Re: Putting toroids on lockdown?


Jim, N5IB
 

Hi Pat,

I haven't used PVA, even for non electronic projects... but...
The dielectric properties of PVA are suspect  see <http://www.edaboard.com/thread242321.html>

Years ago there was quite a discussion on the QRP-L about stuff to coat toroids. At the time I had a Heath Q-meter and did some tests with various substances. Both hot glue and common clear epoxy were not very good. Of course Q-dope was fine, as was the "hot fuel proof" clear finish that airplane modelers used. Clear nail polish was pretty good too, if applied sparingly and allowed to cure thoroughly. The important factor was that you end up with a very thin film.

The effective inductance realized with a toroid is a combination of the inductance of the windings and the inter-winding capacitances. When you coat the toroid with any material whose dielectric constant is greater than that of air (1.00) the interwinding capacitance will increase so the effective inductance will decrease. So any coating material must cure to a thin film to minimize these effects. That's one of the reasons why hot glue and epoxy are poor - they end up more as a blob than a film.

As an aside, one of the reasons the toroid in the Bayou jumper is designed to lie flat on the PCB (with the ground planes removed beneath it) is so that the wire leads can be pulled up snug to secure it all around its perimeter. If that is done there is no real need to coat it with anything. Barring intentional fiddling the turns will stay put.  As suggested in the manual, a sparse coat of nail polish along the top is fine, but I would skip even that. With just about any coating you'll find the tuning has shifted afterwards and some final tweaking will be called for. And you have to give the coating plenty of cure time first, because the dielectric properties change as the cure progresses.

Jim, N5IB


---In 4sqrp@..., <pmk@...> wrote :

Has anyone tried PVA glue to lock down there toroids? I see hot glue is pretty popular and of coarse Q-dope
but I can't seem to keep Q-dope very long as is tends to go bad on the shelf and trying to work with it afterwards is impossible.

For those that don't know what PVA glue is (PolyVinyl Acetate) also known as craft glue. Good for bonding dissimilar materials. Also used in the speaker business as it bonds with the metal/foam or metal/cloth and of coarse paper to plastic. Looks and smells like Elmers.

Also dries clear so if you are putting a clear plastic lens in a cutout for a display this would be the glue to use as it will not melt the plastic if you make a mistake and if you see a little it will not be noticeable. Also not to bad to remove if it needs to come apart down the road.


72

Pat KD4OBQ

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