I've always been interested in winding my own output transformers but never had the confidence to begin....until now.
The main source of information was from Patrick Turners web page and I've basically used his design so that I only had to worry about my winding technique and methodology. I'll leave you to read his notes rather than duplicate them here.
My winding machine consists of a periscopic blood pump with a block of wood connected to the main axle. The wood was cut to the same internal dimentions of the bobbin so that when I slid the bobbin on, the fit was snug. The main advantage of using this machine is that it is variable speed and can be adjusted between 0rpm and about 30rpm. My turns counter was salvaged from a broken hans held measuring wheel.
| Step 1:
Ready to begin winding |
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| Step 2:
One of the secondary layers being wound on part way through the windup.
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| Step 3:
One of the primary layers being wound on part way through the windup.
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| Step 4:
The finished windup ready for wax impregnating. Note: The blue indicates the seconday wires and the red indicates the primary wires. |
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Now that the transformer winding is complete, the next step is to impregnate the transformer so that the windings do no move or vibrate. Most people immerse the coil in hot wax (say 100oC) for a few hours. I've never tried this method as I don't see how the wax can get into the coil other than by capilary action.
My method is to immerse the coil in hot wax (say 100oC) and let the temperate gradient equlibrate through the winding for about 1 hour. Now I create a vacumm of about -28 inches of mercury to draw the air out of the winding. You'd be surprised at just how much air comes out! I've tried to take a picture (see below) to show the air repidly streaming out of the gaps in the bobbin. Next, I restored the pressure to atmospheric while the transformer was still submerged in the hot wax. This had the effect of the wax being 'sucked' into the coil.
| Testing the vacumm with water in the bowl
and without the transformer.
The bowl is a stainless steel cake mixing bowl and the black ring is a gasket is made for the lip of the bowl. A sheet of 100mm thick polycarbonate has been bolted down over the bowl to seal it. On the right you can see the 60ml syringe I used to draw out 60ml of air each time I drew back on the syringe. The blue 'tap' on the end of the syring is used to vent the air drawn from the bowl out to the atmosphere before drawing out another 60ml of air. It takes about 3-5 minutes to reach a pressure of about -25 inches of mercury Note the dial gauge reads anti-clockwise and measures from 0 inches to -30 inches of mercury. The vacumm is near vapour pressure and you can see bubbles of air forming all over the inside surface of the bowl.
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| Drawing out the air from a transformer coil submerged in hot wax. The pressure is about -26 inches of mercury. Note the large air bubbles streaming out of the coil. This photo was taken after about 5 minutes, so you can imagine the amount of air that's come out of the coil! | 
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