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Old spot welding pulse timer controller bare PCB v1

Published: 31 December 2014
Last updated: 09 December 2023

For the new version see HERE.

This was the bare PCB for the spot welder controller, without components, to assemble by yourself.  
Spot welding pulse timer control panel with TFT displaySpot welding pulse timer control panel with TFT display


With this controller, you can set the power of a spot welder. This is done by setting three timings: the pre-weld pulse, the weld pause and the welding time. You can use it for a DIY spot welder made of a microwave oven transformer (MOT), for the Kende DN-100E welder and for the repairing of the Sunkko 788 welder.

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. Because the electronics was not so easy to build, I have made a printed circuit board, which is for sale, together with some other parts:

Spot welder controller bare PCB, not sold anymore

PCB for the Arduino spot welder controller, without components, to assemble by yourself. You don't have to know anything about the Arduino, software or electronics. All you need to do is to solder all the components, connect the board, then power up and press weld button. To make it easy, only standard parts are used that every electronic (e)shop should have, see the partlist. Anyone can build the spot welder controller, it is not difficult and it is nice job too.

PCBsmallSpot welder controller PCB

Programmed ATMEGA328 v6.2

This IC is only for the building kit version. The software can be used with or without TFT display.

ATMEGA328Programmed ATMEGA328

TFT display

TFT LCD module for spotwelderTFT LCD module for spotwelder

There was a lot of demand for the TFT display software, so I made it now. The spot welder board was already prepared for it. Note that the display is optionally:

  • Without display, the weld time can be set by the rotary switch in 9 steps of 50ms. Normally, this is sufficient.
  • With display, the pre-weld time, pause, and weld time and can be set in a large range.

If you program the ATmega328 yourself, you can easily change the weld times in the code.

The new software is suitable for both situations.

Note that the display is much faster than on the video, because of the improved TFT_22_ILI9225 library from Johan Cronje.

Universal Arduino solid state relay / dimmer

This board can be used also as a solid state relays or dimmer for many applications.
The advantages are:

  • Integrated 5V power supply
  • No separate Arduino board needed.
  • Can handle high inductive loads.
  • Zero cross detection.
  • Contains breadboard pads for custom modifications.
  • Optional TFT 2.2" LCD module control.

Microwave oven repair safety precautions

Working on a microwave oven is extremely dangerous. You will normally NOT survive the high voltage, the available power of more than 1000W is enough to kill you instantly like an electric chair. Please read this article first

Microwave-dangerMicrowave oven repair safety precautions


If you have any questions, please post them on

Weld times

  • Weld time: 0ms to 450ms in steps of 50ms.
  • Pre-weld pulse 50ms.
  • Weld pause time after pre-weld pulse: 500ms

Open-source software

The advantage of this spot welder is that the software is open-source unlike other controllers.
Download the software at GitHub.
Here you can also download a project zip file that also contains all necessary libraries.

Circuit description

The PCB contains the entire spot welder controller; there is no Arduino board needed. Solid state relays with normal TRIACs will not work here, because the load is highly inductive. With high inductive loads, antiparallel back to back connected thyristor control works far better than a traditional TRIAC control. Normal TRIACs have turn-off problems with inductive loads, while SCRs always turn off at the negative half-cycle. The RC snubber network R9 and C9 protects the circuit from voltage transient spikes. D1 is a transient-voltage-suppression diode (TVS), without this diode the ATMEGA328 may be damaged by voltage spikes.

Weld current reducing

For welding high sensitive electronic components, it is possible to weld at a lower current by the use of a WH50 power resistor. The resistor value depends on the required current, transformer and mains voltage and must be determined experimentally. Here, the WH50 wire resistor dissipates 600W; this is an overload by a factor 12! For short weld times this is allowed. 
Note: for welding battery tabs, this resistor is not needed.

Arduino spot welder timer circuitArduino spot welder timer circuit

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.

