From: Paul Jebb pfjebb@..............

Date: Fri, 14 Jan 2000 10:09:44 -0500

Hi Sean, Thanks for those suggestions, a lot of nuggets in there. Cheers, Paul S-T Morrissey wrote: > Re counting turns when winding a coil: > > I don't bother to count, but I know the approximate number. > > First of all, I turn the coil by hand on a short shaft under > good light, placing each turn EXACTLY adjacent to the previous, > and delimiting each layer with correction tape strips as I described > a few weeks ago. (need a repeat??). It takes about 2 minutes to > make about 100 turns. And another 10 minutes to epoxy the layer. > A 12-layer coil takes me over 2 hours, depending on distractions > while the epoxy cures. > > This lets you know the number of turns by knowing the diameter of the > wire and the winding length. I use #36 wire from Newark (need a repost > of that info??; they take MasterCard!). "Beldsol" enameled #36 wire has > a diameter of 0.0055", so if I have a winding length of 0.5" with EVERY > TURN exactly placed, I have 91 turns in that layer. Since I don't use > a bobbin form, I step back the end of each layer by one turn, or conversely > start with a slightly larger number in the first layer. So I use a starting > length of 0.615" ( slightly under 5/8") to start with 111 turns; subsequent > layers are 109, 107, 105, 103, 101, 99, 97, 95, 93, 91, and 89 turns in > the 12th layer, for a total of 1200 turns. The average length of a turn > with a 5/8" initial diameter is 2.17", so there is 217 feet of wire in the > coil, and at 414.8 ohms per 1000 ft, I should get 90 ohms, which is about > what I measure. > > For your info: other wire diameters and resistances: > > (diameter for enamel insulation) NEWARK roll length, ft cost > #36 0.0055 414.8 ohms/1000 ft. #36E1321 1/2lb 6400 $27.33 > #38 0.0044 659.6 #36F779 1 lb 19300 $73.49 > #40 0.0034 1049.0 > #42 0.0028 1659 > #44 0.0023 2593 > > Actually, the exact number of turns is not important. What is important > is to get the maximum number of turns inside the gap of the magnet, which > cannot be done with a "scramble wound" bobbin that often results from > using a drill. The required bobbin alone will take up an appreciable amount > of the volume. Also a target resistance value is not necessary. The > trade-off in my opinion is how small a wire you want to work with to get > the maximum number of turns into a given space. #40 and up wire is probably > not amenable to precise layering, but will give more turns and more > resistance. But remember that the output increases only with the square > root of the resistance. 200 ohms will produce 1.4x the signal of 100 ohms. > > If I use the same dimensions as above and #42 enameled wire, which is > about half the diameter of #36, I will get twice the turns per layer, > and twice the number of layers, for about 4800 turns. With the same mean > length per turn, this will be 868 feet of #42, which will measure 1440 > ohms. So I get only 4 times the output (4 x 1200 turns) at 16 times the > resistance (16 x 90 = 1440 ohms). BUT I also have 16 times the Johnson > noise due to the resistance, and it may be difficult to damp with a > resistor if the magnet is not strong enough. > > Regards, > Sean-Thomas > > _____________________________________________________________________ > > Public Seismic Network Mailing List (PSN-L) > > To leave this list email listserver@.............. with the body of the > message: leave PSN-L _____________________________________________________________________ Public Seismic Network Mailing List (PSN-L)

Larry Cochrane <cochrane@..............>