{"id":3260,"date":"2013-03-07T10:30:44","date_gmt":"2013-03-07T08:30:44","guid":{"rendered":"https:\/\/yehar.com\/blog\/?p=3260"},"modified":"2016-02-20T09:57:58","modified_gmt":"2016-02-20T07:57:58","slug":"microwave-oven-spot-welder","status":"publish","type":"post","link":"https:\/\/yehar.com\/blog\/?p=3260","title":{"rendered":"Microwave oven spot welder"},"content":{"rendered":"

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Microwave oven transformer spot welder. Later I have replaced the welding pulse activation switch with a foot switch.<\/figcaption><\/figure><\/p>\n

I made a spot welder out of a microwave oven transformer and a starter cable. Those are quite popular actually. They use a solid state relay to switch the transformer on and off. I use a Fotek SSR-40DA<\/a>\u00a0DC-controlled 40 A AC-zero-crossing-switching solid state relay. It was only 4 \u20ac and free shipping on eBay. First I needed to extract the transformer from a microwave oven and to replace the secondary coil with a starter cable.<\/p>\n

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Microwave oven (was about 1050 W consumed power) gutted. In addition to the transformer (left), some other goodies were found from the inside that I saved for later use.<\/figcaption><\/figure><\/p>\n

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The secondary coil sawed off. There was also another secondary coil (red) that I removed later. After sawing from one side, the secondary coil could be hammered out using a chisel.<\/figcaption><\/figure><\/p>\n

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Some initial trials using a 700 W rated 12 V automotive starter cable. Didn't produce satisfactory welds. The cable was the most expensive part in the project, about 20 \u20ac.\u00a0There wasn't that much copper in the cable originally. Most of the thickness came from the rubbery insulation.\u00a0Perhaps I should have been more patient to obtain the copper from somewhere else, as I ended up stripping the cable anyhow to double the conductive copper area. Minimization of resistive losses in the secondary is key.<\/figcaption><\/figure><\/p>\n

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After making my own cable for the secondary, with twice as much copper, I was still able to fit in a few more rounds. This increased the voltage and reduced resistive losses further.<\/figcaption><\/figure><\/p>\n

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At this point I got a nice 4.6 V AC out of the secondary.<\/figcaption><\/figure><\/p>\n

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One end of the secondary cable is routed \"under the floor\", where it will be grounded to the body of the welder, and then goes to the immobile jaw. The upper jaw is mobile. To reduce wear of the secondary cable here, it is shaped like a spiral. A transparent plastic block electrically isolates the upper jaw from the body.<\/figcaption><\/figure><\/p>\n

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In the bottom, the cable is crimped inside a piece of copper pipe, hammered flat.<\/figcaption><\/figure><\/p>\n

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Another view of the jaws. I wish I'd found perfectly fitting copper pipes for the sockets. Now they are a bit loose. The copper cable goes pretty much all the way to the end of the jaws to minimize resistance.<\/figcaption><\/figure><\/p>\n

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A sturdy 230 V AC fan will take care of cooling. It pushes air out through the front grid at great speed, so that sparks will have less chance of flying into the welder through the grid.<\/figcaption><\/figure><\/p>\n

I wanted to automate the welding timing so I made this simple adjustable-duration pulse generator circuit out of some components that I had around:<\/p>\n

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Welding pulse generator circuit.<\/figcaption><\/figure><\/p>\n\n\n\n
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Spot welder timer circuit<\/figcaption><\/figure><\/td>\n
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CD4093BC pinout in the same orientation (C) Fairchild Semiconductor<\/figcaption><\/figure><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Parts list:<\/p>\n