Inductor tests

[aron]

Ever since I started this hobby, I was always interested in the black art of the inductor. Hardly anyone seems to know what value certain inductors were and from what I have been reading, two measurements seem to be the most important: inductance and resistance. Based on this, I purchased and tested a number of inductors from part numbers mined from the forum and some guesses I made when looking at the spec sheets.

Before you read any further, from what I have been reading, the Fasel inductors Dunlop sells are actually spec'ed pretty good. Although higher than the magical 500mH, they do hover between the 500mH-600mH mark with very low resistance. The yellow seems to have more resistance than the red, but not by much.

The 42TM024 just happens to hit right around the 435mH mark, but with high resistance. The 42TM022 is very versatile with 252mH, 800mH and 190mH in different combinations with relatively low resistance.

The 42TL024 comes in with 1.1H at the primary.

S = secondary
P = primary
o = outside lead
m = middle lead
R = ohms resistance


42TM024
S o+o = 435mH 160R
S o+m = 102mH 76R
Possibly good for midrange circuit??? Possible use in wah?

42TM022
S o+o = 252mH 46R
P o+o = 800mH 60R
P o+m = 190mH 32R
All kinds of uses?

42TL024
S o+o = 196mH
P o+m = 1.1H
http://www.diystompboxes.com/smfforum/index.php?topic=33725.0
Close to 1.5H but not quite.

42TL021-RC
S o+o = 161-187mH
S o+m = 36-43mH
P?? no reading
http://www.diystompboxes.com/smfforum/index.php?topic=22409.msg141895#msg141895

Fasel Red 580mH 15R
Fasel Yellow 540mH 23R

[aron]

hmmm wish I was good in math.

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

[d95err]

Thanks! This page goes to my local harddrive immediately! Inductors are dark and mysterious indeed...

[Paul Perry (Frostwave)]

I think if you have a transformer with two windings, if you put the windings in series but out of phase, the inductances subtact from each other and the resistance adds.
RG will tell me if I'm wrong

[aron]

Thanks Paul. I'm hoping we can do something with this info. I'd like to get a good midrange circuit out of this at the very least.

I need/must finish this pedal!!!!!

http://www.diystompboxes.com/amps/pignose150r/distsect150r.jpg

[aron]

Add this thread info to the tests that Jack has. Unfortunately he didn't list resistance of the inductor, but he does have inductance.

http://www.muzique.com/lab/tips.htm

[rockgardenlove]

I've got a Mouser 42TM013 that I was gonna use for a wah.  Smallbear sells them of course.  I also got an Eleca Halo replica, and I ordered one of your Fasels(yellow I think...it's the order to Portland, OR).  I got two of the Halo kits too from Smallbear.  And of course a stock Crybaby inductor.  I gotta get a meter that reads inductance though...any suggestions? 

Just remembered I have 3 of those 42TM022's I was gonna us for Octavia based stuff.  I'll be watching this thread...I'm giving this next week an inductor theme.     

[Paul Perry (Frostwave)]

I can't recommend a particular unit rockgarden, but I would be looking at a specialist LCR meter rather than a DMM with an inductance range. Because, I suspect that most (if not all) DMMs lump the resistance of the coil in with the inductance when it is measuring. Not so much a problem for a low resistance coil on a high permeability ferrite core, but for the Mouser transformer kind of coil, you will be way off.
Of course, for precision measuring, I suppose you would put a high accuracy cap across the coil & search with an audio signal generator to find resonance, then do the maths. After using a frequency meter to get the resonant frequency accurately...

[d95err]

I think if you have a transformer with two windings, if you put the windings in series but out of phase, the inductances subtact from each other and the resistance adds.
RG will tell me if I'm wrong

What if you use both windings in series and in phase? Does inductance and resistance add?

[R.G.]

