[Elecraft] Neurotic about Toroids

[Tom Hammond]

Hi Lee:

At 03:36 AM 10/23/2005, you wrote:
>I am still considering what my *grown up* rig will eventually be after 
>I've outlived my *first* rig.  I always seem to go back to the K2 and that 
>is where my head and heart are.  But the devil keeps popping up, 
>whispering in my ear that I'll never be able to strip and mount so many 
>toroid coils (and that I'll probably zap an IC).
>
>My main worry is with the stripping process.  The recommended method is the
>melted solder thing where you tin and strip at the same time.  But how do 
>you know you have not tinned the enamel, since the whole thing is covered 
>in solder?

Good question... easy answer... it's kinda like trying to 'wet down' an 
oily plate... the water refuses to stick to the plate because of the oil. 
Same for enameled wire... the enamel will not allow the solder to adhere to 
the copper beneath it! The solder just rolls off.

Tinning a wire which is covered with HEAT-STRIPPABLE ENAMEL is not a 
difficult task... though it does take a bit of time... heat... and possibly 
a good technique.

There are (generally) two types of insulating enamel... heat-strippable, 
and NON-heat-strippable (often referred to as FORMVAR-insulated wire).

Formvar is in insulation VARnish which is very(!) resistant to heat and 
which you will generally have to scrape off the wire before it can be 
tinned. In general, most Formvar insulations are dark (often reddish-)brown 
in color.

On the other hand, heat-strippable insulations are generally much lighter 
in color (golden or a clear red or green), and they will melt (and turn 
black), offering up the bare copper beneath the insulation, once the right 
amount of heat has been applied to them. From experience only, it appears 
that the 'right amount of heat' must be in the range of 700 deg to 800 deg 
F (370 deg to 425 deg C).

You can remove the insulation in any of a number of ways. I'll discuss a 
few below:

1) SCRAPING it off with a (semi-)sharp blade. Hold the blade
    against the wire at right angles and pull the blade to
    the end of the wire, stripping off small strips of
    insulation with each pull. Repeat all the way around the
    wire, taking care to NOT NICK THE WIRE was you strip the
    insulation.

    The main problem I see with this method is that one can
    impart small nicks to the wire which may eventually
    result in weakening of the wire at the nick, especially
    if the wire is in a location where it may be subject to
    being flexed back and forth or in a connector which may
    be installed and removed frequently.

    The other possible problem with this method is that, if
    you are not careful in blade-stripping of the enamel, you
    will leave lengths of un-stripped enamel which can (if
    not burned away during the subsequent tinning process)
    result in a very poorly tinned wire.

2) SANDING it off with sandpaper, emery cloth, or some other
    similar abrasive medium. This method can work pretty well
    if you take the time to do it right. And it will not
    cause damage to the wire as a result of nicking with a
    blade. This method does often take some time to do
    properly and, like blade stripping, can result in
    'blotchy' tinning if you don't apply enough heat once you
    tin the wire.

3) BURNING it off by use of a flame. This method seems to
    work pretty well but once you have successfully melted
    and burned the enamel, you still have to then clean away
    the blackened residue which still adheres to the wire
    itself. you CANNOT merely solder through the carbonized
    insulation. This method takes some time to accomplish
    properly, but it will generally result in a solderable
    wire. You DO have to use some care when using certain
    types of flame... the flame may be so hot, and
    uncontrolled, that you not only succeed in burning off
    the enamel but you also success in 'hardening' the copper
    to the point that it becomes brittle and will break much
    more easily once it is placed into use.

4) MELTING/BURNING it off by using the heat of molten
    solder. I tend to prefer this method, though some find it
    a bit more difficult to accomplish.

    The 'tricks' (if there are some) to this method are using
    a soldering iron which is hot enough, getting the melted
    solder TO the COPPER where it will quickly spread the
    heat, and being patient a bit patient.

    If you have a temp-controlled soldering iron, you'll want
    to set the tip temp up a bit higher than that which you
    use for normal soldering. I tend to use a tip temp of
    about 700 deg F (370 deg C) for soldering, but I bump
    that up to about 750 deg F (400 deg C) when I'm melting
    heat-strippable insulation. If you don't have a
    temp-controlled iron, chances are that your tip temp is
    already at or above the required temperature, so you
    probably won't have to worry about tip temperature.

    Additionally, a larger (width-wise) is recommended,
    though a fine tip will work. You want to melt a blob of
    solder into a clean iron tip, so it will be in good
    thermal contact with the tip of the iron and so the iron
    can continue to provide heat once the wire being stripped
    has been inserted into the 'blob' of solder.

    Heat transfers to the wire being stripped much more
    easily if it can come into contact with a point of BARE
    COPPER rather than if it is required to pass through the
    insulating enamel in order to get to the copper. As a
    result, once I have melted a small blob of solder onto
    the tip of my iron, I'll start the stripping process by
    inserting just the CUT end of the wire into the solder
    blob, allowing it to come into direct contact with the
    molten solder and to begin heating up the enamel.

    Within a few seconds of the bare copper touching the
    molten solder, I'll see some smoke and the enamel will
    begin to bubble up. At this point, I'll slowly insert
    more of the wire length into the solder blob, again
    waiting for the enamel to melt and burn away. I continue
    this until I have melted the enamel as far up on the
    length of the wire as needed, and then I slowly remove
    the wire from the solder blob by backing it out of the
    blob just as I had inserted it. Te end result is a tinned
    wire with a bit of 'dross' (burnt enamel) remaining at
    the upper end of the tinning and possibly a bit of burnt
    enamel on top of the tinner portion of the wire. I can
    now use my thumbnail to remove any races of remaining
    burnt enamel from the tinned wire.

    Finally, if you happen to have a heated vacuum-type
    desoldering tool, such as the Hakko 808, you can very
    easily strip your wires by merely applying a bit of
    solder to the nozzle of the stripper, then slowly
    inserting the cut end of the wire into the nozzle. Almost
    instantly, the enamel will start to melt and burn away,
    tinning the wire as it is inserted further into the
    nozzle. Then, once you have inserted the wire for enough
    to fully tin it, press the vacuum switch while you remove
    the wire from the nozzle. Almost too easy.

Finally, when tinning toroidal inductor leads, try to tin them up just a 
bit PAST the edge of the toroidal core!!!  Many builders stop tinning once 
they reach the bottom edge of the core. This all too often results in some 
unstripped wire being pulled down into the PC board hole once the toroidal 
inductor has been installed and its wires have been pulled taut in 
preparation for soldering. You do NOT want unstripped wire inside the 
plated-thru holes of the PC board. Doing so can result in poorly soldered 
joints which can (and will) fail at a crucial time. When I tin toroidal 
inductor leads, I'll bend them out at right angles from the body of the 
core, tin them almost all the way up to the body of the core, and then, 
once they're cool and cleaned, bend them back down against the body of the 
core. Of course, you don't want to risk having leads tinned in this manner 
shorting out against each other, but the way in which Elecraft has you 
winding your toroidal inductors, helps to ensure that the leads will remain 
well separated one from the other, so shorting between individual tinned 
leads should be a minor concern if they are installed into the right PC 
board holes and if they are then pulled taut before they are soldered.

I see I've gone on much too long (again). So I'll stop here. But just 
realize that tinning heat-strippable wires is NOT difficult, and certainly 
not impossible, if you just use some care in the process.

73,

Tom Hammond    N0SS