Impedance / Turns Ratio Question

[dorrisant]

I'm a bit confused here... not unusual, right?

I have an audio step down TX that I want to duplicate.

I get 1k DCR on the primary and 77 DCR on the secondary. Primary is marked with a dot and the primary is being used on the output side, so I call it a step-down TX.

If I put 10 VAC on the primary, I get 0.277 VAC on the secondary side.

My calculations say that it is a 36:1 turns ratio. Does that sound right?

Then looking at this data sheet from Xicon, I see that the 42TM017-RC looks to have the correct (same) resistance measurements at 1k and 80Ω.

The "Turns" column indicates 6.627:1. This doesn't jive with my above calculation, and if you divide 1k by 80 you get 12.5.

What am I missing here?

[sluckey]

There is no relationship between DCR and turns ratio.

[pdf64]

The winding resistances are somewhat immaterial here. That’s more of a power thing. You just need something with suitable turns ratio, and that can support the impedances you’re working with, ie sufficient winding inductances.

[dorrisant]

There is no relationship between DCR and turns ratio.

I have never used DCR for this calculation until I saw the spec on their sheet. I believe you... It is as relevant as DCR for pickup coils. Still, what am I missing?

[dorrisant]

The winding resistances are somewhat immaterial here. That’s more of a power thing. You just need something with suitable turns ratio, and that can support the impedances you’re working with, ie sufficient winding inductances.

How can a turns ratio be derived from their data sheet? Does their data make any sense to you?

[sluckey]

How can a turns ratio be derived from their data sheet? Does their data make any sense to you?

There's no need to derive anything. The turns ratio and impedance ratio are stated in the data. The also provided the DCR numbers just to mess with you.

[dorrisant]

So, you are in effect saying that none of these will work and that my assessment of a 36:1 is correct.

[pdf64]

Yup

[trobbins]

Are you saying the part you are testing is a 42TM017-RC for sure ?
If so, then did you apply and measure across the total primary and secondary - given they have centre-taps. Ie. do you get the same voltage ratio if you apply voltage to half a primary and measure across each half-secondary winding?

[sluckey]

So, you are in effect saying that none of these will work and that my assessment of a 36:1 is correct.

READ MY LIPS... I said what I said. No more, no less.

You are looking at the 42TM017-RC. It has a primary Z of 25K and a secondary Z of 600. You can calculate turns ratio from that info. Disregard the DCR data.

25000/600=41.666 This is the stated impedance ratio.

square root of 41.666 is 6.45 That is the calculated turns ratio. Pretty close to the stated 6.627:1 ratio.

[dorrisant]

Not testing a Xicon, was looking at the data sheet for a possible replacement... Nope!

I am trying to figure out what TX I would need at the end of a signal chain to connect to a mic preamp. I am looking to copy a circuit and I know that the output coupling TX is 36:1. Keep in mind that the TX side with more winds is connected to the XLR output, so it is a step up TX going from less winds as the input to more winds as the output. The output side shows an inductance of 13H and the input side of the same TX shows 291mH. Inductance measurements were taken with my Extech 380193 in Series Mode. From the size of this little guy it seems to be of the 200mW size, like the Zicons from above. I am trying to reverse engineer this one so I know what to clone it with. Sorry for any confusion. I should have been more clear from the outset.

I have this TX here on the bench, loose and ready for any type of measurement required.

Can anyone tell me what variable(s) to plug into a formula to figure this out. Can the impedance ratio be determined from the given info? I have worked with mic preamps in the past but have never been able to wrap my mind around the impedance ratio as opposed to the turns ratio.

Turns ratio is a relatively easy thing for me to pull from an unknown coupling TX but I want to know how to convert that to an impedance ratio so that I can select something currently available.

Sluckey, I understand, no worries and thanks for that. But I'm not sure what the output impedance should be for this device. Therefore I am not sure what impedance ratio to look for on any data sheet. Sorry if I'm dense sometimes, rest assured it isn't most of the time.

Is it true that if i knew the device's target impedance along with the turns ratio that I could derive the impedance ratio?

[sluckey]

Turns ratio is a relatively easy thing for me to pull from an unknown coupling TX but I want to know how to convert that to an impedance ratio so that I can select something currently available.

You were halfway there in your original post. You applied a test voltage and measured primary and secondary voltages. That gave you the voltage ratio which is the same as turns ratio. All that's left to do is get the square of the turns ratio, which is the impedance ratio. In your case 36 squared is 1296, so the impedance ratio is 1296:1.

Is it true that if i knew the device's target impedance along with the turns ratio that I could derive the impedance ratio?

All you need to determine the impedance ratio is the turns ratio. Now say that ten times.

[dorrisant]

I hear you...

Now what impedance should I look for as the output of this device?

10 x less than the input of the next device... ?

[bepone]

My calculations say that it is a 36:1 turns ratio. Does that sound right?

all ok

What am I missing here?

nothing , dont look in resistance, DCR is just telling about the wire gauge, the AC measurements counts here

[bepone]

How can a turns ratio be derived from their data sheet? Does their data make any sense to you?

nobody will put turns number in any datasheet, it is a "secret" for the transformer, specially for expensive output transformers.. everyone can copy then