What is a 'demagnetization coil' ?

To remove remanent magnetism that might otherwise result in core saturation.

Graham

So the coil must *actively* give a kick to remove remanence ?? And in cases when the coil is passive, the term is wrongly used. If that interpretation is false, please correct me! Thanks,

A classic example is the degausing coils in a TV to to demagnetize the screen. If a magnetic area builds up it can distort the electron gun beams.

Its usually on for a second at power up and is in series with a thermistor to turn it off.

When you drive a transformer winding with a voltage, you magnetize the core. If you drive it in one direction too long, it will saturate, meaning that it no longer acts like a transformer (or an inductor in the case of a flyback) and the winding will appear shorted, usually destroying parts in the driving circuit. So the trick is not to drive too long in one direction and then drive in the other direction. But many topologies can't drive the tranformer in the other direction. Topologies like the flyback let the transformer "reset" on its own. The problem is if you just drive the winding and let it go (a truly passive reset), there is no place for current to go with the power switch (mosfet) off and the primary winding will produce a huge voltage trying to reset, also destroying parts. Here is where the reset winding comes in. It is usually a tap off the primary with a diode to ground specifically used to dump reset energy fro the transformer. This is an active reset.

One of the posts was already on the right track but I wanted to elaborate. I have never heard it called a demagnetization coil before, just a reset coil.

Traver

Thanks for a good answer. However, what you describe above is the case I would call 'passive', because the coil does not *actively* kick energy into the core to remove remanence. The term 'reset coil' or 'protective coil' would be fine in this case. What I would like to know is whether the term 'demagnetization coil' would require something more, like a circuit (capacitor, diode) to bounce back energy in order to actively remove remanence. Or maybe the term 'demagnetization coil' shouldn't be used at all in the context of flyback converters, because it implies a wrong idea. (??)

saturation.

I agree. True "active" reset circuits have a controlled swtich like a mosfet turning on purposely to drive energy out of the core. Although the "passive" reset is cheaper and easier.