# Old inductor

#### ceidas

Joined Dec 26, 2011
50
Hey everyone,
I found an old inductor coil in a ferrite rod and I noticed that there are 4 ends. I dont get it. Isnt this suppose to be 2? All the inductors have two ends, right?

#### bertus

Joined Apr 5, 2008
22,305
Hello,

Can you post a photo of the inductor?
It is possible thet it has two coils.
One main coil and a coupling coil.

Bertus

Joined Dec 26, 2010
2,148
If this is a ferrite rod antenna, as commonly used by AM radios, it is very common for the coil to have four connections.

What you very likely have is a coil wound in two sections, effectively forming an impedance matching transformer.

One section of the coil has more turns, perhaps about 60 of them for medium waves. This coil is made to resonate with a capacitor, typically a variable type to allow the coil to be tuned to the required frequency.

The other winding section has far fewer turns. This allows the coil to be transfer the signals it picks up effectively to a relatively low-impedance device such as a transistor, without damping the resonance of the coil too severely.

The impedance is changed as the square of the turns ratio, so if the second coil had four turns, and the transistor had a 1kΩ input impedance, the resonating coil would see an input impedance of 1kΩ*(15)$$^{2}$$ = 225kΩ

This is more than 100 times the 1885 reactance of a 300μH coil at 1MHz, so a reasonably sharp resonance should be obtained.

#### ceidas

Joined Dec 26, 2011
50
http://img694.imageshack.us/img694/8167/13012012058.jpg
http://img850.imageshack.us/img850/4443/13012012061.jpg

These are two pictures of the ferrite rod, with the second one zoomed to the right coil with the more turns. And yes, I need this coil for an AM radio project but i cant figure out which coil is the one with the more turns.

http://img252.imageshack.us/img252/1115/13012012062.jpg

This is also the variable capacitor i got and i really dont understand this one. I googled every letter or numbers written on it but i just cant find any datasheet. In addition this is not one variable capacitor. It is four in one and I dont know what connections form what capacitor and how many Farads have each one.

EDIT!

I figured out a way to know which inductor is which. I measured the resistance with an ohmeter and the longer inductor had 3 ohms more resistance. Now, my only problem is the capacitor, which I dont know anything about how it is constructed.

Last edited:

Joined Dec 26, 2010
2,148
The rod antenna looks as if it has separate coils for two wavebands, although the coil on the left with the apparently fewer windings might be for connection to an external antenna, for instance for use in a car.

The coil on the right-hand side looks like a "tapped" medium-wave type - were the ends of the twisted wire on the right separated, or used together? You need to be sure, as a tapping needs to remain joined or the coil will not work. On the other hand, joining together the ends of a separate secondary will also spoil the results.

A tapped coil is not quite so flexible in use as one with a separate secondary. If you want a separate secondary like that you could of course add one yourself. If you are not using the tapping though, it has to remain joined to itself - and to nothing else.

The variable capacitor looks like one from an AM / FM radio. It most likely has four sections, two for the AM circuit, of which one at east will be just right to tune your coil. The other sections of the main variable capacitor will be for FM: these will be much lower capacitance and may best be ignored.

Note the common or ground connections running down the middle of the capacitor - all sections have their common connections in parallel.

The larger capacitance AM sections will be those furthest from the mounting base, as a longer ground connection is permissible at the lower frequencies.

#### ceidas

Joined Dec 26, 2011
50
Thanx for your answer but I still dont know which one capacitor is which one... Is there a practical way to figure this out considering the absence of a capacitance meter?

Joined Dec 26, 2010
2,148
Probably wiring it up to the coil and checking the tuning range is as easy as anything else.

Note that some of the connections may go only to the little trimmer capacitors - set by the little screwdriver adjustments in the top.

The common connection (to the moving vanes) is likely to be the strap up the middle of the side in view. The other connection you need should be to one of the more substantial sets of fixed vanes: there seems to be a tab about a third of the way down from the top which looks promising - but there should be a similar one elsewhere.

If you can post some different views of the capacitor this may become clearer.

Joined Dec 26, 2010
2,148
The OP has a capacitor out of a multi-band set. Its connections are more complex.

#### ceidas

Joined Dec 26, 2011
50
Yes, it is a lot more complex than Bertus pics but thanx anyway.

I have a good feeling that i found the capacitor i was looking for and now I am trying to apply it to the AM radio circuit i made but (again) it doesnt work...

http://img43.imageshack.us/img43/1685/amrfr.jpg

Any ideas?

Joined Dec 26, 2010
2,148
Yes, it is a lot more complex than Bertus pics but thanx anyway.

I have a good feeling that i found the capacitor i was looking for and now I am trying to apply it to the AM radio circuit i made but (again) it doesnt work...

http://img43.imageshack.us/img43/1685/amrfr.jpg

Any ideas?
That does not look like a very good circuit. Where did you get it from?