Maximum thyristor current

The maximum RMS on-state current for the BT151-800R is 12A. However, for short weld times we can take the non-repetitive peak on-state current, this is 50A for weld times below 0.5s. For a normal microwave oven transformers the thyristors are adequate oversized. Take stronger thyristors if you are using a large weld transformer.

User guide (only for the OLD spotwelder)

With TFT display:

  • 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 to weld.

Without TFT display:

  • After power on, the LED blinks 4 times to show that the Arduino is alive and than is stays on.
  • Set the weld time by the rotary switch S5: the weld time = position number * 50ms.
  • Press the weld button to weld. To show the welding cycle, the LED is turned off during the weld pulse.

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. To protect the WH50 power resistor (for 600A), don't weld longer than 1 second.
(*note*) In the old software version 5.3, the switch should be in position 1..9. 

Continuously modeContinuously-mode

Modifications for using the TFT display

IMPORTANT: Cut the trace (see below) before you solder the TFT screen!

These parts have to be mounted for the TFT display too: 

  • Menu control tactile buttons S2, S3 and S4. Attention: mount at the other side!
  • Attention: mount the TFT display on the other side! 

TFT displays available on eBay

There are TFT displays with 9 and 11 pins, so there are two different TFT connectors on the PCB.

Note that a link to eBay may refer to an incorrect display if a seller has changed the product in the meantime. Check that the display is 2.0” / 2.2" 176X220 ILI9225 and that the connector pinout corresponds to the circuit. I try to keep the links up to date.

  • TFT LCD Module 2.2" eBay (11pins). It seems that this display is no longer being made.
  • TFT LCD Module 2.0" eBay (11pins).


  • Cut the trace of the solder jumper JP3 (before you solder the TFT screen!), see image 1.
  • Connect a wire from JP6 pin11 to S5 pin1, see image 2.
  • Connect IC1 pin27 to pin28 (this switches the software to TFT), see image 3.
  • Remove rotary switch if it was mounted already, or put it to position 0.
  • LED1 is no longer needed, it still does work but will give less light. You can remove the LED if it was already mounted.

TFT modification 1TFT modification 1TFT modification 2TFT modification 2TFT modification 3TFT modification 3

It has turned out that some TFT displays have a different orientation. Therefore, I made a new software version in May 2017. The orientation can be changed now by pressing the up and down buttons while switching on the mains voltage.


  • Check that the voltage across D2 is 5V.
  • If the LCD display text is upside down, start up while pressing the up and down button.
  • Only for the OLD spotwelder: Set the weld time to maximum. To test whether the programmed ATMEGA328 works, check if the LED blinks 4 times after power up and than stays on. If the weld button is pressed, the LED should turn off a moment. If this works, the fault is in the thyristor section, the zero cross detector, or the switches S1 or S5.
  • If you want to measure the output voltage, use a load, for example a light bulb. Don't use a LED lamp.
  • You can test the zero cross detector as follows: If the spot welder only works in continuous mode but not in normal mode, then the zero cross detector is not working.

Part list

All components are standard; every electronic (e)shop should have them. The total cost of the components is about $20.
You can order all items with just one click. Note that the ATMEGA328 and the TFT display are not on the partlist:

Conrad (send just to Europe): Conrad whish list
I have made a bill of materials (BOM) for Mouser that you can upload to your shopping cart, select Service & Tools > BOM Tool > Upload Spreadsheet and follow the steps. Check if the quantities are correct.
If parts are temporarily not available, you can usually take an equal part that fits to the PCB. The TVS diodes 1N5908 are not necessarily required.

Conrad wish listConrad wish list

(note) If the TFT display is used:

  • Don't buy the rotary switch S5.
  • Buy 1 tactile switch.
  • Don't buy LED1.

