# DC-Transformer circuit help

1. Feb 15, 2014

### Kuzey Cem

I recently learned that if you send direct current through the primary coil of a transformer, no power would be transferred to the secondary circuit? Why is that?

2. Feb 15, 2014

### Averagesupernova

Can you tell us why it works for AC? If so, then you should know why it does not work for DC.

3. Feb 15, 2014

### FrankFerrese

Theoretically, it would work. Since the flux in the core of the transformer increases by the integral of the applied voltage (Faraday's law), in an ideal transformer, the flux would just continue to increase. However, if you tried this with an actual transformer, you would drive the core into saturation and create a lot of heat.

Dr. Frank Ferrese

4. Feb 15, 2014

### meBigGuy

A changing current produces a changing magnetic field, which in turn can induce a current in the secondary. If the current is not changing, the magnetic field is not changing, so no current is induced in the secondary.

When you first apply a DC voltage the current ramps up, so the field is changing and an initial current is induced in the secondary. But when the current stops changing (limited by resistance, for example) there will be no current induced into the secondary.

5. Feb 15, 2014

### dlgoff

6. Jun 22, 2016

I dont know why but i have read on other sources that people use ac/dc transformers on high voltage dc applications of 60v or more. they used the simple wall-warts to convert the high dc voltage to lower dc voltage.
however even then, they still said that some wall-warts worked and some didnt.
Dont know why that works with higher DC voltages but know it does. sometimes!

check out this link: people on that forum use them a lot.
https://endless-sphere.com/forums/viewtopic.php?f=2&t=75171&p=1135736&hilit=wall+warts#p1135736

7. Jun 22, 2016

### Tom.G

8. Jun 23, 2016

### David Lewis

Referring to the picture, if most of the magnetic flux stays inside the iron core, how do moving field lines cut through the secondary coil?

9. Jun 23, 2016

### SDRFG

The inductor has 0ohm resistance at dc

10. Jun 23, 2016

### davenn

no, that is incorrect. There will be resistance and is easily measured with an Ohm meter
for a transformer primary, it can easily be several 100 to several 1000 Ohms

Last edited: Jun 23, 2016
11. Jun 23, 2016

### Staff: Mentor

The field lines do not have to be moving. The changing magnitude and direction (+/-) of the primary current generates a changing flux value, which in turn induces the changing secondary output voltage.

12. Jun 25, 2016

### David Lewis

If the flux value (concentration of field lines) changes, do the magnetic field lines move?

13. Jun 25, 2016

### jim hardy

It's a quirk of the universe that the "lines" need not "cut" the conductors,
only that they be encircled by the conductors.

The field itself is more of a continuous fluid than discrete lines. It goes everywhere , just changes its intensity as you get farther from its source.
But drawing lines helps us visualize it and gives us something we can count to gauge its intensity.

A "line" is a called Maxwell, one of them per square centimeter is called a Gauss which is roughly the magnetic field strength of Earth that tugs your compass needle around.
A field strength of ten thousand Gauss is called a Tesla, about the field strength of a good loudspeaker magnet.

Any help ?

old jim

14. Jun 25, 2016

### David Lewis

Many thanks, Jim. I posed a difficult question that has been puzzling me for a while. Good answer.

15. Jun 25, 2016

### jim hardy

Thanks for the feedback !