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Why use Teflon wire in HF power toroid ?

Discussion in 'Homebrew and Kit Projects' started by AA7EJ, Sep 15, 2012.

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  1. AA7EJ

    AA7EJ Ham Member QRZ Page

    I am gathering info and material for my home-brew ATU. The design I am using recommends to use Teflon wire in the autotransformer toroid converting the 50 ohms to 12.5 ohms.
    It uses 14 gauge wire for “high power 1kW” coupler.
    I am going for 100 to 250 W.
    I am trying to find out what would be the advantage of using Teflon, especially when from experience I know it is not that easy to work with and string thru even 2 inch toroid and is relatively hard to find in small quantity.
    The electrical properties may have something to do with it, but I am really not in favor to use “an exotic” part just because it has .01 db less loss than using other insulation.

    Any to the point commentaries are as always appreciated and welcome.
  2. AB9LZ

    AB9LZ Ham Member QRZ Page

    The wire provided in my elecraft BL1 balun kit ( for winding the balun) was plain old doorbell wire. "Carol 20awg CL2 sunlight resistent" to be specific. I just pulled it apart to re use the core in another project.

    73 m/4
  3. W9GB

    W9GB Ham Member QRZ Page

    TEFLON (PTFE) insulation or sleeping is often used in BAL-UN and UN-UN construction for legal power limits (and HV) --- and addresses the Clueless Ham issues (destroying cores with excessive power and mismatches that produce HV points).

    I understand the difficulty in finding small qty. of PTFE wire for balun winding.

    I took a "hint" from Bob at Balun Designs.
    He uses Teflon (PTFE) sleeving, specifically --
    Bob constructs his windings with Thermaleze wire inserted in Teflon tubes (PTFE sleeving)
    which provides a breakdown voltage of 10,000 volts.

    Teflon sleeving is easier to acquire from a number of sources.

    Mark, AB9LZ is correct that you can used common copper wire (insulation typically rated to 300 V) for QRP power levels and for the Elecraft BL1 kit, up to 150 watts RF.
    Last edited: Sep 16, 2012
  4. G4COE

    G4COE Ham Member QRZ Page

    The real reason silver plated copper conductors is used in Teflon coated insulation is that teflon requires a high temperature to extrude, the temperature being that high copper would instantly oxidised.... and we should know the advantage of silver plating.

  5. G4LNA

    G4LNA Ham Member QRZ Page

    Newark seems to have plenty. I buy mine from its sister site Farnell Electronic Components in the UK.
  6. G4LNA

    G4LNA Ham Member QRZ Page

    Or, if you want the stranded stuff from Newark, scroll down the page a bit.
  7. K8ERV

    K8ERV Ham Member QRZ Page

    I didn't know TFE insulation was used for sleeping. I'm gonna try that.

    TOM K8ERV Montrose Colo
  8. AA7EJ

    AA7EJ Ham Member QRZ Page

    Thanks for all replies, I do appreciate most of it.
    I am still struggling with QRZ discussions which turns into jokes, sermons or otherwise highjack the thread. Despite being very opinionated I still believe in freedom of speech and opinion, so be it, I live with it.

    The coupler / tuner I am building was published long time ago and is basically LC filter with autotransformer ( 50 to 12.5 Ohms) at the input, not a balun. The design goal was to couple TX to short ( whip type) antennas with minimum adjustments.
    The design calls for using twisted pair and Teflon tape to cover the toroids. It uses stack of toroids. ( Increase power handling capacity of the transformer?) One of the design requirements is to maintain physical positions between the wires to maintain desired characteristic impedance. Since the characteristic impedance depends, besides wire dimensions, on distance between the conductors, the insulation thickness is important.

    BTW. I did try to twist unknown (Aircraft?) stranded wire of 22 gauge and managed to break the outer insulation so the inside brownish tape poked thru.
    After much search I found scrap of 22 gauge “appliance wire “ with TFE insulation. Will try it next.

    As far as what wire size – I need to do some search on how to calculate voltage and current on the output of 100 W TX at resonance, or just put a scope on dummy load with known PA setting on my Ft 857.

    Or maybe there is something like that on the web HI HI HI .

    But I need a refresher course on transformers in general , anyway.
    And not on the type my grand kids play with.

