# Pulsed DC into inductor turns to AC?

1. Oct 15, 2011

### hobbs125

What is it that causes pulsed dc into an inductor to turn into ac?

Are their specific frequencies at which this occurs (i.e. inductors self resonance frequency)?

2. Oct 15, 2011

### vk6kro

What you can get is a ringing effect just after the rising and falling edges of the pulse.

Like this:

The ringing on the lower edge does go negative, so you could call it AC.

On large coils, and with large currents, these negative going pulses can have very high amplitude.

The frequency of the ringing would be the same as the resonant frequency of the inductor.

3. Oct 16, 2011

### hobbs125

What if the square waves are at the inductors resonant frequency. Then the inductor would effectively be a parallel tank circuit. In this case wouldn't the inductors output be an ac waveform?

4. Oct 16, 2011

### yungman

I thought DC mean stable, unchanged current. Pulse is a pulse even it go from a set low voltage to a set high voltage. A pulse train can be separated into different frequency components(AC) with a DC component super imposed onto each other. There is no pulsed DC.

Last edited: Oct 16, 2011
5. Oct 16, 2011

### Phrak

You say "pulsed dc" applied across an inductor. But this an incomplete question. Is it a low impedence voltage source, or a current source? Does the impedence change with applied voltage and removal of the voltage/current source?

You have another source of confusion: there is no difference between the applied voltage/current across an inductor and the output. They are the same.

6. Oct 16, 2011

### vk6kro

DC means Direct Current or unidirectional current, not constant current.

See Wikipedia:
Direct current (DC) is the unidirectional flow of electric charge.

You can have a DC pulse. That is what comes out of logic gates when they vary rapidly in output from 0 volts to 5 volts. It isn't AC because it has only one polarity.

What if the square waves are at the inductors resonant frequency. Then the inductor would effectively be a parallel tank circuit. In this case wouldn't the inductors output be an ac waveform?

There would certainly be a component at the resonant frequency.

The square wave would lose all its high frequency components and become more like a sinewave at the resonant frequency.

This would only work if the inductor was fed from a large resistor in series with the signal generator.

7. Oct 16, 2011

### sophiecentaur

As Phrak says. The question is incomplete. This is just a general arm-waving discussion about non-specific situations. There are better things to do than argue about the etymology of electrical terms - some Engineers being what they are, they can be very sloppy in their terminology at times.
A waveform, in general, will have DC and AC components. Nothing more to be said, usefully, unless the waveform and the circuit producing it are specified.