Spot welder controller for the Kende DN-100E (Harbor Freight) prevents blown fuses
Avoid blowing up fuses
A serious problem is that circuit breakers or fuses in the breaker box are blown frequently when using spot welders. By the use of the here described spot welder controller, this will be avoided.
Harbor Freight / Kende Welding Machine DN-100E
The Chinese spot welder Kende DN-100E, made by Zhejiang Kende Mechanical & Electrical Co. Ltd is a popular spot welder for home use. It is sold under a number of other names in different countries; Harbor Freight is the brand name in the US. The picture shows one sold by a Swedish importer. Prices differ a lot from country to country but it is one of the cheapest you can buy and considering the quality/price it is an unbeatable offer.
Harbor Freight / Kende Welding Machine DN-100E
Oliver Neubert shows here how you can weld a car with the spot welder controller and the Kende spot welder. The steel plates are both 1.5mm thick. The preweld pulse is 1s, the pause is 60ms and the weld pulse is 5s.
Welding 1.5mm thick steel plates with my spotwelder controller and the Kende DN-100E
Of course the power is limited because of the 230 Volts mains but for simple work, normally carried out by the do-it-your-self man or women; it will do its job as can be derived from a number of YouTube testimonies. With a trick it is also possible to spot-weld aluminium, see at YouTube.
Arduino spot welder controller
This Arduino spotwelder controller can be used for the Kende DN-100E, see the article HERE.
Spot welder controller for the Kende DN-100E prevents blown fuses
Note that you need to replace the thyristors by stronger ones e.g. the TN5015H-6T (or 6I), which max. RMS current is 50A.
Limitations of the Kende DN-100E control
The Kende spot welder is robust and simple in design, not much can go wrong. It has an adjustable handle for pressing the work pieces together and a simple toggle switch to switch-on the mains. For spot-welding you keep the pieces pressed together by gripping the handle and with a finger of the same hand you can operate the toggle switch as long as necessary. After some trial and error, reasonable spot welds can be made. The drawback of this simple manual control without timing is that the repeatability of the welds is generally poor; for welding thin material it is useless. It further has a problem in that, depending on when the toggle switch is pressed in relation to the mains sinus period, the current surge can be quite high, so that even with a high current main fuse, repeated blowing the fuse cannot be avoided.
Improved spot welder timing control
The quality of a spot weld depends on a number of welding parameters. Most important are: the pressure of the electrodes on the work pieces, the total welding energy input and the duration of welding current flow: the total welding energy input should be delivered sufficiently fast so that the material melts quickly in a small area and heat conduction into the surrounding area is negligible.
For reasons of simplicity, I decided to focus on the welding power control.
Instead of the manually operated switch, an adjustable time-switch could be used. However, in particular for short switch on times, the repeatability will still be poor. Due to the sinusoidal nature of the current flow, when the power is switched on successively and somewhat randomly at different points in time during the sine wave, the total amount of power delivered to the weld is not necessarily the same for any given set run-time. Such a timer will also not reduce the repeated blown main fuse problem.
When searching the internet for a better time switch, I came across the Arduino spot welder controller developed by Mr Albert van Dalen in the Netherlands. Mr van Dalen developed his spot welder controller for a DIY battery tab resistance fine-spot welder, based on a modified microwave transformer, but obviously that controller could also be used for the Kende spot welder when replacing the thyristors for more powerful ones. I was very happy to see that Albert had thought of reducing the weld transformer inrush current as well, so this controller was exactly what I had been looking for!
Arduino spot welder controller 
Arduino spot welder controller 
Spot welder controller inside
Assembly, testing and further modification
Mr van Dalen sells the printed circuit board and the pre-programmed ATMEGA328 for a very reasonable price. The other parts are easy to obtain (Conrad for Europe) and assembly is an easy job, not the least thanks to the high-quality circuit board: all parts fit extremely well.
I ordered my controller at the end of 2016 and that controller has a 10 position switch for setting the weld time in steps of 50ms, thus from 0 to a maximum of 450ms welding time. In January 2017 Mr van Dalen developed his controller further so that now, with a newly programmed ATMEGA328, also longer time periods can be set by means of small push buttons. I will also make a second controller with that newly programmed microprocessor.
The only modification made to the circuit was the use of 30A thyristors (CMA30E 1600PB). The controller immediately worked very well. No blown fuse and perfect repeatability of the welds.
However, for very thin material (0.2mm stainless steel) 50ms was still too much. Having first thought of re-programming the controller for smaller time periods it was noted that in his spot welder Mr van Dalen had put a switch and resistor in the primary circuit for switching to a low current mode. That solution was adopted and in fact two 10 Ohm 100 Watt resistors and 4 switches are used for even more control possibilities:
Power limit resistors
Below are some pictures of the controller and different welds made. The controller is connected in series with the spot welder. A foot switch is used, so you keep your hands free.
3mm stainless steel rods to 2mm ring
3mm stainless steel rods
Good repeatability, 0.2mm stainless steel
0.2mm stainless steel to 2mm rod 
2mm rods welded with different weld time
I want to thank Paul Alting van Geusau from München for writing this article.