Why Not DC?

totally right. ga.

In order for electrons to be moved in the secondary, the magnetic field of the primary must move (grow and shrink). The magnetic field in the primary only moves when the current varies. Therefore, all coupled coils are effective only with varying currents. (normally associated with varying voltages or AC)

Quit asking us to do your homework for you.

You can do homework, which is what other people want you to know.

You can wonder about things that may not be in your instructors wheelhouse.

Lynlynch doesn't like homework anymore, and lives in an oyster shell.

Most of us still wonder about things we don't know, so keep asking.

We don't mind not answering if it's too tedious.

Thank you for that answer. I have knowledge in some areas but not all. My weakest is electricity and I read CR4 specifically to bone up on that knowledge. Sometimes the only thing I understand are the answers to 'home work' questions. I have learned a great deal reading the blogs on CR4 and look forward each day to the questions in my email. This is late because I have been out of town, but felt a reply was in order.

I did't say transformers used to produce magnetic field and you yourself said a solenoid magnet is a designed component to make a magnetic field and it works with both AC and DC.All i know is whenever current flows through a conductor a magnetic field is produced around it weather it is AC or DC. clear me here you are saying that you cannot produce a magnetic field with DC supply to a solenoid and it cannot be induced in other conductor.

Please reread posts 1 and 4; they should be really easy to understand. Also, your tagline saying "everybody lies" is offensively silly.

I, and others, already did. A steady DC current to a solenoid (or other winding) does create a magnetic field, but it is an unchanging field. This is dead simple, but clearly you did not understand it.

If you wish to give up, I encourage that in strong terms. Try some other line of work. And, once again, please ditch your offensive tagline.

I agree with the others. It is very simple, a CHANGING magnetic field induces a current. cant get any simpler than this.

Re Tag line 100% agree. "Silly"...too soft.

He is not even original as he is trying to appear...And actually he is biting his own tongue.

The famous Greek philosopher Solon of Athens, is the one who deserve the credit, He said:

"All Greeks are lier's. I am A Greek."

So dear Mallar, draw your own conclusions.

Answer is already given, but you should read and try to understand

http://en.wikipedia.org/wiki/Inductance#Coupled_inductors

as it provides a good fundamental description.

k guys i give up

Edison's favorite was DC but he gave up to AC finally for these reasons mostly.

I don't think Edison ever gave up... which is why we can't find Tesla in the history and invention books.

The short answer is YES, transformers CAN be (and ARE) used with DC power supplies. The DC supplies have to be SWITCHED, though. The principal problem with using transformers with switched DC supplies is signal distortion if the switching frequency's too rapid: the square-wave primary signal comes out more and more as an exponential-shaped one as frequency increases. The secondary signal looks like the voltage between the terminals of a capacitor in an RC (resistor-capacitor) signal switched to a DC battery. (Someday I'll figure out how to add diagrams to my messages here.) That being said, special high-frequency transformers are frequently used to isolate circuits in which square-wave signals (digital information bits or chopped power-supply signals) circulate. Such applications includes: communications, where baluns ('balanced-unbalanced'), which serve to match/transform the different impedances of communications channels (cables, waveguides); uninterruptible/electronic power supplies, in which mains AC power and backup DC power (from batteries) gets converted to DC, switched at fairly high frequencies (for transformers), for example 400 Hz, and then reconverted to 50/60 Hz AC; and others. So, people: remember the aphorism "There are no stupid questions, only stupid answers." In this case ... uuuuhhh .. uuummmm ... :-S Don't slag the askers of questions! DreadZontar

thak you very much sir.now i am pretty clear...

Pleasure's all mine. :-) DZ

There is one thing not mentioned here that people have overlooked. DC can effect how well a transformer works. Here is a thought experiment to demonstrate.

