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Dual pulse battery tab spot welder controller

Published: 31 December 2014
Last updated: 02 June 2023

This advanced controller was used by many people and has a unique reduced inrush current circuit that prevents main fuse blow up.

The advantages of the spotwelder

  • Open source software available on GitHub
  • See the open source PCB Eagle files on GitHub.
  • Front panel for Hammond 1591XXSGY enclosure on GitHub.
  • Reduced inrush current prevents main fuse blow up
  • Menu controlled operation with TFT display
  • High power alternistor triac
  • Can also be used as a reflow oven controller in the future, see here.
  • Single board that fits into a standard Hammond Electronics enclosure
  • No separate 9V transformer needed
  • Front panel 3D print file available
  • Temperature sensor DS18B20 input (software not yet implemented)
  • Rotary encoder input (software not yet implemented)

You can use it for:

Dual pulse spotwelder controller  in Hammond caseDual pulse spotwelder controller in Hammond caseDual pulse battery tab spotwelder controller Dual pulse battery tab spotwelder controller

Microwave oven repair safety precautions

Working on a microwave oven is extremely dangerous. You will normally NOT survive the high voltage of 4000V, the available power of more than 1000W is enough to kill you instantly like an electric chair. Normal insulated tools are not safe, the high voltage may strike through it. Please read this article first

The microwave oven high-voltage is 5 times as dangerous as the overhead line of a train, see the video here:

Microwave oven repair safety precautionsMicrowave oven repair safety precautions

This product is no longer for sale since 7-2020. 

But there are now cheap spotwelder controllers for sale in China, see here for an example:

Spotwelder controller China

Circuit description

See for more information about the triac en the circuit: Arduino universal AC power control board.

Zero crossing detector prevents main fuse blow up

There are many spot welder controllers for sale from China. These are pretty good and cheap, but they do not have a zero crossing detector as this controller. A zero crossing detector is necessary for reducing the weld transformer inrush current, to prevent the mains fuse blow up.

Maximum triac current

The maximum continuous current for the T3050H-6I is 30A. However, for welding, the non-repetitive current is important. Normally, there is enough time between each weld to cool down the transformer and the triac. For a welding pulse of 0.2s, the allowed current is 150A and for 1s this is 100A. Anyway, the main fuse of your house will blown-up at more than 100A.
For small spot welders, such as those built with a microwave oven transformer or the Kende DN-100E, the alternistor is adequate oversized. 
If the triac is not strong enough, you can easily solder a much larger one on the PCB.


I have developed the D.I.Y spot welder because I needed a special spot welder for the building of my Maxun One solar bike. It turned out that the spot welder is being built by many people worldwide.

DIY battery tab spot welder with controllerDIY battery tab spot welder with controller

Spot welder parts

Besides the board, all you need further are the microwave oven transformer and all the other mechanical parts and cabling.

Front panel for the Hammond Electronics enclosure with self tapping screws

Front panel with self tapping screwsFront panel with self tapping screws

Electrode holder radiator earth clamp set

This makes the mounting of the welding electrodes very easy. Size 20mm x 20mm x 12mm

ClampRadiator earth clamp set

Welding electrode set
Pure red copper, length 45mm, diameter 6mm. 
It is important that the electrodes are made of the right material, not all types of copper are suitable. Note that there are special copper alloys for spot welding, such as Class 2 Chromium Copper 18200 or Chromium Zirconium Copper 18150, but these are just needed for example at car production. Pure red copper welding electrodes don't stick to nickel tabs. You only need one set of electrodes in your life, because the wear is negligible, unless you have to weld thousands of batteries. You have to shape the electrodes yourself with a bench grinder.

WeldpinWelding electrode set

The big advantage of this spot-welder timer is that the control is menu-driven and therefore very simple. A manual is actually unnecessary.

Wiring and mounting


  • Always test the controller first with a light bulb instead of the welding transformer. Don't use a LED lamp.
  • If you want to measure the output voltage, use a light bulb as a load.
  • Note that it often happens that the transformer of the Sunkko has a short circuit, that will blow-up the triac. Always check first the transformer by using a on-off switch instead of the controller and try to weld. If the main fuse blows, then the transformer is broken.

No connectors are required because only screw terminal blocks are used.
Note: The in and output connectors are for AC voltage and there is no + or -.
Note: The power connectors are not faced-out at the edge of the PCB because else it wouldn't fit into the small Hammond enclosure.

Spot welding pulse timer controller wiringSpot welding pulse timer controller wiring

  • MAINS: 110VAC to 240VAC input
  • OUT: output to the welding transformer
  • Foot switch (optinionally)
  • J6: rotary encoder (software not yet implemented)

Weld times

The time-table has a logarithmic distribution. The distance between the steps is 20% to 30%:
0, 10, 20, 30, 40, 50, 60, 80, 100, 120, 150, 200, 250, 300, 400, 500, 600, 800, 1000ms.

The welding times are different for 60Hz mains: 8.3, 16.7, 25.0, 33.3, 41.7, 50.0, 66.7ms etc. But because these are not round numbers, the fictional weld times for 50Hz are still used for 60Hz. So at 60Hz, a welding time of 1s is actually 830ms.