Inductors do seem mysterious, partially because more than in any other kind of part, the imperfections and side effects show through. You can make a pretty perfect resistor if you're careful. You can make a modestly perfect cap with some effort. Mostly you can ignore the extraneous effects on both Rs and Cs in audio work. You can't with inductors.

An inductor is any conductor which forms a magnetic field - which is all of them. A circular magnetic field encircles every conductor. If you hold a wire in your right hand with your thumb lined up with the wire pointing in the direction that current flows, your fingers point in the direction in which the M-field circles the wire. M-field is proportional to current.

If you form the wire into a loop, then the M-field in the middle of the loop is all in the same direction, and reinforces. The M-field on the outside is all spread out and less intense. If you make many loops with a relatively common center area, the M-fields all add in the middle, and you can make an area of intense M-field.

As you build up M-field, you force energy into the M-field. It's stored there. When you stop forcing energy in, the M-field relaxes and the energy comes back out. Since it went in as a current, it comes back out as a current. This is "inductive kickback". The energy stored in the M-field is going to come back out - Mother Nature told it to act that way - and it does not much care what voltage it takes. So it will go to ANY voltage to make the current come back out. It happens that if you put a voltage across an inductor, that starts the current across the inductor increasing. The inductor impedes the change by the relation V = L* di/dt. In fact, this is one definition of inductance: L = V* dt/di, or inductance is the voltage divided by the rate of change of current.

You can measure inductances this way with a pulse inductance test. You put a pulse of voltage across an inductor and watch the current on a scope. The current starts at 0 and rises in a linear ramp. You can then compute di/dt directly from the scope trace, divide that into the voltage, and you have the inductance value.

When you put AC currents into an inductor, you're continously changing the M-field, and that takes work. So the inductor impedes the flow of AC current. This impedance is Xl = 2*pi*F*L big. The inductor has no impedance to DC, only its resistance. It takes a while to load up to a certain current, but when that current stops changing, the inductor M-field is constant, and it does not impede the DC flow, except by its resistance.

We use ferromagnetic cores in inductors because these materials concentrate M-field and make the volume of space they occupy much more M-field friendly. For the same turns of wire, filling the loops with iron may increase the inductance of the same wire coil by a factor of several thousand. Obviously, we ...like... iron cored inductors. They're much smaller and cheaper than the huge coils of wire we'd otherwise have to use.

But Mother Nature told us that there is no such thing as a free lunch (that's the Second Law of Thermodynamics, loosely translated). The iron materials which increase inductance bring a whole set of quirks and odd imperfections of their own. They saturate (there's a limit to the field strength they can take) and they distort (hysteresis losses in the material have this effect) as well as heating up (due to eddy current losses).

So what do we make of this mess?

(1) Don't use inductors unless you have to. This is the solution adopted by the industry. Lots of brainpower has gone into how to not use an inductor.
(2) If you have to, you need to know that the signal you will use in the inductor does not step over the bounds of what the inductor can do. That is, it must not saturate the inductor or burn it up by overheating the resistance or the eddy current losses in the core.
(3) For us amateurs, look for inductors which have been made cheaply for something else that we can pervert to the use.

For lots of us, that's a small tranformer, which is where we started this thread.

Paul, you're mostly correct. If I have a wound inductor core with two windings, I can measure the inductance on each winding and get inductances I'll call L1 and L2. These are proportional to the number of turns squared, that is L1 = Kc*N1*N1, where Kc is the core constant that makes the numbers come out right. L2 = Kc*N2*N2.

If we hook N1 and N2 in series aiding, then the total turns is N1+N2, so Ltotal = Kc*(N1+N2)*(N1+N2), or Kc*(N1^2+N2^2+2N1*N2) So you actually get a bit more than L1+L2. If you hook N1 and N2 in series opposing, then the resulting Lt  is Kc*(N1-N2)*(N1-N2) = Kc*(N1^2+N2^2 - 2*N1*N2). So the inductances don't strictly add and subtract. But they do get larger and smaller in the directions you propose. You get two different inductances in the direction you expect, but it's not a direct add and subtract of the inductances, as a result of inductance being proportional to the square of the turns.