Say that a transformer that takes 1 ampere of RMS AC current at 120V (these acronyms should be obvious) at some pure sinusoidal frequency produces and converts this to 6VAC RMS pure sinusoidal wave and 20 amperes of current into an appropriate load. If the load is reduced the core saturates and the current limits to 21 amperes of current and an examination of the voltage with an oscilloscope will show a distorted sine wave on the secondary but nothing significantly different in the primary voltage sine wave. Now, when one goes back to the correct load the secondary waveform returns to the 6VAC 20 ampere pure sine wave. If one adds a DC 0.5 current to the 1 ampere RMS AC current on the primary side of the transformer then one will get less than the 20 amperes of current on the output side. Looking at the secondary voltage waveform with an oscilloscope one will see that one half of the sine wave (above or below 0) looks fine. However, the other side of the waveform will look like the earlier saturated core waveform.

So DC can affect an AC coupling signal though a transformer but the DC component itself does not propagate through. A critical thing this thought experiment set up was that the AC signal was close to saturating the core. If the core is not close to saturation, then no change in the AC signal output will happen with a DC primary additional current until somewhere the core saturates. At no time though does a DC signal appear on the secondary.

As stated in a reply to induce current and get voltage in the output (secondary) of a transformer the magnetic field must be changing, increasing or decreasing as in an AC wave where the current in the driven coil (primary) is changing and this magnetic field continuously increases and decreases and comes in contact with the secondary coil inducing a current. As in many DC-DC or DC-AC converters like used in a vehicle from the 12 VDC battery circuitry chops the DC resulting in a change. This chopping might be at 400 Hz or even 60 Hz or even 50 kHz. The higher the freq the easer to filter to make a higher DC voltage supply. Like in the old days of tubes that required 200-1000VDC to operate DC-DC supplies were made, but the primary 12 VDC had to be chopped. First used was a DC motor driving a generator (dynomotor), then a vibrator which was a simple relay that when turned on turned itself off which turning it back on and it set there vibrating chopping the DC so it could be fed to a transformer to step up the voltage. Next came the transistor oscillator for this and is still used in the DC-AC converters. Next came transistors that normally work very well on 12 VDC and all this chopping business went away.

obviously a very thoughtful contribution. I'm not qualified to judge the accuracy, but I certainly a stimulating post nevertheless. ga.

Chris

Thanks Chris. Yes, it is thought provoking. A DC transformer? Hah, got my brain going at any rate! BTW the DC transformer responds to AC as well, perhaps up to few kHz (it's limited to some fraction of the switching frequency, which might be 50 or 100kHz). I say AC, but that's AC riding on top of DC, because the circuit can't work with bipolar signals, at least not without adding a few more parts. What to do with such a capability, who knows?

But the DC Bus-Converter part of the idea is extremely useful. For example, start out with a standard high-power off-line to 28V converter, and generate all kinds of other useful high-current supply voltages. For example, we used three Bus Converters to create the ideal odd amplifier supply voltages needed for driving 3-axis magnetic-field coils.

Another useful idea, a bipolar power supply. Start with a say 72-volt off-line DC supply, hook up a Bus Converter with the duty cycle set to 50%, for 36V, which creates a center tap for the 72V supply. Now ground that center tap, and you have a +/-36V high-current power supply, perfect for use with a home-made stereo audio amplifier (2x 125 watts into 4 ohms). This works because the Bus Converter as a center tap is happy both sourcing and sinking current, as required for your speaker ground.

It is - provided you "switch" the dc 'alternately' (plus and minus) before the transformer. Basic designs

This, if you like, is a 'rotary dc transformer' rotary converters

Perhaps you'd like to study the people who Discovered all this 'alternating dc transformer' stuff? Faraday, Henry, Callan,

DC power can produce the constant magnetic field but changeable magnetic field required to induce EMF. AC power magnetic fields changeable according to supply frequency.

The flux produced by DC is Constant i.e. not alternating. According to faraday's law either conductor or flux must be in motion. Transformer is static so windings r also Stationary. If we use d.c. there will be no emf induction n there's also possibility of burning of winding coz reactance offered to d.c. will be less for winding designed for a.c.

No one has mentioned the most common DC "transformer" - the automobile ignition system. Collapsing magnetic field - 12 VDC to 30,000 volt pulses.

because that operates on a changing magnetic field. the large voltage is generated the 'collapse' aka rapid change in the magnetic field.

Chris