Hi all,

Even though I'm an electrical engineer, I've been struggling with the following: for a microphone isolator with a transformer I've been trying to come with a way to create a 1:1 transformer, but with a different input and output impedance. Now, if I'm not mistaking (it's been a while since I had transformers in school), the impedance is directly influenced by the amount of turns on the primary and the secondary coil of the transformer. I'm therefore kind of lost on how to get different impedances while maintaining the 1:1 ratio.
According to the datasheet of several audio transformer manufacturers, they are able to create a 1:1 transformer, but with an input impedance of let's say 1 kOhm, and an output impedance of 250 Ohms. Does anyone know how this is even possible?
Cheers!

 

I am not sure it is even proper to talk about the absolute impedance of the primary and secondary, only the ratio and that is the square of the ratio of the turns. The actual impedance comes from the source. And the load should match the source impedance times the ratio for max power.

But I am no expert on this, others may have better info.

 

I do not believe it is possible to have a 1:1 transformer with different impedances on the primary and secondary.

 

I am not sure it is even proper to talk about the absolute impedance of the primary and secondary, only the ratio and that is the square of the ratio of the turns. The actual impedance comes from the source. And the load should match the source impedance times the ratio for max power.

But I am no expert on this, others may have better info.

Thank you for your reply, I will look into that.
Cheers!

 

See answer #2 to this question on stack exchange

Primary/Secondary Transformer Impedance, and Impedance Matching - Electrical Engineering Stack Exchange

 

I do not believe it is possible to have a 1:1 transformer with different impedances on the primary and secondary.

That's what I was thinking as well. However, if I check the datasheet of, let's say the JT-MB-CPC (https://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-mb-cpc.pdf), it specifies 2 different impedances with a 1:1 ratio. Or could those impedances be the result of the testing/measurement circuit?
Cheers!

 

See answer #2 to this question on stack exchange

Primary/Secondary Transformer Impedance, and Impedance Matching - Electrical Engineering Stack Exchange

That looks VERY promising. Thanks!

 

That's what I was thinking as well. However, if I check the datasheet of, let's say the JT-MB-CPC (https://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-mb-cpc.pdf), it specifies 2 different impedances with a 1:1 ratio. Or could those impedances be the result of the testing/measurement circuit?
Cheers!

Yes, well not everything is the same on both sides. I'm not an expert on transformer construction, but the datasheet does not imply that the impedances are the same on both primary and secondary and in particular the capacitances from winding to shield and case are different. You also need to remember that impedance is frequency dependent and is far from constant across the audio spectrum. Lastly, impedance matching is done for maximum power transfer. That is not what is happening here so the fact that they are not the same is less important than you think it is.

 

Normally when the impedance of a transformer is specified, it refers to the input and/or output impedances it is designed to work with, not the actual impedance of the transformer's inductance and stray capacitance.

Thus an audio interstage stage transformer with a rated 600Ω impedance is designed to work with a 600Ω load.
It has little directly to do with the transformer's measured impedance which is much different.

 

If a transformer has a 1:1 ratio then the impedance looking into the secondary will be the same as the impedance connected to the primary.
The primary inductance will determine the frequency response at the low-frequency end: the leakage inductance and self-capacitance will determine the frequency response at the high-frequency end.
If you have a transformer with a 1:2 ratio then the impedance looking into the secondary will be four times the impedance connected to the primary.

 

Yes, well not everything is the same on both sides. I'm not an expert on transformer construction, but the datasheet does not imply that the impedances are the same on both primary and secondary and in particular the capacitances from winding to shield and case are different. You also need to remember that impedance is frequency dependent and is far from constant across the audio spectrum. Lastly, impedance matching is done for maximum power transfer. That is not what is happening here so the fact that they are not the same is less important than you think it is.

The datasheet implies that the impedances are not the same at all at 1 kHz. But I will take a closer look at how to get the maximum power transfer. Thanks for your replies!

 

Normally when the impedance of a transformer is specified, it refers to the input and/or output impedances it is designed to work with, not the actual impedance of the transformer's inductance and stray capacitance.

Thus an audio interstage stage transformer with a rated 600Ω impedance is designed to work with a 600Ω load.
It has little directly to do with the transformer's measured impedance which is much different.

Hmmm, that would make sense, even though it's a bit confusing. Thanks!

 

If a transformer has a 1:1 ratio then the impedance looking into the secondary will be the same as the impedance connected to the primary.
The primary inductance will determine the frequency response at the low-frequency end: the leakage inductance and self-capacitance will determine the frequency response at the high-frequency end.
If you have a transformer with a 1:2 ratio then the impedance looking into the secondary will be four times the impedance connected to the primary.

Exactly, which is why I've been kind of confused about some datasheets.
Thank you for your reply!

 

Exactly, which is why I've been kind of confused about some datasheets.
Thank you for your reply!

A transformer that claims to be a 600Ω transformer with a 20Hz low-frequency cutoff, is no different to a transformer that claims to be 1.2k with a 40Hz low-frequency cutoff; but what happens at the high-frequency cut-off is rather less predictable.
(This applies to signal transformers. When the voltage time constant gets high enough to reach saturation it's a different ballgame)

 

It has little directly to do with the transformer's measured impedance which is much different.

Yes, I think that is the source of the confusion. There are two different impedances one might talk about.

 

A transformer reflects impedance on one side to the other by a Square of the Turns Ratio.
If have a 1:2 transformer with 75 Ohms on the "1" side, the "2" side will offer 75 * (2^2) = 300 Ohms.

 

A transformer reflects impedance on one side to the other by a Square of the Turns Ratio.
If have a 1:2 transformer with 75 Ohms on the "1" side, the "2" side will offer 75 * (2^2) = 300 Ohms.

Yeah, that is what I understood as well. But the information from the datasheet was conflicting with this property. Or at least to me it seemed to be conflicting. Anyway, I have some reading material for the upcoming week. Thanks for responding.

 

But the information from the datasheet was conflicting with this property.

It says that the input impedance looking into the primary with a 1k load on the secondary is 1k. That's correct.
It says the output impedance looking into the secondary (with the 1k resistor disconnected) is 250Ω. The secondary contributes its own resistance of 50Ω, the primary adds another 50Ω and the source contributes 150Ω.

 

It says that the input impedance looking into the primary with a 1k load on the secondary is 1k. That's correct.
It says the output impedance looking into the secondary (with the 1k resistor disconnected) is 250Ω. The secondary contributes its own resistance of 50Ω, the primary adds another 50Ω and the source contributes 150Ω.

Indeed. Thank you very much for breaking it down for me.

 

Indeed. Thank you very much for breaking it down for me.

No, I misread that. The input impedance looking into the primary side with 1k load on the secondary is 1.0k to 1.15k.
That's 1k of the resistor, 50Ω from the primary resistance and 50Ω from the secondary resistance making 1.1k in total.