Quote:
Originally Posted by Nico
Try this one, its cheap to build, and all parts probally set you back no more than $15. The heating element can be purchased from Jaycar, part no. WW4040. Also required is a length of heat resistant tubing to feed the element into, part no. WS5504.
The heating element is a special type of wire known as Nichrome wire whereby its resistance measured from one end increases as length increases. Its the same kind of stuff used in toasters, although you would definitely not want your element to glow red hot.
I feed the wire through the tubing then using some insulating type of flexible foam/rubber mat (roughly 25mm wide x dia of OTA) taped the tubing (with the element contained within) to the rubber/foam with a long strip of gaffa tape, then just folded the edges round the back to make it neat.
You'll need to solder the ends of the nichrome wire in order to make a good connection with the output of the PWM circuit. I tried a few ways (nichrome wire is difficult to solder) and found that by lightly removing the surface of the wire I was able to solder it to a couple of female spade terminals (remove the plastic insulation before soldering, and apply stacks of heat + solder) once cooled down just slide the insulation back on, or just use tape!
Stuff the circuit (made mine on a piece of viro board, also at Jaycar or dicksmith etc..) into some kind of plastic jiffy box and mount the led through the lid (so you can see the PWM in action), connect the load (element) and you're away.
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The resistance wire on those old boiling jug immersion heaters is ideal for the elements. You can pick them up pretty cheap at supermarkets and hardware shops. Cut off a length to give you 20 ohms resistance, about 9-10 watts at 12volt. Mine were made by cutting a 25mm wide piece off a PVC pipe and cutting it lengthwise to allow it to clip over the scope tube. Drill a couple of small holes in each end and and anchor one end of the wire by looping it through the holes. Wrap the wire evenly spaced around the 25mm (takes a couple of tries to get it right) and feed the other end through the holes again. Feed the connecting wires through the holes at each end to anchor them and solder to the element. I wrapped mine with some tubular hemming material from the local sewing shop and put a strip of waterproof tape on the outside. Works a treat and the soft material prevents scratching of the OTA. Does'nt get hot enough to damage the material but will melt plastic tape at full output if put in close contact with the wire. There should be enough length on the jug element to make at least 2 heaters for a 4" OTA.
Cheers
Bill