experiments with inductors wound on resistors

[5v333]

sitting at home with the C flu and was experimenting with RL isolators in circuit and have made some RL isolators from winding copper wire around resistors.
partly for isolating different ground points with good results.

i made two inductors from two different kind of resistors and measured them with a resonating LC setup. I kept the resistor unhooked so i could measure the inductance by it self.

R1 = vishay CMF/RN/RL 2-3W 470R
R2 = 2-3W green unknown filmresistor 1.3K

after i calculated the inductance i then calculated the AL of both and they varied a lot.

the brown vishay resistor had an AL of about 26
while the green resistor had an AL of about 2.9

the vishay is the one to use, atleast for me.

over and out.

 

[efinque]

2-3W resistor sounds very large.. is this a power amplifier Thiele network by any chance?

Anyway, I don't have a clue of inductance values, there's a formulae for wire thickness, coil diameter and number of turns where in general when one is made smaller the other has to be increased. I remember from a FM radio build that you could "tune" the rx bandwidth wider or narrower by adjusting the coil length.

Interesting subject as I read about inductors in EQs and amplifiers, I was thinking of equipping my DJ mixer build with a similar parallel RL circuit to improve the frequency response etc.. although it already has output transformers, it has a rather large summing bus which could house a per-channel Thiele network.

I think the problem is finding suitable sized inductors (here the best bet is to coil my own as matched pairs) and dealing with the RFI, magnetic fluctuations etc, maybe put them in shrink tube or something.

Iirc I used enamelled copper wire but I can't remember if it makes a difference whether you sand it or not (with very small currents in a moderately sized RF coil it could I think as it's technically an insulator, not just a very good one in the amounts usually found in copper wires)

EDIT : I think when using enamelled copper wire (ie. inductors) you may benefit from sanding at least the ends or junctions to avoid bad connections, cold solder joints etc.

 

[abbey road d enfer]

sitting at home with the C flu and was experimenting with RL isolators in circuit and have made some RL isolators from winding copper wire around resistors.
partly for isolating different ground points with good results.

i made two inductors from two different kind of resistors and measured them with a resonating LC setup. I kept the resistor unhooked so i could measure the inductance by it self.

R1 = vishay CMF/RN/RL 2-3W 470R
R2 = 2-3W green unknown filmresistor 1.3K

after i calculated the inductance i then calculated the AL of both and they varied a lot.

the brown vishay resistor had an AL of about 26
while the green resistor had an AL of about 2.9

the vishay is the one to use, atleast for me.

over and out.

That's weird. Assuming similar diameter and length and same number of turns, tehre should not be this huge difference. Unless one of the resistors is wound on a magnetic core, which I doubt, or is a wire-wound type using magnetic wire, which I also doubt. As far as I can see, these two resistors are not so different in construction and size.
What inductance have you measured and how? You say "with a resonating LC setup"; can you describe the setup and results?

Winding inductors on resistors, typically for amplifier stabilizing purpose, is a convenient way of producing a low-Q inductor.

 

[5v333]

one had 25 turns and the other about 35.

i put a 1K resistor in series with the inductor and a cap in paralell with the inductor.
signal generator to the resistor and ground under the C//L.
probed in the middle with a scope.
sweeping and looking for te first resonance.
890K in one case, 1.2M in the other.

F = 1/ (2π√(LC))
L = 1/(C(2πF)^2)

i then calculated AL on an internet calculator.


hmmm... maybe a dissection of the green resistor is about to happen..


ive read on some forums about some resistors are magnetic and that the vishay resistors are more or less non magnetic. end caps etc...
could that have something to do with it?

my understanding of AL is limited, i just know that you need to know it when you roll your own stuff.


my aim is around 3-4uH.
its for RF controll in the NYD EQ.

 

[Audio1Man]

When you add a core to the coil the inductance increases. The Vishay is a ceramic core with end caps. The lead wire is welded to the cap.  Break open the Vishay and look @ the construction. Try using a 2W carbon resistor as the coil form (no plated steel, check with a magnetic).
Duke

 

[5v333]

i think i have some carb comp laying around. more of that tomorrow if i have the strength.

the vishay was not CMF/RN/RL but a CPF. its more or less the same...

the green resistor is prob a ceramic resistor. and might be non-inductive...

Just saw in the datasheets of RN/RL and they have a spec non-magnetic version. last three digits tells if it is .

 

[efinque]

Resistors in practice do have inductive/capacitive properties, depending on the manufacturing process (I was just reading about this the other day, another thing is thermal noise, physically small resistors generate more heat with large currents which may manifest itself as noise etc)

Afaik carbon film resistors are the most widely used these days.. basically there's a thin conductive helix inside the component around an insulating core. They're often light-yellow/brownish.

Then there are metal film resistors which seem to be disliked by some, they have a metal coil (=film) wound around an insulating core. They're usually blue.

Carbon composition resistors which are basically containers filled with carbon are in my experience very rare these days but still used in some vintage equipment although I found them marketed for applications requiring a 50's guitar tone. I've seen some inside old radios etc, they're usually dark red/brownish.

Then there are metal oxide resistors but I don't recall seeing one in my life, maybe I have but I never paid attention to it. They're blue too like the metal film resistors, only larger I guess (the ones I found were rated at 2W)

Ceramic wire-wound resistors (I tend to classify them just as power resistors which usually have heatsinks though) are similar in construction to the carbon/metal film resistors but they use wire instead.

Many manufacturers do bulk runs and afterwards sort the components within specified tolerances etc so there is a margin of error, from what you described 470R vs 1,3K shouldn't make that much of a difference.. what is AL anyway, audio level?

And RF control.. in an EQ?

 

[5v333]

I found the formula for AL (inductance factor).
Doing the math myself, i get almost the same value for both cores (resistors).
So maybe i didnt pay attention or the calulator was acting funny..
AL is about 2.7-2.9

 

[abbey road d enfer]

I found the formula for AL (inductance factor).
Doing the math myself, i get almost the same value for both cores (resistors).
So maybe i didnt pay attention or the calulator was acting funny..
AL is about 2.7-2.9

That's what I thought. It didn't make sense otherwise.