DIY Spot Welder – Max Weld


The previous version of the spot welder was limited to work only on 230vac. This could be modified to be used with 120 as well by an experienced person but maybe not that easy for the novice users.


Max Weld now incorporates a jumper to select 115/230vac with the following new features

  1. 120/240 voltage selector
  2. Micro Weld resistors or fuses for 18650 packs
  3. LED indicators for PULSE, READY and POWER
  4. Dual SCR design
  5. Input and output are fused
  6. TVS diodes protection D10, D11 and D13
  7. AC/DC Optical isolation
  8. Optional RC grade servo connector
  9. Wide range weld time from 10ms-500ms
  10. Infinite Weld cycle
  11. On-board serial programming port
  12. 2 (line) x 16 (character) I2C LCD
  13. Precision controlled servo Trim pots
  14. Audible feedback
  15. 4 modes of configuration
  16. All values are saved to the EEPROM except for the Weld Time


Features in brief

Dual Thyristor design

Unlike the previous version that uses a TRIAC which is designed to conduct in both directions in a AC circuit which makes them ideal for any AC load control. Max weld has two SCRs rated 16A each. The two scrs in a back to back configuration makes the overall design more robust and reliable which is controlled by just one optocoupler making them ideal when driving inductive loads such as Motors and transformers.

Here our load, a transformer, is highly inductive hence we need the load to switch OFF at the opposite cycle when it was switched ON, also In-rush current that needs to be reduced. In other words if the LOAD was switched ON at the positive half cycle then the LOAD has to be switched OFF at the Negative half cycle. An SCR will only conduct for one cycle at anytime on the other hand a TRIAC will conduct for the next half cycle also there by making them less suitable for this application for an inductive load but maybe achieved by other methods. Read more about In-rush current.

 RC grade servo to lower/raise weld head (optional)
For those with a creative mind, Max weld has an optional connection for a RC grade Servo which can be used to lower and raise the arm of the spot welder making it ideal and a low cost alternative to the pneumatic spot welder where a compressor is used. A servo has enough torque to raise a 3kg load or more. All it needs is constant current supplied by a uBEC or anything similar that can provide a constant current. The code within the Atmgea328P precise control of the servo making it the best of its kind for precision spot welding.

Wide range weld time from 0ms – 1000ms
Max weld spot welder has a wider range to control the weld time from 0ms up to 1000ms (when using welding time above 300ms, monitor temperature of the SCR and if required add suitable heat sink or a fan within the enclosure) with increments of 10ms there by making it ideal to control the current for higher rated MOT’s. On an inductive load, the last turn-off polarity influences the next turn-on, hence switching on the MOT at increments of 20 helps with this phenomenon. More about ‘Reverse polarity ‘ follow the link to page 4 & 5.

Spot welding fuses or resistors on 18650 packs ?

No problem Max Weld can be used as micro welder too. Just dial down the W2 pulse and set the desire W1 pulse in config mode and begin. 5ms would be something to begin with and 10ms can blow the leads of the devices.

Infinite weld cycle – auto welding
This feature helps in building battery packs for e-bikes and other battery powered gadgets. Once the weld button is held down for more than 2seconds and released welding begins and continues to weld with a 800ms delay between each weld cycle.

On-board Serial programming port
Max Weld can be programmed as needed with the on-board serial port making it more versatile and customisable with any standard USB-Seral connector or with the Arduino. There should not be any IC on the Arduino board itself and the mains power to this PCB needs to disconnected when using this port with a 5v supply from the PCB or ignore the Vcc when powering the board. A FTDI interface such as this can be used with the appropriate driver installed thereby omitting the need for an Arduino board. When serial connector the CTS pin should be  grounded.

Arduino compatible 16×2 I2C LCD
The previous version of the spot welder had 8 LED’s to indicate the weld time from 100 to 450ms. This limited the option to set weld times less that 100ms. The LCD now shows

  • The current mode of operation
  • Pre-weld time (W1), pause time (P) and the Weld pulse set (W2)
  • Weld arm (servo) position

Audible feedback
An audible feedback using a buzzer indicates when an operation is completed and when the weld time is incremented or decremented for W2.


The working of this PCB is no different from the previous version. There are three terminals namely

  • Mains – to power the PCB.
  • uBEC pwr is an 12v 8A rated ac to dc converter which powers the uBEC (DO NOT CONNECT THE uBEC DIRECTLY TO THIS TERMINAL).
  • LOAD to which the MOT is connected to.

