Clifton Laboratories 7236 Clifton Road  Clifton VA 20124 tel: (703) 830 0368 fax: (703) 830 0711

E-mail: Jack.Smith@cliftonlaboratories.com
 

Revision History:
03 June 2009. Original
11 August 2009. Added photo of home made Q-Dope after settling
22 June 2010. Added comment on source of precipitate found at the bottom of the home made Q-dope bottle

Table of Contents
When_is_it_necessary_to_use_a_coil_retaining_compound
General_Cement_Q-Dope
Making_your_own_Q-Dope
How_to_Apply_Q-Dope
Electrical_Performance_Measurements
Homebrew_and_commercial_Q-Dope;_post_setup_transparency
Stratification

In response to a question posted at the Softrock reflector concerning what should  be used to coat a wound toroid inductor to retain the wires in place, I suggested home made Q-Dope solution be used.

This brought questions concerning the mechanical and electrical properties of homebrew Q-Dope compared with the commercial Q-dope manufactured by General Cement. It also brought forth suggestions for alternative compounds, including beeswax and hot glue from a glue gun.

I've previously compared my homebrew Q-Dope against GC's product and concluded there was no material difference, but I thought it worth repeating those measurements, and expanding them to include beeswax and hot glue.

The purpose of a retaining compound is to fix  the wires in place so they do not shift as the inductor is handled, or installed. Should the wires shift, in certain applications, and with certain cores, an undesirable shift in inductance might occur.

I also should note that in some cases it is necessary to retain the wound inductor to the printed circuit board to prevent it from being ripped loose by mechanical shock such as might be incurred during shipping or if the PCB is inadvertently dropped. This page does not discuss this type of retention. For this purpose, my preference is electrical grade (non-corrosive) RTV. Many otherwise useful adhesives will not stick to the solder mask employed on printed circuit boards.

For another view on retaining compounds, see the similar series of measurements made by Roy Lewellen, W7EL, at http://www.qrp.pops.net/w7el.asp.

The first question that should be addressed is "is it necessary to use a coil retaining compound at all"?

My preference is  to use a retaining compound only when necessary. That is when the wire and core size are such that the wire does not hold itself in place after winding. With, for example, a 0.5 inch diameter toroid inductor core (T50 size) and No. 22 AWG wire, a retaining compound is not usually necessary. In contrast, a 0.68 inch diameter toroid core and No. 26 AWG wire almost always requires retaining compound. 

Special circumstances may dictate applying a retaining compound even if the wire is relatively rigid. For example, if a toroid inductor is adjusted to a particular inductance value by spreading or compressing turns, retaining compound is necessary to fix the turns in place, particularly if the inductor is to be handled after adjustment, such as tinning the leads and forming to be inserted into a PCB.

Solenoid coils are more likely to need retaining compound, of course, unless particular attention is paid to a solid mechanical attachment at the winding ends.

For many years, the retaining compound of choice has been General Cement's "Q-Dope." I don't know when it was introduced, but the oldest electronic parts catalog in my collection, a 1942 copy of Radio's Master Encyclopedia, shows Q-Dope as being available at $0.21 for a 2 oz bottle, while the associated  thinner cost $0.15 for a 2 oz bottle.
 

What I look for are:

• The packing peanut must be white. Pink peanuts have an antistatic treatment and may or may not be usable. I've not  tried them. Green peanuts are supposed to be biodegradable and are not usable.
• Test a peanut to see if it dissolves in water. If so, it's not plastic.
• Some peanuts have a large pore structure and look like a figure "8." I do not believe these are polystyrene.
• Other Styrofoam products, such as coffee cups, may be used. I've not tried these as I have a large quantity of usable packing peanuts on hand.

Before launching on production, I would test the peanut with a bit of toluene to see if it dissolves cleanly.
 

Eventually (and this will take far more peanuts than you thought at the outset) the solution will start to thicken. Add peanuts until the solution has the consistency of pancake syrup. If you overshoot and the solution thickens too much, add a small amount of toluene to thin it.
 

