Here is a repeat of the design abd construction of a compact coil to be used with the rare-earth magnet/soft iron washer magnet design. The original is from December 1999. Regarding winding a compact coil for a seismometer as either a direct velocity sensor or as a feedback force coil (using the magnet assembly made with parts bought from McMaster-Carr). One of the perplexities of making a home made seismometer is the need to create an efficient coil and magnet transducer to use either as a basic velocity sensor or as the forcing element of a fedback broadband system. The resources of the amateur builder are constrained to making the best of "found" hardware that can be modified or or often ingeniously adapted. For a coil-magnet transducer, the magnet is most likely to be used "as is", and a coil created to fit it. But the coil itself needs to be created on a form that provides the greatest number of turns actually inside the magnet gap. But generally, a "found" FORM will take up a large portion of the available space in the gap. If one examines a speaker coil, the form is a thin cylinder of craft-like paper and often aluminum (to dissipate heat) that is no more than about 0.010" thick. THe rest of the circular gap of about 0.093" is about evenly occupied by about 50% for the two or four layers of the coil winding with the rest for clearances on each side. If the speaker coil IS used for the seismometer, the low resistance limits the velocity output. The tight clearance is a problem for linear motion of the coil for the partial arc movement of the seismometer boom. So there is a need to be able to wind a coil with a much higher number of turns that still fits the speaker magnet (or another magnet made with found hardware). With enough searching, a smooth cylinder form with the proper outside diameter (OD) can be found. For example, for the 10" speaker, the ID of the gap is 1 15/16 and the OD is 1 5/8", so the ID of the coil is about 1 17/32" and the OD is 1 19/32". There are 40 turns per layer of each two layer, 4 ohm coil. Winding a new coil with #36 magnet wire would get 8 layers of 90 turns each into the same space, for a 4.5x increase of the output to about 50 volts/meter/second.. So I have found a way to wind a coil that consists entirely windings of the coil on a temporary tubular form, with epoxing of each layer. After the temporary form is removed, only windings remain to fill the magnet gap. This method is not easy, and will try your patience, but if you take your time, the result is predictable and rewarding. The form is a very smooth tube of about 0.010" OD less than the required finished ID; the sources are everywhere; take your ruler to the store. Pill jars might be used, but be careful of the taper used in molding them. Any very smooth tube will do. I have peeled the label from a CRazy-Glue tube for a 5/8" id form that I need for a compact transducer using bought parts for the magnet.. (The dimensions below apply to this coil). The initial setup requires that the first layer be securely held in place but in a manner that it will slide off the form. So a label is installed >sticky side out< on the form. Start with a paper label cut in length exactly to the circumference (2.0") but about somewhat wider (1.0") than the planned winding (0.625") and installed sticky side out on the tube with strips of regular tape outside the planned winding area. I fold under the ends of the tape to facilitate removal, as well as to hold the free end of the starting turn. Set up the wire roll (#36 Beldsol) ("Beldsol" wire has an enamel insulation that melts free of the wire when it is soldered at 700deg.F) on a smooth rod across the opening of a box so that it unwinds freely. Then pass it though a small pad of paper to control the tension by putting various weights on top of the pad. Start the coil with the free end looped under the temporary tape. I wind the coil by turning the top towards me so I can see each winding. Turn the coil form by hand and make sure each winding falls exactly next to the previous. I have found that using a portion of the final coil mounting with a 1/4" shaft at each end of the forming tube allows some amount of "twirling" to speed the winding process. I get "magnet wire" as it is called from Newark (800-463-9275). The #36 Beldsol is Belden # 8058, a 1/2 pound spool of 6400 ft, which should make about 30 of the coils I described. (Or a single 2656 ohm coil at 415 ohms/1000 ft). The Newark number is 36F1321, for $28. (A more elaborate winding setup can be imagined, but may not be worth the effort to create one or two coils. This proceedure takes about 15 minutes per layer, including waiting on the epoxy. ... About 2 hours for an 8-layer coil, or one RAMS game for the 12-layer version). One can go nuts trying to count the turns, so just use the winding length divided by the wire size to determine the number of turns. A 0.625" winding length of #36 wire of 0.0055" diameter is about 120 turns per layer. The initial coil length should be at least half the number of layers longer. Once a layer is complete, I lay the form and coil on a tablet at a right angle to the direction to the wire spool and use a pair of rulers and a weight to prop it up so the winding stays under tension and in place. For epoxying the layer, I use the "five minute" epoxy by Devcon that comes in the parallel syringe tubes. It takes about a pea size amount of each resin; if it is over-mixed, it sets up too fast. I use a small screwdriver to apply a thin layer to the winding layer, keeping the tension on the free end to keep it in place. (This requires rotating the form so that one additional turn results). Then I immediately wipe all the excess from the winding with a finger, wiping it clean on a towel each time. This pushes the epoxy between the windings and results in a completely smooth outside finish to the layer. Any delay will find the epoxy starting to gell: you have about 1 minute to finish!. Prop up the coil again with the winding under tension and allow the epoxy to set for about 5 minutes. (Test the epoxy set-up on your mixing pad). (The un-set epoxy will wash off your finger with warm soapy water). The problem now arises in winding the second layer back towards the starting point: the wire is very difficult to see laying over itself. And if any winding is out of place: ie: not laying exactly adjacent to the previous turn, dips or lumps in the winding will result, and subsequent layers will take up too much room. So I have found that laying three or four stripes of typing correcting film across the winding makes the new winding quite visible. I use the "Mono Correcting film" for three stripes evenly spaced around the circumference. It then becomes very easy to see each turn, and even back up a turn if white space is seen between turns. I then wind back toward the starting point, stopping one turn shy because a turn is added in applying the epoxy. Each layer must be one turn less at each end in this "formless" method construction. Again, the coil is propped up to keep the wire under tension, the epoxy mixed, applied, and wiped smooth, as above. I apply the next set of white film stripes offset from the first so the slight added thickness is distributed evenly around the coil. This process is repeated until the coil is built up to the desired OD for clearance in the magnet gap. Mistakes will be amplified in later layers, so don't make them. To date I have made a 12 layer coil of about 1200 turns and 98 ohms. (5/8" ID by 3/4" OD; starting length 5/8", finishing length (of 12th layer) 1/2". The output is 10.5 Newtons/Ampere with the "bought" magnet I have described. Once the coil is complete, let the epoxy cure for several hours. Then remove the temporary taping and slide the coil off the form toward the tapered end (even the KRazy glue tube is tapered slightly). Carefully twisting the tube helps, but be careful not to damage the coil. Then carefully remove the original sticky side up label from inside the coil, being careful about the free ends of the winding. I then epoxy the winding to a short piece of fiberglass tube with a 1/4" fiberglass threaded stud in the center for mounting. Regarding the direct solderability of Beldsol at 700 Deg F, I have found that lightly scraping the enamel insulation with a razor-type knife facilitates the removal of the insulation when soldering it to a terminal, which reduces the chances of melting the socket or pin or whatever if it is a PVC material. For temperature control, I use the Weller WTCPT solder station that uses interchangeable tips with different temperature cut-offs (where the curie point of the tip is reached at a given temperature and it becomes non-magnetic and releases a magnetic switch that turns off the heater current; if you have one of these, this is why it clicks all the time). This Weller soldering system is much more affordable than those with electronic When I get some time to update the web site, I will post drawings and photos. Regards, Sean-Thomas __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>