Can you use a dc circuit in a transformer?

1. Mar 26, 2005

T@P

can you use a dc circuit in a transformer? will it work the same way?

2. Mar 26, 2005

Andrew Mason

I think your question is: can you use a transformer in a DC circuit? The answer is 'yes', but it won't act like a transformer. It will act like a coil of wire and will get hot if you put much current through it. And you will get an arc when you open the switch (due to the self inductance). There will be no current in the secondary [Edit: except when the DC current in the primary is changing - eg. when you open or close the switch]

AM

Last edited: Mar 27, 2005
3. Mar 27, 2005

Danger

Won't it work in an interrupted DC circuit, as having a buzzer or such like in series, because the collapsing field is what induces the secondary?

4. Mar 27, 2005

Andrew Mason

You are quite right that a transformer can be used in an interupted DC circult. That is, after all, how a coil works in a car engine. It turns 12 V into the high voltage needed for the spark.

But I don't think a buzzer uses a transformer. It simply uses a relay that pulls a switch on loaded spring that then cuts off the current to the relay and the cylce repeats.

AM

5. Mar 27, 2005

Danger

My fault for being lazy. I was sort of short-handing my post. What I meant was that you could use something like a buzzer in the circuit to interrupt it (like the points in a car) and thus obtain a transformed, but not steady, DC voltage.

6. Mar 28, 2005

T@P

oh so i need an ac circuit for a transformer... i see. thanks.

7. Mar 30, 2005

Danger

No... no. Sorry if that was a bit misleading. The way a transformer works is that the primary circuit around the armature sets up a magnetic field. That field collapses when the current is removed, and at that point induces a current in the adjacent secondary windings. A straight DC current is never removed, and so doesn't do anything. If you shut it off, you get one very quick pulse at the secondary. An oscillator, buzzer, etc. continuously makes and breaks the circuit, and every time it's broken you get a pulse. Make the interruptions close enough together, and you get a pulsed current at the output which in most practical cases is smooth enough to use. The reason that AC is used most of the time is that it's already pulsing @ 60Hz, and therefore needs no additional components such as breaker points.

8. Mar 30, 2005

T@P

Thanks for the clarification. is that why some transformers make alot of noise? (the buzzer going on and off?)

9. Mar 30, 2005

Clausius2

I heard it is because the continous crashing and vibrating of the metal plates of which the magnetic nucleus is formed. As alternating magnetic flux gets through the nucleus, it provokes magnetic forces inside it which make internal vibrations of such compilled plates.

10. Mar 30, 2005

Danger

The transformer buzz itself is often amplified as well, since it's probably mounted solidly (screws, etc.) to a larger structure to which the vibrations are transmitted. It's a lot quieter if you mount it on stand-offs of some shock-absorbent type.

11. Mar 30, 2005

Billy T

It is a little more complex than that. For example, the secondary can be in a region where there is never any magnetic field. Take a look at post 48 and 50 of thread "first stars - how big - black holes now?" (in the cosmology section) to see specific design with no magnetic field ever present near the secondary coil and to understand better how a transformers really works. That off thread bit about transformers was to show that people often accept with out thinking "feel good" words that have nothing to do with what is really happening - In case of tranformers, many believe that "lines of flux cutting thru the secondary" cause the secondary current. Some who believe this even know that these "field lines" are not even real. They were invented by Faraday, and he knew they were not real, only an aid to thought.

12. Mar 31, 2005

Danger

You've just been lying in the weeds waiting for me to do that, haven't you? It's all very cool, and I don't have time to read it right now, but it doesn't really impact the original question here. For practical purposes of sticking a transformer into a circuit, the collapsing field is adequate. :tongue:

13. Mar 31, 2005

T@P

I got overwhelmed by the posts in the cosmology section...
correct me if im wrong, but isnt the principle of a transformer that a changing electric field makes a magnetic field, (the AC current through the first coil), and the flux through the second coil produces an EMF in that coil. in relation to the "feel good" words described by the author of posts 48 and 50 (sorry i dont remember), i just dont see them. can anyone explain? sorry if im a stubborn learner, but this is all kind of new to me. :yuck:

14. Mar 31, 2005

pervect

Staff Emeritus
Your understanding of a transformer is basically correct, T@P - except that it'd be better to say that an electric current causes a magnetic field. A changing electric field does also generate a magnetic field, but this is important in capacitors, not in transformers. Ind transformers it's the current that causes the magnetic field.

As far as Bill_T's point goes, don't worry about it too much. He's just saying that if you concentrate the magnetic field, you can have a net magetic flux through a loop of wire without having the magnetic field intersect the copper. But the magnetic field doesn't have to intersect the copper, it just has to pass through it.

15. Mar 31, 2005

roger

How does the current cause the magnetic field ?

16. Mar 31, 2005

Andrew Mason

This question is beyond the scope of this thread and probably should be a new thread. It is not a trivial question and can the answer can have as much depth as you want. The answer has to do with Special Relativity.

AM

17. Mar 31, 2005

T@P

either that or you can point to maxwell's equations and laugh.

18. Mar 31, 2005

Andrew Mason

Maxwell's equations describe the magnetic field and quantify it, but do not explain how it arises.

AM

19. Apr 1, 2005

T@P

ok you win :) theyre just funny especially since they look so menacing with all the integrals =)

20. Apr 1, 2005

Danger

Go check out GD. Anything involving Integral is menacing.

Of course, he'll probably delete this post...

Last edited: Apr 1, 2005