Q-Dope may be applied either with a small brush or by dipping the coil into Q-Dope solution. If you don't have a suitable brush, check the children's art section at Wal*Mart or a hobby and craft store such as Michaels.

A thin coat is all you need. I use one of the multi-jointed alligator clip "third hand" devices to hold the coil after dipping in Q-Dope or when brushing Q-Dope on. Let the coil remain suspended by the leads until dry.

Q-Dope dries in around 30 minutes at room temperature.

When working with Q-Dope, whether homebrew or "store bought" please remember that it is flammable and should not be used near open flame or other ignition sources. In addition, use Q-Dope only in a well ventilated area to avoid exposure to the toluene fumes.
 

To assess the effect of coil  retaining compounds on an inductor, I wound five similar inductors, each consisting of 20 turns of No. 22 AWG magnet wire on T50-2 (red) powdered iron cores.

I identified these inductors as A through E and measured their inductance and Q with an HP 4342A Q-meter, operating at 7.9 MHz.

I  then applied retaining compound  to these inductors:

• A—no retaining compound. Used as a control standard
• B—dipped in homebrew Q-Dope
• C—dipped in GC Q-Dope
• D—dipped in molten beeswax
• E—coated with molten hot glue from a hot glue gun

I then measured the inductance and Q after the retaining compounds solidified.

The photo below shows the five test inductors after treatment. (Cores A and C are flipped so the unpainted core face is upward. All inductors are wound on T50-2 cores.)
 

Likewise, it was impossible not to experience some shift in the windings in the process of removing the inductors from the Q-meter, coating them with the various retaining compounds and remounting them on the Q-meter for the second set of  readings. Hence, changes on the order of 1% to 2% between pre- and post-treatment should be regarded as being essentially unchanged.

With this error range in mind, the results show the only retaining compound causing an apparent shift in inductance is beeswax. With respect to Q, somewhat surprisingly, the worst Q loss was with the GC Q-Dope. I regard the beeswax and hot melt glue Q changes as being statistically valid.

Whether these Q and inductance changes are material, is another question. It would be a rare application indeed where an inductor with a Q of 229 works but one of 211 fails, representing the worst case Q shift.

I believe it is correct to say that based on this limited set of measurements, there is no reason to doubt the homebrew Q-Dope is as effective as the GC commercial Q-Dope.
 

The "true" Q of an inductor with distributed capacitance C_D is related to the apparent Q, Q_A, measured on a Q-meter with resonating capacitance C by the following formula:

As the distributed capacitance, C_D, increases, Q_A, the apparent Q, increases even if the true Q of the inductor remains constant.

A similar effect is seen with apparent inductance and true inductance.

I mentioned that the homebrew and commercial Q-Dope have similar translucence and that the color as applied to an inductor is essentially identical. The photo below shows test inductor B, coated with homebrew Q-Dope and C, coated with GC Q-Dope. There is little if any difference in transparency after drying.

After my home made Q-Dope was undisturbed for a month, I noticed a distinct stratification. The top 80% or so of the solution was  clear whilst the bottom became milky or chalky color as seen in the photo below. The photo does not show it well, but the white color is from very small particles that have settled to the bottom of the bottle.

I assume the white powdery reside results from something like powdered talc added to improve handling when the peanuts are blown from molten Styrofoam. (See E-mail message below for a better answer.) In any event, whatever the non-dissolvable residue is, it has no significant effect upon the electrical or mechanical properties of the solution to the best of my ability to measure it. And, if the solution is allowed to settle for a week or two, you can always use only the clear portion.

It would be possible to decant the clear top fraction, of course.

My wife's dad is a PhD organic/polymer chemist and a few years ago guided me in making QDope.

I too had the white residue in the bottom of my bottle and he said that it was titanium dioxide that the manufacturer of the Styrofoam cups used to brighten the whiteness.

Chris
KD4PBJ