If a part is out of stock at Conrad, take the alternative, see below:

Qty Value Parts Description Conrad number / alternative
3 100nF C1, C4, C5 Capacitor 531855-89
2 22pF C2, C3 Capacitor 457167-89
1 100nF 400V C9 Capacitor 459861-89 / 1468330-89
1 1nF C6 Capacitor 451602-89
2 100uF 25V radial C7, C8 Capacitor 445453-89
1 Perpendicular 6p JP2 Connector 733879-89
2 Terminal block 5mm JP7, JP8 Connector 731091-89
1 Molex KK or else JP1 Connector 740155-89
1 Molex KK or else JP1 Housing 740549-89
2 Molex KK or else JP1 Terminal 741112-89
1 16MHz HC-49 Y1 Crystal 155164-89
2 1N4007 D3, D4 Diode 162272-89
1 LED 5mm (note) LED1 Diode 184543-89
2 1N5908 D1, D2 TVS 5V 168279-89
1 MOC3052M IC3 IC 1265985-89
1 78L05 IC2 IC 175676-89
1 ATMEGA328PU IC1 IC 92026-89
1 Bridge rectifier B1 Rectifier 140725-89 / 140730-89
4 330 Ohm R3, R6, R7, R8 Resistor 1089141-89
1 1M R5 Resistor 1089183-89
1 220 Ohm R4 Resistor 1089139-89
1 10k R2 Resistor 1089159-89
1 4k7 R1 Resistor 1089155-89
1 100 Ohm 1W R9 Resistor 419443-89
1 Rotary 10 pos (note) S5 Switch 705684-89
4 Tactile switch (note) S1 (S2, S3, S4) Switch 701749-89
2 BT151-800R / 2N6509TG T1, T2 Thyristor 151330-89 / 151409-89
  Gerth 304.09 for 230V TR1 Transformer 1092870-89
  Block AVB 2,0/2/9 *) for 120V TR1 Transformer  
1 28pin DIL Socket IC1 Socket 183812-89

Mains voltage 230V or 120V

*) For 230V 50Hz, use the Gerth 304.09 transformer 1092870-89 from Conrad. For 120V 60Hz (United States, Canada) , use the transformer Block AVB 2,0/2/9, available at among others Newark and Farnell. Instead of the AVB 2,0/2/9, you can use the AVB 2,0/1/9 too.

For 120V, the optional WH50 power resistor has to be about 10O, if you need a weld current of 600A. 

Solder wire

Today, lead-free solder is standard used in the industry. However, it is not the best solder, it is just less toxic. The best solder for electronics work is still the old fashioned 60/40 alloy. It is much easier to use than lead-free solder and it has excellent characteristics and therefore it is still legal in military and medical applications, where the quality outweigh the health risks.
For electronics work, I recommend: flux-cored 60/40 tin/lead with a diameter of 0.5mm - 0.6mm

PCB assembly

There can't go wrong much, but you need to use a soldering iron for electronics work and work accurately, see the images for the result. 
The middle pin on the 9V side of the transformer Block AVB 2,0/2/9 has to be cut, otherwise it will not fit to the board.

ComponentsSpot welder components (not complete)Eagle-TSTop side
Eagle-BSBottom sideSpotwelder controller top sideArduino solid state relay top side
Spotwelder controller bottom sideArduino solid state relay bottom sideFTDIThe FTDI connector should be angled
If you don't want to hand solder these boards yourself, Quick PCB Assembly's prototype services can do the job for you at an affordable price. They are located in the USA.

Wiring and mounting


Mounting1Mounting the PCB in the cabinet

The Arduino chip can be programmed in three ways:

  • Just buy the programmed chip here.
  • With an Arduino board by which you can program the chip yourselve.
  • Using a FTDI adapter, see the picture. Google on "FTDI USB To TTL Serial Converter Adapter Module For Arduino". With this adapter connected, the spot welder controller board behaves as a regular Arduino Uno and can be programmed in the same way as the Arduino Uno.

FTDI-programmerFTDI programmer