For measuring inductors, the easiest for me has always been a pulse inductance test, but I have oscilloscopes. It may be easier for you to measure if you put a known frequency into a series resistor/inductor and measure the voltages across the resistor and inductor with an AC meter. The voltages are proportional to the impedance of each, so you can compute the impedance of the inductor by the voltage divider rule and from that L = Xl/2*pi*F.

[Paul Perry (Frostwave)]

Thanks for clearing that up, RG!

[alfafalfa]

But why does my inductor not work the way I want in this wah ??

Some time ago I experimented with a diy inductor, I had these big cores ( 1.2 inch in diameter ) and used  0.20 wire (in cm) . I made it exactly 550 mH ( according to my capacity meter).
It sounds good but when I use it with distortion I want it to amplify a certain frequencyband so I can get my guitar to feedback with it. The problem is I can't seem to get a smooth enough frequency range to make this happen.
What should I do ?

[alfafalfa]

Here another picture, couldn't  paste it in the same message.

http://i113.photobucket.com/albums/n233/alfafalfa/wah1eigeninductor.jpg

Alf

[Paul Marossy]

It sounds good but when I use it with distortion I want it to amplify a certain frequencyband so I can get my guitar to feedback with it. The problem is I can't seem to get a smooth enough frequency range to make this happen.
What should I do ?

It sounds like your Q is kind of low. Do you have a resistor in parallel with the inductor? If so, try removing it and see what effect that has.

[R.G.]

But why does my inductor not work the way I want in this wah ??

The short answer that's not very helpful is that an inductor always works the way Mother Nature's rules say it will work, not the way you want it to. But you knew that.

Some time ago I experimented with a diy inductor, I had these big cores ( 1.2 inch in diameter ) and used  0.20 wire (in cm) . I made it exactly 550 mH ( according to my capacity meter).

You have a capacity meter that measures inductance too?
I know they exist, but they are usually quite expensive. Maybe inductor meters have gotten cheap while I wasn't looking.

It sounds good but when I use it with distortion I want it to amplify a certain frequencyband so I can get my guitar to feedback with it. The problem is I can't seem to get a smooth enough frequency range to make this happen. What should I do ?

When you say "smooth enough frequency range" what do you mean?

By the way, it's not good to use steel bolts through inductors to hold them together. You want plastic bolts to avoid the steel putting its own little magnetic spin on things. The steel through your inductor is about a 5X better "conductor" of M-field than the ceramic ferrite of the rest of the core. I suspect that the inductance may change when you change out the bolt.

As to possible reasons for the oddities, Paul's got one of them - Q factor. "Q" is the "quality factor", the ratio of inductance to resistance. If the Q is too low, you can't ever make a sharply resonant filter out of it because the resistance eats up the resonant energy. Series resistance or parallel resistance both damp the inductor.

It could also be things like self capacitance, although that's probably not what is happening in this one.

[alfafalfa]

Okay RG you have a good point there.
I will remove the steel bolt first , see what happens.
The wah has a Q factor trimpot but it doesn't work very well  either. So I will check the schematic again.
My meter can measure capacity but also inductance  and temperature among other nice features.
I bought this meter because it can measure the picofarad range.
My Fluke can't .

http://i113.photobucket.com/albums/n233/alfafalfa/capmeter.jpg

Alf

[Paul Marossy]

I really, really, really, really, really, really need to get something that measures indcutance.

[jonathan perez]

I really, really, really, really, really, really need to get something that measures indcutance.

is there a specific one were all talking about, here?

[puretube]

start with a 555...

( & search: "grid-dip")

[aron]

is there a specific one were all talking about, here?

Did anyone check the DIY FAQ 

Uh, what is the resistance of that hand-wound inductor?