#### ceidas

Joined Dec 26, 2011
50
I made it. Whats wrong about it? It has two amplification stages and the CC amplifier is for rejecting the negative cycle and for low impedance output.

Joined Dec 26, 2010
2,148
There are several things that could be commented on. The first thing is that you will probably not get good amplification at 1MHz with two stages biassed at about 250μA and rather big load resistors.

More critically, are you really expecting to demodulate the signal at the final stage which drives the loudspeaker (or earphone)? There are several reasons why this is unlikely to work, most importantly that a relatively large RF signal will be needed to drive a non-linear process in an output stage. Normally, detector stages work at very low currents, residual RF is smoothed away at the detector output, and a separate audio amplifier raises the level of the demodulated signal.

It is also undesirable to have the earphone live at RF. The speech coil has inductance, and fields radiated from the speaker and its wiring are liable to cause feedback. At the very least, a capacitor should be connected across the output, large enough to bypass the RF but small enough to et the audio through.

Have you simulated this?

#### ceidas

Joined Dec 26, 2011
50
No, i havent but according to you there must be a lot of stages in an AM amplifier. Then how all these schematics on the net do this particular job with 2-3 transistors?

I decided to leave my idea, for the moment, and use a tested solution, like this http://www.diy-electronic-projects.com/projects/78/2sradio.jpg But still nothing. I am using 2n3904 transistors and all I get are high pitch annoying sounds.

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#### bertus

Joined Apr 5, 2008
22,305

Joined Dec 26, 2010
2,148
No, i havent but according to you there must be a lot of stages in an AM amplifier. Then how all these schematics on the net do this particular job with 2-3 transistors?

I decided to leave my idea, for the moment, and use a tested solution, like this http://www.diy-electronic-projects.com/projects/78/2sradio.jpg But still nothing. I am using 2n3904 transistors and all I get are high pitch annoying sounds.
No, according to me a simple TRF radio could be made with one transistor. Greater gain can be obtained with more transistors, but quite good earphone results can be obtained with just one, especially if a "reflex" circuit is used so that the transistor amplifies first at radio frequencies, then following a diode demodulator the same transistor amplifies the audio. I built many simple radio circuits when I was a schoolboy, over forty years ago.

The main problem in your first circuit is that you do not have a stage set up with proper conditions for demodulation.

The second circuit has an input transistor working at a very small current. It may perform as a demodulator. Layout will be critical: ideally there should be just a tiny amount of positive feedback to improve the gain, but too much will cause oscillation.

As a result of the way the second transistor loads the output of the first, a Colpitts oscillator is produced. If oscillations are too much in evidence, try reducing the supply voltage, or perhaps increasing the 330 ohm resistor feeding the collector of the second transistor.

#### ceidas

Joined Dec 26, 2011
50
I replaced the 330 ohm resistor with a 2.2K potentiometer and no matter what I do high pitch annoying sounds is all i get.

edit: By the way, i see a lot of detectors implementing detection with diodes. How they do that, when a diode needs 0.7 to start passing significant current?

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Joined Dec 26, 2010
2,148
I replaced the 330 ohm resistor with a 2.2K potentiometer and no matter what I do high pitch annoying sounds is all i get.
I note that there is no capacitor shown across the power supply: there should be one, physically close to the resistor that was originally 330Ω. 0.1μF should do.
After that, you might try a lower supply voltage: 4.5V?

There are many other possibilities, from layout problems (physical arrangements of components) to wiring errors, faulty parts or parts having unsuitable characteristics.

Using a solder-less breadboard for this kind of circuit is a possible source of trouble. The capacitance between adjacent strips can easily be 10pF. If you could post a picture of your layout, it would help us to advise you.

#### ceidas

Joined Dec 26, 2011
50

Joined Dec 26, 2010
2,148
There are quite a few reasons why this might not work. Crystal radios (simple detectors without amplification) need to meet a few requirements, some of which are not met here. There are others that need to be checked.

1. A substantial antenna is needed, preferably about 30 metres / 100 feet of wire, as high as possible outside the building. The ferrite bar antenna will not pick up enough energy, unless you happen to be on the doorstep of a large AM transmitter.
2. A good ground connection is also required.
3. Low-impedance earphones are unsuitable. Sensitive high impedance headphones are needed. These were much more common in the past than nowadays, but old sets in working condition are not that hard to find. Alternatively, a piezoelectric crystal earpiece will work, in parallel with about a 47kohm resistor.
4. Especially for high-impedance magnetic phones, a capacitor is needed across them, perhaps about 47nF.
5. A germanium diode is probably best as the rectifier,but these are hard to obtain now. Small Schottky diodes are a more available substitute.
6. If all you have is a silicon transistor or PN diode, then biasing will help, but the bias must be applied in such a way that it does not place a heavy load on the input. Your circuit causes a 1kΩ load, which is not good. Bias can for instance be fed in fed in to the "ground" end of a coil. I can show a diagram of how to do this if you want.