transformer coil inductance

poiureza · Post by **poiureza** » 08 Dec 2016, 13:02

I'm trying to simulate a pickup in front of a classic fuzz with a transformer according to the AMZ guide.

I'm a bit perplex about the inductance values :
From the data sheet, coil 1 (primary) has :
- 10K impedance @1kHz
- 480R DC resistance

Thus 10K = 480 + 2.pi.F.L (looking at amplitude values only)
Thus L = ~ 1.6Henry

But my inductance DMM gives me 4.5H across the lugs of the primary coil.
How comes ?

I can imagine some influence of mutual inductance due to the second coil but my estimate of that is about 0.1H.
Still pretty far from explaining the 2.9H difference.

Any idea ?

Primary :
2500 turns (0.06mm diam)
10K impedance @1kHz
480R DC resistance

Secondary :
400 turns (0.08 mm diam)
1K impedance @1kHz
125R DC resistance

Manfred · Post by **Manfred** » 08 Dec 2016, 17:38

The primary 10kOhms impedance means the transformed secondary impedance of 600Ohms by the the transfer ratio 3.771:1,
The read value of 4.5 Henry is the no-load inductance.
In that case the transformer acts as core type inductor.
In the load mode this inductance is in parallel with the load impedance.
The inductive reactance @1kHz has an impedance of about 28kOhms and has no longer an influenz,
because it is many times larger than the load impedance value of 600 Ohms.

Thus 10K = 480 + 2.pi.F.L (looking at amplitude values only)

By the way, the formula of the serial connection form inductor and resistance is a so called vector addition.
Z = sqrt [R^2 + (2*pi*L)^2]

XC-600134-205894.pdf: (63.5 KiB) Downloaded 36 times

Manfred · Post by **Manfred** » 09 Dec 2016, 06:29

Sorry, I forgot "f" in the formula,

Z = sqrt [R^2 + (2*pi*f*L)^2]

poiureza · Post by **poiureza** » 09 Dec 2016, 14:39

Thanks Manfred !

Yes, my formula was wrong

(late at night and not enough coffee).
Btw the secondary has a 1K impedance specified in the data sheet.
It's a 10:1 transformer (not a mouser part)

I still don't see how we go from 28K calculated primary coil impedance to the specified 10K primary coil impedance
(and 2.8K calculated to 1K specified for the secondary) but I don't want to bother you with this
(transfer ratio is 2500/400 = 6.25 btw)

I'm only going to use the primary in coil/core mode (secondary will remain open).
Eventually my real question is probably : what is my circuit going to see ?
4.5H inductance and 28K impedance ?

Manfred · Post by **Manfred** » 10 Dec 2016, 11:13

Sorry, I am busy in the moment, the reply takes a bit time,
the reply would be gave within the following few days.

Manfred · Post by **Manfred** » 14 Dec 2016, 22:46

Hi,

Primary :
2500 turns (0.06mm diam)
10K impedance @1kHz
480R DC resistance

Secondary :
400 turns (0.08 mm diam)
1K impedance @1kHz
125R DC resistance

It is not perfectly clear, if the primary impedance of 10kOhms should be the impedance the transformed secondary impedance of 1 kOhm,
or if each impedances of both sides were measured without any load.
The inductance of the secondary side must be Ls = Lp / (transfer ratio)^2 = 0,115mHenrys.
Please check that using your DMM.

You read the value of 4.5H, without any load.
As already mentioned, in that case, the transformer acts as core type inductor
an your circuit see this read inductance.

poiureza · Post by **poiureza** » 15 Dec 2016, 13:10

Manfred wrote: You read the value of 4.5H, without any load.
As already mentioned, in that case, the transformer acts as core type inductor
an your circuit see this read inductance.

Thank you very much.

My DMM reads ~0.35H for the secondary when primary is left open.

Btw the reading takes a lot of time to stabilize (like 10 seconds), almost as if fluxes needed to stabilize when the DMM injects probe voltages.
This is really strange because, from what I know, an open transformer coil is not supposed to interact with the other (loaded) one.
And my DMM works fine as it gave the correct reading within a split second when I checked a real (single coil) inductor from another project.

Manfred · Post by **Manfred** » 15 Dec 2016, 21:20

Hi,

got your transformer a continuous coil winding or a coil winding with tapping?

poiureza · Post by **poiureza** » 16 Dec 2016, 08:32

You mean a central leg for splitting ?
Yes, the secondary has that.

When I measure the inductance of these partial secondary coils I get ~0.15H for each of them (with primary open)

Manfred · Post by **Manfred** » 16 Dec 2016, 16:38

poiureza wrote:You mean a central leg for splitting ?
Yes, the secondary has that.

When I measure the inductance of these partial secondary coils I get ~0.15H for each of them (with primary open)

Yes, I meant that.
First of all, it expressed doubts about the transfer ratio, it is okay to me.

poiureza · Post by **poiureza** » 19 Dec 2016, 08:27

Shooooot you're right !

I checked the transformer specs again and I hadn't seen that 400 turns was for half the secondary.
The complete secondary has 2x400 turns

One last thing : Ls = Lp / (transfer ratio)^2 , is this formula valid when the primary is open (not used) ?

Manfred · Post by **Manfred** » 20 Dec 2016, 11:39

One last thing : Ls = Lp / (transfer ratio)^2 , is this formula valid when the primary is open (not used) ?

No, it isn't.

transformer coil inductance [SOLVED]