# Wind high impedance transformer at home

Suppose I need to make transformer at home, but I am unable to wind 100k turns obviously, then can I just connect a resistor in series with a winding to achieve desired impedance? Will it have serious problems like bad frequency response? I understand that resistor would not behave like an inductive load, so I also thought maybe it is possible to connect inductor in series for impedance matching?
Or is there any other way to make high-impedance(~10kOhm) transformer winding at home? It would obviously require insane amount of hand-winding even to get to 1kOhm :(

UPD: See the comments, apparently I got the concept wrong.

• What do you want to do with the transformer? I mean, why do you want it to be high impedance? Also, what turns ratio do you plan to use? – mkeith Feb 20 '15 at 5:38
• It's basically isolation transformer(1:1 or 0.8:1, something like that) for use to connect line-out of PC soundcard to external amp. So frequency response I need is basically 20Hz - 20kHz. High impedance is needed because line out expects load around 10kOhm(can be little less, not much of a difference). – ScienceSamovar Feb 20 '15 at 5:52
• Do you have a drill? Or any power tools? A simple bobbin winder should be easy to make. – Connor Wolf Feb 20 '15 at 6:03
• Why do you think you need 100K turns? The reflected impedance on the primary side is the load impedance times the turns ratio squared. – John D Feb 20 '15 at 6:04
• Connor Wolf - it would be so much pain in the ass and in the end would be such an ugly thing(I need it to be pretty small, and 100k turns on small core would be weird), I would rather spend 20 bucks on ready made transformer. – ScienceSamovar Feb 20 '15 at 6:12

Just warning you, it's the most tedious, life-endingly boring task you could possibly imagine. I like to imagine that they had people wind Tesla Coils as punishment during the Inquisition.

www.digikey.com

Two days and you'll have a high-impedance transformer in your hands.

If not, buy an electric drill, get a bobbin, or a piece of cardboard with tape face-up on it, and enjoy the next day or so.

And yes, a resistor is not going to act like an inductor, but to the primary it will look like it if you put it on the secondary in line with your load, because you are essentially floating the other side independent of the primary. The current is proportional to the load and turn ratio, if you have the same ratio of turns, it's proportional to the load, just like anything.

On top of that, a resistor is unchanging with frequencies of our level, so it's wayyyy better.

It's the same reason you can turn a MOSFET on carrying 10,000 volts with a 15V floating signal. The reference is not 0, it's V. The reference for your primary's current output is not the primary itself, it's whatever is on the secondary.

A well done 10000R impedance coil is hell to wind, especially if you aren't operating at a high enough frequency to warrant it, but it's doable.

For example's sake, I have a 200~ turn coil at 100kHz and got

$$X_L = 2 \pi (100000)(0.025)$$

$$X_L = 15000$$

At your frequency, with my coil, you only get

$$X_L = 2 \pi (20000)(0.025)$$

$$X_L = 3141$$

So you'd have to wind three of mine, just about. So 600~ turns of 28(?) gauge copper wire for a radius of 25cm and a center hole of 5cm radius.

It's really kind of horrible to wind that much.

• I just wound 2 x ~600 turns of AWG #39 this morning before coffee. No big deal, I used a small (3") lathe. Now winding that many turns manually on a toroid would be a pain. – Spehro Pefhany Aug 7 '15 at 15:29

Are you able to purchase ready-made transformers or is this something that you must wind yourself?

The problems you are going to run into are: what kind of steel are you using for your core laminations and how do you keep track of the number of turns that you have wound on the bobbin.

Winding the bobbin is easy but it might be tedious. Most people use some form of motor-driven winder with a variable speed control. An electric drill will do in a pinch.

FWIW - most people use 600:600 transformers for audio isolation these days. Most modern audio equipment has output impedances somewhere between 22 Ohms through about 100 Ohms. 47 Ohms is very common.

These low output impedances will drive a 600 Ohm transformer very nicely.

• But this kind of 600:600 ohm transformer are usually for some basic stuff like voice transmission, and have frequency response up to 3-4kHz, so no use for good audio. There are some ready-made transformers that works well up to 20 or even 100kHz, but usually manufacturers ask big money for them, like 50-100 bucks. – ScienceSamovar Feb 20 '15 at 10:02
• I'm going to suggest that you visit my company's website www.trinity-electronics.com and have a look at the audio isolation transformers that we sell. Our customers include Television and Radio broadcasters as well as professional audio installation and rental companies. Although our transformers are mounted on PC boards with connectors, we are happy to sell you bare transformers at a lower price. Do be aware that it is really easy to destroy the transformers if you attempt to solder wire to the leads without a PCB to stop them from moving. – Dwayne Reid Feb 20 '15 at 14:56