# OK I'm very confused now about AC

• posted

Hey,

Right, I'm a newbie to the world of practical electronics. The evil institution that I'm currently attending has told me (with a very large amount of complicated calculus-based background) that inductors and capacitors are basically useless with anything except an AC source. Now, I'm seeing cap's and inductors being used in circuits running from DC supplies. If a capacitor is basicaly an open circuit to DC, what the hell is going on?

Is there anything I cannot do with a DC supply? If I buy a dc supply right now, will I find myself wishing I had bought an AC supply some time in the future?

Not to boast, just to increase my chances of getting a satisfying answer, I'm pretty solid on theory so don't leave anything out because you think I might not understand. Read: Newbie to the practical side, intermediate on the theory.

• posted

The circuits may run from DC supplies, but they produce outputs which change with time - which is to say, in the broadest sense - AC.

The capacitors and inductors prevent the AC being generated in one part of the circuit from propagating through the power supplies (including the ground connection) and generating un-desired AC in other parts of the circuit.

You are probably looking at reservoir capacitors and filter inductors, which should give you a clue as to what is going on.

------------ Bill Sloman, Nijmegen

• posted

Either you misunderstood what the teachers said, or the teachers didn't explain this the right way.

With few exceptions, there are no 'purely DC' circuits, not even (perhaps especially!) power supplies.

Inductors and caps enjoy a lot of use because of their transient features. The examples that follow are pretty simplistic, and hardly scratch the surface of where and why these components get used.

As an example, let's look at the stuff running from the DC power supply. All those circuits (if they are doing anything remotely interesting) are generating switching noise that couples back into the supply (and disturb their own local version of the supply). Caps are used for two main purposes in this area

1. Transient suppression. Get rid of those nasty transients generated by the chips themselves.

1. Bulk bypass. In this application, we're using the cap as a small 'battery' to provide curent to the device when the main power supply droops (perhaps the load is too high - that certainly happens in some designs).

The vast majority of such caps are used in the transient response area. This ignores the bulk caps and loop filters on a switchmode supply (but that's a subject for another day).

Inductors, likewise, have characteristics we can use. As another example, when I have designed very high speed interconnects (2.5 -

10Gb/s), the drivers are universally 'CML'. This is current mode logic, which means the driver 'consumes' a (fairly) constant current regardless of the output state. To prevent the high speed switching from interfering with this (desired) constant current, I put small inductors in series from the power supply to these chips. (An inductor tends to oppose changes in current).

Of course, switch mode supplies require an inductor for other reasons.

As I said, the uses for caps and inductors are wide and varied, and this barely starts the conversation.

Hope that helps clue level increase.

Cheers

PeteS

• posted

Hey Moikel,

Yes, an ideal inductor becomes a short circuit for an ideal steady-state DC, and an ideal capacitor becomes an open circuit for an ideal steady-state DC. But how does one arrive at a steady-state wihtout passing through a transient condition? In other words, if you're going to use your DC supply you've got to throw the switch at some point in your lifetime. And when you do that you create a transient condition.

Remember also, that our best physical devices only approximate our ideal models of them. In the world that we inhabit direct current sources always include lesser amounts of alternating currents. Inductors and capacitors are used in DC supplies in order to help filter out these residual alternating currents.

Best, Mike

• posted

...and a copy of Horowitz and Hill's "Art of Electronics".

At least back when I was in engineering school, the classes taught theory, they didn't teach beans about practical engineering. You'll understand more about what and why from reading these two books than from all your classes combined. (Not that the theory is unimportant; just that it's insufficient.)

• posted

Go to your local library, and check out a copy of the ARRL Radio Amateurs Handbook.

Tam

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Either they lied, or you got it wrong, or they are using "AC" to mean anything other than pure DC (of infinite duration).

There is no practical example of pure DC in existence, since that would require perfect unchanging amplitude for all eternity. Real DC applications are dealing with ripple voltages and currents and loads that vary all over the place. All these variations can be described as various AC components added to pure DC.