  • Pre-weld time: 0ms to 1000ms 
  • Weld pause time: 0ms to 1000ms
  • Weld time: 0ms to 1000ms

To protect the microwave oven transformer, the maximum welding time is 1s. For longer welding times, use the continuous mode.

Here are some oscilloscope images of the measured weld current:

Weld current for weld time 10msWeld current for weld time 10msWeld current for weld time 20msWeld current for weld time 20msWeld current for weld time 30msWeld current for weld time 30ms

Interval time

We also have to take into account the minimum interval time between successive welds. The welding transformer and the alternistor need some time to cool down. When we weld batteries manually, the electrodes must be accurately moved from one weld to another each time, this ensures sufficient interval time.

Settings for nickel plated steel strips of 0.15mm thickness

Most used are nickel plated steel strips with a thickness of 0.15 mm, these can be welded the best. You may have to experiment with the weld times and the electrode force, but start with these values:

  • Weld electrode force 0.4kg (4N)
  • Pre-weld time 50ms
  • Pause 500ms
  • Weld pulse time 100ms (50 to 250ms)
  • Electrode tip diameter 1.5mm
  • Electrode outer distance 5mm

Controlling high-power spot-welding machines

High power spotwelder controllerHigh power spotwelder controller

The controller can also be used for professional, high-power spot-welding machines. Here, longer welding times are common, up to 3s. The alternistor heatsink may get too hot when using very long welding times with short intervals. In case of doubt, check the temperature of the heatsink after the voltage has been removed

Built-in power supply

The controller contains a mains power supply and works on all mains voltages worldwide from 110V to 240VAC, 50 and 60Hz. You only need an extra microwave transformer. 

Standard Hammond enclosure

The PCB fits exactly into the Hammond Electronics 1591XXSGY enclosure. See the dimensions for drilling HERE.

Arduino spotwelder controller in Hammond caseArduino spotwelder controller in Hammond case

The dimensions of the board are 101mm x 70mm. If you want you can cut the PCB just behind the buttons to 85mm x 70mm.

Front panel for Hammond Electronics enclosure 

The spacers for the (special) self-tapping screws from the original Hammond front panel are very weak, I have reinforced these:

Reinforced PCB spacers for the self-tapping screwsReinforced PCB spacers for the self-tapping screws

Information for nerds

See this article for more details Info for nerds.

Spotwelders from other users

See the article HERE

Spotwelders from other usersSpotwelders from other usersMiroslav Kolinsky1

Repairing all Sunkko battery spot welders 788H 709A 709AD 737B

The electronics of the Sunkko battery spot welders are not made well and it often breaks. You can repair it by replacing the electronics by this spot welder controller.

Note that it often happens that the transformer of the Sunkko has a short circuit, that will blow-up the triac. Always check first the transformer by using a on-off switch instead of the controller and try to weld. If the main fuse blows, then the transformer is broken. In that case, the transformer have to be replaced, you can take a transformer from a microwave oven.

Sunkko 788H repairSunkko 788H repairSunkko 709ASunkko 709ASunkko 709ADSunkko 709ADSunkko 737BSunkko 737B

Kende DN-100E improvement

The Kende DN-100E does not have an electronic control of the welding time. Therefore it is hard to make high-quality weldings with a good repeatability. With the controller you can greatly improve the weldings with the Kende DN-100E , see here how to do.
Kende DN 100E spot welderKende DN 100E spot welder


There is a forum about the spotwelder controller (read 35000 times), see HERE. It contains answers to practically all questions.

Operation of the zero crossing detector circuit

This circuit necessary for the reduction of the weld transformer inrush current, to prevent the mains fuse blow up. The current through the weld transformer, which is an inductive load, will lag behind the voltage by 90º. This is the reason that, to reduce the inrush current, the optimal switch-on time is not the zero point of the mains sinus but the top of the sinus. To be able to do this, the Arduino has to detect the mains zero crossings, this is done by U3. The delay time in the software "sinusMax_us" is 4583µs, this strange number is the average of 1/4 cycle of 50Hz and 60Hz. 

PhaseInductive load phase shift

Setting the power of the welding current is done only with time control and not also with current value control. At some spot welder controllers, the current value can be set too, but because this works with phase control, this will increase the inrush current with the risk of blow up the main fuse. Weld-time setting has the same effect.

High voltage!


Pay attention: the PCB is directly connected to the mains, just the low voltage part is safe. Usage is at your own risk.

User guide

  • Use the up, down and select button for setting the parameters, see the video on YouTube.
  • The settings are stored in the EEPROM.
  • Press the weld button or the foot pedal to weld.

Continuous mode:

There is a possibility to weld as long as the weld button is pressed. To enable this mode, set the welding time to 0 (*note*), press and hold down the weld button simultaneously during power on for 1 second and than release the button. Now you can weld continuously without the timer. Take care: the transformer will be overheated easily, so use this feature only for testing or for measuring the weld current.

Rotary encoder

Instead of the menu buttons, a rotary encoder may be used. The hardware is implemented on the PCB, but the software still has to be made for this.