time average of the resulting current signal, and that would reduce the speed at which switching between 0 and 1 could happen, due to time taken to measure the average (one period of the AC signal).

 

[Neandethal00]

Also even if we had diodes for AC voltage (though I don't see how),

Two diodes in parallel with opposite polarity will conduct AC current during positive and negative half cycle, losing only .7V in both polarity. It's like Triac.

 

[singhvi]

Two diodes in parallel with opposite polarity will conduct AC current during positive and negative half cycle, losing only .7V in both polarity. It's like Triac.

Like I said the one in reverse bias will breakdown at high voltage if you had that kind of configuration

 

[DragonPetter]

There's no such thing as stronger between AC or DC, power is generally produced in AC form (except solar power and maybe few others which do not involve generators). Instantaneous power is voltage(t) X current (t), doesn't matter what the functions in time are. But AC current does lead to losses due to oscillating magnetic fields and hence eddy currents and radiation.

Now why electronics use DC power, well the diodes used in electronics cannot sustain currents in reverse direction as they would breakdown, diodes do not have the same resistance in forward and reverse direction, and therefore they are useful only in one direction from the point of view of electronics.
1s and 0s are represented by voltages, a certain range (eg 0 - 4 V) is a zero signal, and (5 +) is a one signal. Current in diodes increases exponentially with voltage and hence they are useful in such an application, as the difference in current through diode at <4 V, and >5 V would be huge and easily detectable. Also the reason why AC voltage is not suitable or necessary.

Also even if we had diodes for AC voltage (though I don't see how), one would have to measure the time average of the resulting current signal, and that would reduce the speed at which switching between 0 and 1 could happen, due to time taken to measure the average (one period of the AC signal).

When you apply a logic change to a diode, say going from 5 volts DC to 0 volts DC, you have now applied an AC signal to the diode. I think you are mixing up bias with AC, because even from 5 to 0 volts, there is an AC component, it just has a DC bias. FETs, the most common switching component in digital electronics today, do not even act like diodes when on, their Drain-to-Source channel conducts in both directions.
I don't say this to discredit what you mention, but rather I think your gave an insight that semiconductors require a DC bias to operate which is a function of the Vth or Vbe, and the AC source only gives enough bias for a fraction of its period.

 

[sophiecentaur]

Like I said the one in reverse bias will breakdown at high voltage if you had that kind of configuration

?
By that argument a rectifier circuit would never work. You would always use a diode with appropriate reverse breakdown voltage for the application. That can be hundreds of volts for many common, cheap diodes.

But what would two crossed diodes achieve that a simple piece of wire wouldn't (unless you wanted a 0.7V p-p square wave)?

 

[newtwoophysic]

It has nothing to do with zeros or ones, but the fact DC is easier to changed voltage in small appliances

 

[sophiecentaur]

The reason why they were not designed to is because physics indoctrinated everyone with the false concept that their is only positive electricity.

What a crazy statement. Could you quote me somewhere where it says this in a textbook?

I might point out that the word 'electricity' does not refer to a quantity at all. It's really a 'lay' term and refers to a general field of study and is not used by Physicists when they are being precise. (It doesn't have a unit, as such, so it's not a quantity)

 

[DragonPetter]

My main point is that since all power is transmitted as AC to begin with (even solar is first inverted), the moment we decide to use DC, we have lost 10% to 50% of the original power in the waveform. A full wave rectifier + capacitance + whatever switching or linear regulator that is required means we have already wasted power. So yes AC power is always stronger than the DC regulation being created from it.

If we are talking about power electronics, motors, pumps, etc... the advantage of AC can not be ignored. As others have stated the reason low voltage electronics do not use AC for their operation is because they were not designed to. The reason why they were not designed to is because physics indoctrinated everyone with the false concept that their is only positive electricity.

Not all power is transmitted as AC. Furthermore, there are losses with transferring AC to your wall just as there are losses in rectifying AC power into DC power. You have to accept energy losses either way. None of these are the real reasons why we use DC power sources in our digital electronics.

Physics did not indoctrinate anyone that there is only positive "electricity". This is way off. We use positive and negative voltages in electronics.

 

[singhvi]

?
By that argument a rectifier circuit would never work. You would always use a diode with appropriate reverse breakdown voltage for the application. That can be hundreds of volts for many common, cheap diodes.

But what would two crossed diodes achieve that a simple piece of wire wouldn't (unless you wanted a 0.7V p-p square wave)?

Maybe some rectifiers do work on high reverse voltages, but I am pretty certain that the fast response ones, used in processors sure don't withstand those voltages

 

[sophiecentaur]

Perhaps not but this thread is talking about Power, n'est ce pas?
Let's try to stay on topic. A rectifier will be used for the voltage appropriate to its specification. You wouldn't use a sensitive detector diode in a car battery charger, that's for sure.

 

[likephysics]

Hi,

Would I be correct in saying that we do not use AC current in electronics because in the circuits, the presence of current would be a "1" and no current would be a "0". Thus with DC on = "1", off = "0". With AC, the current becomes zero 60 times a second (for 60 Hz) and thus would not be useful in electronics.

Virogen,

With the electric devices, there is no concept of input or output. You just power it up and it rotates, lights up or does something similar.
But with electronics, there is input, output and power.
The input part like audio, video is an oscillating signal (AC).You cannot manipulate these signals with AC power. you need DC to power up your electronics.
The output is a louder,clearer audio, hi-def video etc.
You just cannot amplify an oscillating signal like audio with another oscillating signal (AC source). you could kinda modulate, but that wouldn't be pleasant to your ears.

 

[sophiecentaur]

Virogen,

With the electric devices, there is no concept of input or output. You just power it up and it rotates, lights up or does something similar.

.

That is not really a distinction. The 'input' for a simple electrical system is the state of the on/off switch. A more complex 'electronic' system may have a. seemingly, just as trivial an input signal in the state of, say, the shift lock key of a keyboard. The only difference is one of complexity.

 

[pgardn]

Perhaps not but this thread is talking about Power, n'est ce pas?
Let's try to stay on topic. A rectifier will be used for the voltage appropriate to its specification. You wouldn't use a sensitive detector diode in a car battery charger, that's for sure.

Actually the first post was not a power question, n'est-ce pas? ;) Kinda got carried that way. So since the original question did spiral out of control I will continue with a question:

How would one play voltage games with DC power transmission over distance? Transformers seem to be an easy way, but if one had to, what would the best way to accomplish this using DC in "our" current state? current = at present. Say from a power plant to a city 15 miles away?