I am not going to make an exhaustive list of things you might do to see which ones do not include the possibility of a DC supply.

supply some

Instead of the DC supply? I doubt it. In addition to the DC supply, possibly. If you want a versatile DC supply for lots of experimental work, get one that has both adjustable voltage regulation and adjustable current regulation. I also like ones with both a voltage and current meter that I can watch at the same time, rather than one meter with a switch. The next step up might be to get a dual supply so you can work with a positive and a negative supply with a common ground.

What do you hope to use the supply for?

• posted

A DC supply does not imply that all voltages and currents in the circuit are stable and unchanging. This should become apparent to you when you learn about active devices next term.

Consider something like a radio receiver. The supply is DC, but the signals passing through the circuit are AC (at a variety of frequencies: RF, IF, AF).

Consider an oscillator. (Very roughly) this will make AC from DC.

Consider a computer motherboard. It will have some inductors and lots of capacitors. The inductors are used in switching DC/DC converters. Very roughly speaking, these chop up a DC power supply to get AC, then transform it to a different voltage, then rectify it back to DC, but at a different voltage. The capacitors are used for power supply filtering and decoupling.

Regards, Allan

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To be painstakingly nitpicky, a DC current or DC voltage never varies. It existed at the beginning of time and will last until the end of time.

Obviously such a thing doesn't really exist :-).

We use what we call DC supplies that stay nice and steady while they're on. Filter caps like you see are a step towards that, for filtering out AC.

DC circuits by themselves are really pretty boring. Most interesting circuits have waveforms/pulses/etc.

Most circuits that work on AC waveforms are powered by DC. The most useful "AC supply" you'll come across is a signal generator.

Tim.

• posted

Moikel wrote: If a capacitor is basicaly an open circuit to DC, what the

dV/dT

If I buy a dc supply

Yes. And vice versa.

Paul Burke

• posted

That is only true if you take the ideal definition of AC as anything that changes - ever. Even a DC power supply is turned on and off occasionally, and it has ripple on the input to the filter. Often caps and coils are used in circuits where something is trying to change and you don't want it to. When you realize that every DC circuit is also an AC circuit in some sense, then the theory and the practice can be reconciled.

-Robert Scott Ypsilanti, Michigan

• posted

Not really 'supply' just 'current'. You can generate waveforms from DC, usually square waves. That's how we clock CPUs after all. Toy blinking-lights usually generate the on-off waveform with a transistor-capacitor based circuit. You can find such circuit for analysys by googling "blinking lights circuit". A classic one is something like this:

Modern electronics, of course, uses resonating crystals to generate square waves.

Anything that keeps switching between one voltage level to another (on/off etc) is AC. Even if it is not a sine wave. Square waves are AC too. That's how you get to use caps and coils on DC circuits. You can generate AC from DC (note 'generate' not simply 'convert' as when you convert DC to AC).

You can even buy inverters that can generate usable 220V AC (110V if you're Japanese or American) from DC supply. Usually you'd attach these to car batteries or some other suitably large DC supply. Inverters are just fancy high voltage versions of blinking lights circuits.

The only time I've worked with an AC supply is when I was at university. Even then, most of my lab work uses DC. You'll be needing a DC supply anyway since most low votage electronics projects uses it.

These days I usually buy an AC/DC wall-wart for my DC supply in my projects as they are cheap. I just make sure my circuits are regulated using the appropriate regulator: LM7805/LM7809/LM7812 etc. And make sure to put in a big capacitor on the supply inputs to my circuit to smooth out the AC ripples (them cheap supplies aren't clean).

But I do digital electronics and noisy DC supplies doesn't bother me much. If you do analog, you may want to invest in a real DC supply.

By the way, if you think you're good with theory then check out the blinking light circuit I linked above and analyse how it works. It's quite cool actually, I built one for arts&craft when I was 12. Of course, back then I didn't understand how it works :-)

• posted

Not really 'supply' just 'current'. You can generate waveforms from DC, usually square waves. That's how we clock CPUs after all. Toy blinking-lights usually generate the on-off waveform with a transistor-capacitor based circuit. You can find such circuit for analysys by googling "blinking lights circuit". A classic one is something like this:

Modern electronics, of course, uses resonating crystals to generate square waves.

Anything that keeps switching between one voltage level to another (on/off etc) is AC. Even if it is not a sine wave. Square waves are AC too. That's how you get to use caps and coils on DC circuits. You can generate AC from DC (note 'generate' not simply 'convert' as when you convert DC to AC).