    Cheers Vaclav
  9. AD6KA

    AD6KA Ham Member QRZ Page

    For Teflon insulated wire, I go to Apex Electronics.
    It's a BIG surplus place about 30 minutes from
    my QTH. They have pallet racks piled with spools
    ofany size & color of Teflon and almost any other
    wire you want.
    Price is $5.00 per pound plus shipping.

    Their stock is NOT all in their online catalog,
    so give them a call or email with your needs.

    If you are ever in the area, drop in, it's a FUN
    place to poke around in. Don't tell the XYL
    "I'll only be there 20 minutes" because I guarantee
    you will get distracted by all the cool stuff they have there.
    73, Ken AD6KA
  10. K7MEM

    K7MEM Ham Member QRZ Page


    Just a little warning on working with Teflon insulated wire. Teflon insulation is very heat resistant and doesn't burn or disfigure when it is being soldered. But Teflon is not resistant to pressure. Too much pressure during winding or bending will cause the Teflon insulation to be displaced (cold flow).

    Years ago, we used Teflon insulated wire for the power to a rack of PC/hand wired boards. The wire was used to connect the +10 volt power and grounds. Each cable cable bundle consisted of 10 - #10 Teflon insulated wires. At one end they were grouped into a very large solder lug and at the other they were distributed across the card rack to the board connectors. We had about 24 of these racks and when QA inspected the harnesses, they found that all of them had cracked and had to be replaced. That kept me busy on a third shift for several weeks.

    We also used a #30 Teflon insulated wire for some board building. The wires were generally routed from chip to chip in the gaps between the chips. To keep everything neat and secure, the wire was bundled at intersections, and during long runs, using waxed nylon lacing string. If you made the tie up too tight, you could easily introduce shorts that were very difficult to find.

    Just take care when using Teflon insulation.

    Martin - K7MEM
  11. G4COE

    G4COE Ham Member QRZ Page

    Teflon ear plugs - used in ears when er' indoors starts to snore!

  12. AA7EJ

    AA7EJ Ham Member QRZ Page

    I use #30 wire for many tasks when building logic circuits- mainly wire wrap. I do not have the original packing , but I think it is Kynar brand. Some I got in RS years ago. I have not tried to solder it recently, so I really do not remember if it melts using 25 W iron. Is Kynar Teflon insulated? Just curious.
    Thanks for the comment on cold flow. I just twisted a 22 pair of TFE wires and wound it on the 2 inch toroid but it keeps unraveling ( Teflon is slippery!) so I was going to overtwist it a little. Now I will watch for this cold flow, wound't want to start my tests with shorted turns.

    73 Vaclav
  13. WA9SVD

    WA9SVD Ham Member QRZ Page

    When you "buy" a piece of equipment, such as a tuner, you learn nothing more than how to "twirl the knobs," if even that. (Plenty of hams can't even do that all that well.)
    If you build something, such as a tuner, you KNOW how the parts are assembled (did that capacitor lead REALLY get properly soldered?) and can trouble shoot problems, with luck. The experience of assembling and wiring your own equipment, is, to quote a famous (or infamous) credit card company, "PRICELESS." With prudent shopping, you CAN usually still build for less than buy.

    I'd rather trouble shoot a tuner I built myself than try to put a MFJ tuner (purchased new) into working order because of poor QC, when design and packaging of the design means you have to remove a half dozen components just to reach the part of the circuit in question.
  14. K7MEM

    K7MEM Ham Member QRZ Page


    You are correct. The #30 wire used for wire wrap does not have Teflon insulation. Wire wrap posts are square with rather sharp edges, making it unsuitable for anything with Teflon insulation. The Kynar insulation's don't usually melt but the often show a burnt edge.

    And, yes, the smaller gauge TFE wires will have some spring to them. That is partially due to the insulation but it might also be due to the wire. You could possibly hold your windings in place by wrapping them in tape as you wind them. Personally, I would go with a wire that holds it's twist.

    For most of my current work, I use enameled wire. Enameled wire is usually soft and will hole it's twist, but there are other precautions to consider. Using it with cores, I usually wrap the core with tape before I do any winding. I just had a lightening strike that blew up the 80 Meter resonator on my 5BTV and melted the variable capacitors in my tuner. I rescued the cores from the tuner but rewound them with enameled wire. It appears that the cores survived the hit because my rebuilt tuner seems to be working fine.

