# How to drive a transformer

#### MisterBill2

Joined Jan 23, 2018
9,802
Well, the impedance of most cables is in the range of 50 to a couple hundred ohms. And if I were making a cable identifier, I think would actually want a constant current source. You start with the worst case length and type of cable, and how much signal you need to be able to detect it at the far end, work backward from there. Also this seems like the sort of thing that would be better

Well most cables have impedances in the tens to hundreds of ohms. 50 to 150 ohms would probably cover a good bit of the cable out there.

As to signal strength, I guess it depends on how long the cable is and what you're using to pick it up at the far end. But I can't imagine needing even one watt. Personally, I think I would go with the current source which would self-limit if there was a short.

Also, this seems like the sort of thing that would be battery powered. If so then why would you need isolation? Isolation from what?
What sort of cables are those with such high characteristic impedance? I can see a cable with the pairs joined at the far end and then a magnetic probe stuck between the conductors inducing a voltage in a pickup coil. Really a good scheme. And with an isolated drive current it can detect damaged insulation as well. And we will not be matching impedance to 3 or 4 significant digits. So two transformer windings like I mentioned will work well.

#### Ian0

Joined Aug 7, 2020
3,756
Well, the impedance of most cables is in the range of 50 to a couple hundred ohms.
That's a LONG cable: 17mΩ/m for a 1mm^2 cable. Say it's a signal cable and it's 0.25mm^2. That's more than half a mile!

#### Ian0

Joined Aug 7, 2020
3,756
So it's a cable identifier. . . .
If it needs to be isolated, just run it from a battery as @transconductance suggested.
Why does it need to be a sinewave? Won't a squarewave do? Now it's a battery and a 555.

#### bug13

Joined Feb 13, 2012
1,996
So it's a cable identifier. . . .
If it needs to be isolated, just run it from a battery as @transconductance suggested.
Why does it need to be a sinewave? Won't a squarewave do? Now it's a battery and a 555.
Sine wave, isolation and 5W was the spec I was given.

#### bug13

Joined Feb 13, 2012
1,996
And with an isolated drive current it can detect damaged insulation as well.
I am interested to know how to detect damage insulation. That would be good if I can do that.

#### Ian0

Joined Aug 7, 2020
3,756
Sine wave, isolation and 5W was the spec I was given.
It's a poor engineer that doesn't question a ridiculous spec.

#### bug13

Joined Feb 13, 2012
1,996
It's a poor engineer that doesn't question a ridiculous spec.
You maybe right.
And also I understand that my job is to deliver what they asked for.

#### Ian0

Joined Aug 7, 2020
3,756
You maybe right.
And also I understand that my job is to deliver what they asked for.
What they asked for? Or what they actually want? !

#### bug13

Joined Feb 13, 2012
1,996
What they asked for? Or what they actually want? !
I’m on my phone so I didn’t want to re type it all. If you refer to my OP, I need to put about 5W sine wave into a cable which is shorted at the end over an isolated transformer.

I know how to drive 5W sine wave into a resistive load directly. I just don’t know much about transformer in this application. Eg Driving a transformer and the transformer need to delivery 5W into a resistive load.

I got a better understanding now after lots of replies.

#### MisterBill2

Joined Jan 23, 2018
9,802
It seems like this may be for identifying pairs in a telephone cable that may have many pairs. The old standard provides color codes for at least 600 pairs, which is a lot of wires.
Detecting damaged insulation would be by detecting a voltage between the pair and any other pair, or between the cable shield conductor and the pair being checked. That is part of the reason for isolation. If all of the other pairs are grounded then a voltage between the pair under test and ground would indicate a shorted circuit.
And how can somebody with very little information know that some requirement is not important??
And one more thing is that exact impedance matching is not the most important part of this application, while the isolation is rather important. AND harmonics from a square wave can couple capacitively much better than a good sine wave.

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#### transconductance

Joined Jun 29, 2019
45
That's a LONG cable: 17mΩ/m for a 1mm^2 cable. Say it's a signal cable and it's 0.25mm^2. That's more than half a mile!
I somehow missed that it was shorted at the far end. I was thinking of the characteristic impedance of a pair that's open at the far end.

#### transconductance

Joined Jun 29, 2019
45
I’m on my phone so I didn’t want to re type it all. If you refer to my OP, I need to put about 5W sine wave into a cable which is shorted at the end over an isolated transformer.

I know how to drive 5W sine wave into a resistive load directly. I just don’t know much about transformer in this application. Eg Driving a transformer and the transformer need to delivery 5W into a resistive load.

I got a better understanding now after lots of replies.
If the transformer has a high enough magnetizing inductance, then driving a low impedance resistive load through the transformer is no different than driving it directly.

#### transconductance

Joined Jun 29, 2019
45
I am interested to know how to detect damage insulation. That would be good if I can do that.
Some cable testers use time domain reflectometry to locate defects. This is definitely a couple of orders of magnitude more complicated than generating a sine wave. However, many modern ethernet PHY chips have at least some built-in TDR capability. You might be able to leverage one of those.

#### MisterBill2

Joined Jan 23, 2018
9,802
It is probably a challenge for TDR to work well with audio cables.