Question for the electrically inclined.

Adex_Xeda · Post by **Adex_Xeda** » June 4, 2003, 4:07 pm

I'm constructing some power supplies that convert 115Vac to 24VDC.

One part that came included in the kit was a cylindrical donut shaped piece of ferrous (a magnet sticks to it) metal.

A co-worker said he saw those metal donuts before and that you clamped around a loop of wire (AC or DC?) . The purpose of which was to reduce noise in the power lines.

Question:
Do you clamp it on the AC lines going into the power adapter or the DC lines leaving the power adapter.

Question:
How does it work? I know that when current changes through a conductor a magnetic field is generated. Pushing that magnetic field through a ferrous metal induces eddy currents or something in the metal that reduces high frequency noise?

I'm kind of foggy here.

Perchance any of you guys know?

Thanks in advance

Mawafu · Post by **Mawafu** » June 4, 2003, 4:26 pm

Whichever will be coming closer to electrical circuitry inside whatever it will be installed. We use them on our products at my work for this purpose. For example, one of our products is a very small box so everything is crammed together; we have ferrite beads on all the wiring. Another product has more room inside and only has a ferrite bead on the cable going to the display since it passes right on top of the power supply.

Judgement call in either case, but more often then not I see them on the DC side.

I have never worked on the design of our products (just test equipment) so my opinion is based on observance and not the actual design.

Pilsburry · Post by **Pilsburry** » June 4, 2003, 4:51 pm

I think there is one on my monitor cable from my computer.....not the electrical plug but the actual monitor cable.

In short: I have no idea where you should place it. But, it looks like you can put it almost anywhere, it's just probably optimal to place it in one spot, so if your unsure...my gut feeling is buy some more and put them everywhere. heh.

Willowwing · Post by **Willowwing** » June 4, 2003, 8:39 pm

Basically what your doing first is making a full wave rectifier taking pulsating ac to pulsating dc. Then to a stepdown xformer to 24vdc usually noise should be filtered on the dc side. Using and inductance capacitance circuit you should be able to get the frequency you need. I forgot the formula for frequency cutoff been a few years since I had dc/ac class.

Adex_Xeda · Post by **Adex_Xeda** » June 4, 2003, 10:19 pm

Hehe, yea I know the design of the power adapter, I was curious however about the physics behind this clip-on cylinder of metal.

Adex_Xeda · Post by **Adex_Xeda** » June 4, 2003, 10:52 pm

Hmm I found some information close to the answer, but a few things are still missing from this picture.

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http://hydro4.sci.fau.edu/~rjordan/bust ... -ups_6.htm

[2] When you place a metal ring in a region where a magnetic field is rapidly alternating, the ring may become hot. Why?

Faraday's Law gives us the reason; the alternating magnetic field induces an emf in the ring that also alternates. Since the ring is a conductor, i.e., it has a relatively small resistance, a large alternating current will be induced in the ring also. (These currents are called "eddy currents".) At any instant, the power produced is (voltage x current), which may be large, large enough to cause a heating effect. Note that if the magnetic field is constant there will be no heating effect because there will be no induced emf and no current.

This type of heating - "eddy current" or "induction heating - is used on some stovetops. Beneath the stovetop, which is usually some easily-cleaned, ceramic material, there is a coil that carries a rapidly alternating current. This coil produces a rapidly alternating magnetic field, and when a metal saucepan is placed on the coil then the alternating field induces an alternating emf in the bottom of the saucepan. The emf produces a current and so heating occurs. Note that this is not a "hotplate" because it's not hot; the surface of the stovetop can be quite cold since the heating effect is produced in the saucepan itself!

Will it work with all saucepans? No! The saucepan must be made from a conducting material; the effect works with most metal pans but it will not work with ceramic or pyrex pans.
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