DC inductance?

Thread Starter


Joined Jan 17, 2012
I am interested in electronics, and have been reading about electromagnetic inductance.
I have experimented with ac inductance for the last few weeks, but was wondering what can be done using DC power. I heard that DC can be supplied to a primary induction coil, then once switched off, a voltage spike would appear on a secondary coil. If this is true could a bulb be seen to flash if attached to the secondary coil. I did try experimenting with this but got no results, maybe I did not have the correct amount of windings?


Joined Feb 19, 2009
What you attempted to build was a "boost" switching power supply, unregulated.

Look up "Joule Thief" for a basic boost circuit using an inductor. It's actually a bit of a transformer due to the two sets of windings, but one is only for feedback to keep oscillations going.

Inductors resist a change in current, so they are inherent low pass filters, when the current stops, the voltage across the inductor rises until there's a path for the current to go.


Joined Jan 10, 2012
The lamp needs time to get hot enough to glow, but the resultant pulse from your experiment will be very short. You might be better off just taking the two wires from the secondary and placing them very close to observe an arc. You might also try reversing the primary and secondary. Also, make sure you limit the DC current through the inductance by using a resistor or your power supply's current limit. I have done your experiment using an automotive ignition coil, and have observed a weak arc. Not sure what you are using so don't know if it will actually work.


Joined Mar 14, 2008
An LED will respond to a short pulse much better than an incandescent bulb, if that is what you were using.

Be aware that the pulse from the inductor will be the opposite polarity from the applied voltage.


Joined Dec 26, 2010
A set-up using a transformer primary fed by switched DC can generate pulses of both polarities in the secondary, one way round for the primary on, reversed for the primary off. This can easily damage a single LED, because they tolerate reverse voltages poorly.

It would be safer to use two LEDs in parallel, but opposite ways around. A series resistor should be used to limit the "primary on" state current to a reasonable value for the LED concerned, say 20mA.

The "primary off" current also needs to be reasonable. This will depend on how much primary current builds up with the primary powered, and also on the turns ratio - the other way around than for voltage transformation: voltage step-up = current step-down.
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