You can even buy inverters that can generate usable 220V AC (110V if you're Japanese or American) from DC supply. Usually you'd attach these to car batteries or some other suitably large DC supply. Inverters are just fancy high voltage versions of blinking lights circuits.

The only time I've worked with an AC supply is when I was at university. Even then, most of my lab work uses DC. You'll be needing a DC supply anyway since most low votage electronics projects uses it.

These days I usually buy an AC/DC wall-wart for my DC supply in my projects as they are cheap. I just make sure my circuits are regulated using the appropriate regulator: LM7805/LM7809/LM7812 etc. And make sure to put in a big capacitor on the supply inputs to my circuit to smooth out the AC ripples (them cheap supplies aren't clean).

But I do digital electronics and noisy DC supplies doesn't bother me much. If you do analog, you may want to invest in a real DC supply.

By the way, if you think you're good with theory then check out the blinking light circuit I linked above and analyse how it works. It's quite cool actually, I built one for arts&craft when I was 12. Of course, back then I didn't understand how it works :-)

• posted

the voltage across the cap is the integated charge. Or in other words, the current through a cap is proportional to the derivative of the voltage. The similar is true for the coil. You have to wait until the math taught includes integration and differentiation.

AC supplies are useful too, but are not as simple. What frequency, voltage and impedance range should it be for ?

Rene

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• posted

The capacitor is being used to filter the AC ripple voltage so the output is pure DC, not DC with AC riding on top of it

• posted

hi dude! i think the the capacitor works in both the ac and dc current system for exmple on the filtering system i used in my rodot, i use the capacitor to filter the noise from the motor to protect my ic chip. if you look in the radio system there are more the three capacitors and in the computer motherboard there are still capacitor.if you are still in doult please open up you casette recorder and check the amplifier. it has about three capacitor. use the imformation from google and just tape capacitor. this is how i improved.

about the inductors, this iclude the transformer but the the transtormer are only use in ac current circuit to step up or step down the voltage. if you have the tape player with the ac plung there is a transformer just in the rectifying circuit, so it low down the voltage before it reach the rectifier.

i think this will give ya a clue good luck lameka ( from Namibia)

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Troll away - let's see if you catch anyone.

• posted

They said no such thing.

What the "hell" is going is your inability to discern context. In the context of circuit analysis, the energy storage components are of no consequence to the DC analysis.

I don't think you could brush your teeth with one, for example.

How the "hell" would we know what you might or might not wish for in the future.

You need to work on your listening skills, and, as well, admit there are things you don't understand, which means you should ask questions like this during the lecture.

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Why do the postings via "Google Groups" all sound alike?

robert

• posted

Later on you'll take a circuits class and they'll tell you that there is no such thing as DC; it's just AC with a frequency of zero. That's the loophole that makes inductors and capacitors useful everywhere. :)

In a linear DC supply (the kind with a big fat transformer), the cap is serving to filter the rectified output. Since it blocks the low frequency (the desired DC output) and passes the high ones (the 60 Hz ripple), the cap can serve as a selective short-circuit, passing the noise to ground while the DC goes out. You will find a cap across the power input of almost every DC-powered circuit for the same reason; it's there to filter out noise in the power that could interfere with the circuit's operation.

Most consumer power supplies now are of the switching variety. Very basically (because I'm lazy, not because it's complicated), they use the property of an inductor to resist changes in current by rapidly turning them on and off. The inductor has no choice but to generate significant voltages to keep the current flowing, and these voltages are harvested and stored in a cap to be delivered to the circuit. You can find any number of pictorial explanations on google.

The inductor is essentially the reciprocal compliment of the capacitor. It passes low frequencies (including DC) and blocks high frequencies. So you will often find them at the inputs to audio circuits where they are intended to prevent RF from entering, but allow the much lower audio frequencies in.

If you're shopping for a power supply for research and bench work, a good current/voltage regulated DC supply is invaluable. You will find this serves almost all of your requirements. Unless you are doing power electronics work, a common signal generator is the only AC source you will need. However, if you can get your hands on a Variac and an assortment of transformers, you can very cheaply generate (a very unregulated) AC of any voltage.

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