    The #30 gauge Teflon coated wire that I use to work with was specific to the application. We had 12" x 12" hand wired boards that had to have a thickness that didn't exceed 1/2". That meant wire wrap was out so we used a technique know as "stitch welding". The boards were drilled for standard DIP ICs, but the pad next to each pin was not drilled. The pad was nickle plated to allow the wire to be welded to the pad. We used a machine to do wiring which looked a lot like a sewing machine.

    It sounds a little odd, but as long as you kept the welder adjusted correctly, it was very reliable and fast. We could wire a board with 160 ICs in a single 8 hour shift. We often off loaded the work to the production women who could always beat us, hands down. It was fast because we always first generated a "wire list". The "wire list" was then punched on cards and processed by a computer. The computer would pick out inconsistencies in the "wire list". We would then wire from the "wire list". A net on the "wire list" was always at least 2 nodes but some were much more. But you only cut the wire at the end of net and you never had to strip wires. The welder used the "cold flow" characteristic of Teflon wire to push through the insulation. Not needing to strip wires really sped up the process.

    Good luck with your project.
  15. WA9SVD

    WA9SVD Ham Member QRZ Page

    Just don't be in Apex when we have an earthquake... it would probably take an expedition a month to dig you out!:rolleyes:

    But PTFE ("Teflon") insulated wire is used because of a low amount of expansion with heat, which maintains insulation better, it has a much higher breakdown voltage than normal plastics (even QRP rigs can generate fairly high voltages in the toroid windings with a severe mismatch) and it's abrasion resistant.
  16. AA7EJ

    AA7EJ Ham Member QRZ Page

    The "appliance wire" - that is on the spool, is rated 600V max , I think that will do.

    OK. I " re twisted " the pair and now thinking out loud how to convince it not to rewind when it gets loose from the wise and the drill.
    Two options – superglue or hot glue.
    Superglue should wick in some and if I wound the toroid before it is completely dry it may work.
    On the other hand hot glue remains somewhat flexible for a while, but it will probably goop up the pair
    Either method will be messy, I am tempted to try the old way – using beeswax. At least it would smell better.
    I have heat shrink tube but it is not transparent, I like to see how it looks.
    Maybe I better get one.

    Any other suggestions?

    OT I have bag of polystyrene caps somewhere, but I do not remember how much voltage they will take,
    I think I'll need something around at least 2KV for the switchable bank of capacitors.
  17. AA7EJ

    AA7EJ Ham Member QRZ Page

    years ago I worked for Stromberg_Carlson of old time radio fame. They build telephone switches and the circuit boards were plugged into "back frame" which was wired wire-wrap style by what we affectionately called "Garden-Denver"machine. It was a show stopper on any factory tour!

    It was fascinating to watch it to wire about 3 feet by 1 foot mess of green wires.

    Made a good living traveling the country and fixing known problems where the SAME wire got cut by the sharp post because of too much tension was put on the "run". HI HI HI

    Then they started making PCB backboards and that had two effects on the industry.
    PCB was harder and expensive to modify or fix in the field and the engineers had to work harder too - not to make mistakes.
    Nowadays PCB software ELIMINATED layout engineers totally. Progress.
  18. N0SYA

    N0SYA Ham Member QRZ Page

    Why use Teflon wire in HF power toroid ?

    An transmatch that used a toroid with several turns zapped over. The inductor was ruined till the wire was replaced with teflon covered wire, no more arcs.
  19. WA9SVD

    WA9SVD Ham Member QRZ Page

    600 Volts insulation may be sufficient, but again, even QRP operations can generate high voltages with excessive SWR.
    I wouldn't use glue (especially NOT "super glue") in case you ever want to modify the toroid. You should be able to hold partial windings in place with strips of electrical tape, which can be removed after the winding is completed.
  20. KM1H

    KM1H Ham Member QRZ Page

    I certainly would not use 300V wire for anything over QRP and especially at 100W or more. One only has to read about those $100 5KW baluns that are destroyed at 1200W with what most would consider acceptable VSWR.

    Teflon #18 or 20 is fine at 100W but Id also suggest wrapping the core with Scotch 27 or similar HV tape.

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