What is the Purpose of the Magnets in a Biased Choke?

[Jdo300]

Hello All,

A couple of nights ago, I had the pleasure of cannibalizing an old CRT computer monitor for parts when I came across this interesting little coil on the board (see attached picture). It has a ferrite cap on the end and if you take it apart, there are two ceramic magnets above the coil and under the cap. The coil was also wound around a slug of ferrite. Could someone explain to me why they put the magnets in this coil and what the magnetic field changes the coil's behavior. The only thing I could come up with is that the field might presaturate the core for some reason. Could the field be biasing the coil so that it's easier for pulses or sine waves to go one direction?

Thanks,
Jason O

 

[berkeman]

I haven't seen those before, but the effect of the magnets will be to bias the ferrite slug so that it is on one side of its hysteresis curve. It would bias you to the left or right of the B-H curve, like this one at wikipedia:

http://en.wikipedia.org/wiki/Magnetic_hysteresis

I'm not sure why you would want to do this -- it makes the response to an AC signal non-linear. Could you tell what part of the circuit this component was in?

 

[Jdo300]

I haven't seen those before, but the effect of the magnets will be to bias the ferrite slug so that it is on one side of its hysteresis curve. It would bias you to the left or right of the B-H curve, like this one at wikipedia:

http://en.wikipedia.org/wiki/Magnetic_hysteresis

I'm not sure why you would want to do this -- it makes the response to an AC signal non-linear. Could you tell what part of the circuit this component was in?

Hi Berkeman,

I don't remember what part of the board it was on because I was just randomly desoldering stuff off of it. In case this helps, I remember that there was a small electrolytic cap that was glued up against the coil and I want to say that they may have even been electrically connected on the board but I'm not sure. I can check later when I get home and try to find the spots that I pulled it off from though.

Is there any particular name for this kind of coil? I'm trying to find more information on it.

Thanks,
Jason O

 

[berkeman]

Hah! Learn something every dang day!

I just walked down the hall to run this by another analog engineer here (actually one of the best in Silicon Valley), and he said he hadn't see that type of component before either, but I think he figured out what it is used for.

If you bias the ferrite one direction, that means that you will be able to withstand a larger DC current before saturating the other direction! So the tradeoff is whether you can reliably bias the ferrite one way to soak up a DC current bias the other way, or whether it's cheaper to just use a bigger ferrite to handle the DC current without saturating.

I did a quick google search on magnetic bias inductor, and got some good hits. I didn't spend enough time searching to find a source or datasheet, though.

Hey, thanks Jason. Interesting question, and I learned something!

 

[Jdo300]

Hmmmm now that is interesting . That reminds me of the old Magnetic amplifiers which used a DC bias on the core to regulate an AC current on the secondary coil! Good stuff and thanks for the clarification .

- Jason O