enter image description hereI recently bought a few electromagnetic coils wrapped around ferrites. I was intending to use them to induce a voltage from surrounding magnetic fields. When I went to wire them however, they seemed to have four leads instead of two. Cannot find an answer anywhere. Possibly I am searching the wrong thing? The leads are black and red (+ and -) and then green and a colourless wire. Could this be for AC? As I mentioned, I am a beginner.


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    \$\begingroup\$ A photo would help; there's a good chance a coil with 4 leads is a "common mode choke"; look that up and see if it makes sense. \$\endgroup\$ – BobU Aug 20 '16 at 22:50
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    \$\begingroup\$ Did you buy things without a data sheet or a part number that you can trace to a legitimate data sheet? If so you are pretty much on your own with the stuff you purchased. \$\endgroup\$ – Michael Karas Aug 20 '16 at 22:52
  • \$\begingroup\$ If the coils are wrapped around each other, it's a CM choke or transformer or CT. If separated along the ferrite axis, it could be an LVDT sensor. who knows? got a photo? there are millions of different magnetic parts. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 21 '16 at 0:04
  • \$\begingroup\$ @TonyStewart Attached a photo \$\endgroup\$ – Ember Aug 21 '16 at 1:16
  • \$\begingroup\$ @BobU Attached a photo \$\endgroup\$ – Ember Aug 21 '16 at 1:16

You have a ferrite rod antenna for an AM radio, exactly as the description says. You should have included that in the question originally.

There are several ways it could be configured, but two of the wires (one uncoloured- Copper) refer to the two ends of the main (full length) winding.

The other two are anybody's guess. The best way would be to get the datasheet from where you bought them - or buy new ones, this time with the "no datasheet, no sale" rule.

Failing that, there are two common possibilities.

(1) One of the three wires (probably Black) is connected to Copper, and the other two are not. (Use a multimeter). Then Copper and Black would be connected to the tuning capacitor to form the main resonant circuit, tuned to the frequency you want to detect. They also form the primary winding of an RF transformer.

The other two form a low impedance secondary winding, which you can connect to an RF amplifier (or even just a diode if you're making a crystal set) and/or a long wire antenna.

You'll have to guess or measure the transformer turns ratio but 7:1 is fairly likely.

(2) All four are connected. Then the other two wires are two different taps on the primary winding, not an isolated secondary. You can use one tap for the antenna (and black for ground) and the other for RF amplifier, detector etc. But again, without the original datasheet and applications circuit, it's all guesswork.

To emphasise, both circuits below are just guesses.

You certainly won't be able to detect DC magnetic fields with them, but you might pick up AC (50 or 60Hz) with them, untuned.


simulate this circuit – Schematic created using CircuitLab


You may have:

  • A transformer. This is the case if there are two independent windings. If leads A and B are connected, and C and D, but no connection between A-B and C-D, then this is what you have.

  • If a transformer, it could be the special case of a common mode choke or balun. These are not intended to transfer power from one winding to another, but to present high impedance to common mode signals and low impedance to differential signals.

  • A multi-tapped coil. These have various purposes. One of them is to make a auto-transformer. In this case, there will be connections between all four leads. By using a ohmmeter sensitive enough to measure the resistance of any of the coil segments, you can figure out the sequence of leads from one end to the other.

    There are only six combinations of two leads. The two ends will be the pair that has the highest resistance between them. Using one end lead as a common, the others will have increasing resistance in sequence from that end.

  • \$\begingroup\$ I don't think so, I probably was not descriptive enough, but it is definitely not a transformer. This is what is says on the packet: FERRITE ROD W/COIL AM RADIO \$\endgroup\$ – Ember Aug 21 '16 at 1:14
  • \$\begingroup\$ @Ember you should but that information in the question. The product description will greatly help people answer your question correctly. \$\endgroup\$ – user2943160 Aug 21 '16 at 1:33

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