Why electronics do not use AC.

[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?

 

[drmagtri]

What do you mean by voltage games, transformers do not even work with DC signals. So then we would have to use many high voltage semiconductors in series instead... this is much much less reliable than the simple transformer.

 

[pgardn]

What do you mean by voltage games, transformers do not even work with DC signals. So then we would have to use many high voltage semiconductors in series instead... this is much much less reliable than the simple transformer.

Yes, I am fully aware of this.
Voltage games "step up, step down" sorry for the confusion.

So I am asking how one would do this DC wise, and you answered with what I thought. I was really looking for something that could be adjustable to suit voltage/current as needed. This would require what kind of setup? Variable resistance that is sensitive to... how could this be done in the least painful manner? Has it been done somewhere?

 

[drmagtri]

Yes, I am fully aware of this.
Voltage games "step up, step down" sorry for the confusion.

So I am asking how one would do this DC wise, and you answered with what I thought. I was really looking for something that could be adjustable to suit voltage/current as needed. This would require what kind of setup? Variable resistance that is sensitive to... how could this be done in the least painful manner? Has it been done somewhere?

I have built a commercial switching regulator circuit which can take anywhere from 30V to 5KV and regulate 60V on the output at several amps... it is possible but it is extremely expensive and orders of magnitude away from the reliability and efficiency of a transformer.

 

[pgardn]

I have built a commercial switching regulator circuit which can take anywhere from 30V to 5KV and regulate 60V on the output at several amps... it is possible but it is extremely expensive and orders of magnitude away from the reliability and efficiency of a transformer.

So its a royal pain, thanks.

 

[drmagtri]

So its a royal pain, thanks.

Yes, if you wanted to do this.. you need to talk to CREE who is leading the industry with High voltage mosfet die.