Setting up for first time use

a. Setup W1 and W2 pulse time


Assuming that you’ve already setup a weld table on a plane surface and the welding machine is upright and leveled to the table we can go ahead and setup the welding process.

A good weld is achieved by adjusting the weld time and adjusting electrode pressure (which i will be explaining latter). The Pre-weld, otherwise know here as W1, is the important weld in a dual pulse spot welder. To get a near accurate pre-weld, in the normal mode or the manual mode turn the knob to reduce W2 time to 0ms now the second weld (W2) is disabled. Now enter the configuration mode and go to option W1.

Here set the minimum W1 time (1ms) to start with. Save the configuration. Leaving W2 at ‘0’ do a test weld and see if it welds but can be removed easily. If it is a weak weld then you increase W1 time to get a weld that is easily removable with your hand.

The W1 should not weld the nickel tabs strong, it is just to clean the weld tabs for the second weld W2. Once you have the appropriate W1 time then you can go ahead and set your W2 time as well. Increase it from 0 to a spot where your electrodes do not stick to the nickel tab after the weld and that you get a strong weld.

Do not over power W2 as increased weld current can make a hole on the cell causing chemicals to spill out, the cell may explode due to high current as well.

I’ve found that the below time gives a good and strong weld nugget.

W1: 8ms
W2:  20ms

b. Set the servo arm
The Servo should be mount separately that would hold both electrodes such that servo arm moves the weld electrodes up and down. The up travel, the ‘End point’ can be adjusted to any point from 0 to 130 where 0 is disabled. The same goes with down travel that is the ‘start point’. Here the start point should be adjusted carefully as that is when the electrodes make contact with the nickel tabs and the down pressure is very important for a weld nugget to be formed. Adjust the start point such that there is a little pressure when the electrodes are touching the nickel tabs.

Modes of Operation/Working

The default screen after initializing shows name of the product, Pre-weld (W1), Pause(P),Weld (W2) time and the status of the PCB.

Following are the modes of operation,

Mode 1 :- Manual Weld
Press and release the weld button to do single welds. During this operation a primary weld ‘W1’ is done to prepare the nickel strip for the next weld. After 450ms pause  ‘P’ a second weld ‘W2’ is done depending on the weld time set by the POT.

Mode 2 :- Automatic Weld
This weld has the same operation as the Manual Weld with an added feature to do multiple weld with a 2 second delay between each weld cycle (one manual weld is consider as one weld cycle). This is initiated by holding down the weld button for more than 2sec and not more than 5 seconds to activate. Holding down the weld button for more than 5 Seconds would activate the Servo Preset mode.Note that in this mode the transformer and the electrode may get hot. If noticed, wait till temperature subsides and then continue.

Mode 3 :- Configuration MODE


Hold down the weld button till 5 beeps are heard. This mode basically has three subcategories

  1. Set Start point:


This mode is activated by turning the knob to Start. This mode sets the down travel of the servo arm, in other words how much should the weld head move before the it makes contact with the Nickel tab. The adjustment of the start point is done through the potentiometer R15. Once the desired travel is set press the Weld button to save the value to the EEPROM. To exit without saving set the value as same as the stored which will exit to configuration mode.

  1. Set End point:


This mode is activated when knob reads 2 which is used to set the upward travel of the weld head. The End point or the return travel is adjusted via potentiometer R14. Pressing the weld buttons saves the current value.To exit without saving set the value as same as the stored which will exit to configuration mode.

  1. Testing the Start and end point:

This mode simply tests the above two configuration by activating the servo. Moves the servo arm down and returns after a second.

  1. Set W1 time:


This mode sets the pre-weld time w1. The first pulse removes surface inconsistencies and contaminants. The pre-weld setting is done by using the potentiometer on the PCB. Pre-weld time can be set from 0 to 50ms. Unlike other modes this setting is not saved instantly. To save the setting press the weld button and the setting is saved to the EEPROM of the ATmega328 and that is permanent even if the PCB is reset. Writing to the EEPROM is limited to a certain number of times hence use them wisely.

Serial – USB Programming port
Switch off power to the PCB always when programming, failing to do so would activate the SCRs to conduct continuously, if left un-noticed the SCRs and transformer would over heat.

The wiring from the PCB to the Arduino are straight forward as labeled
RST – To REST pin on Arduino
RX – To RX pin on Arduino
TX – To TX pin on the Arduino
Vcc – To 5v on the Arduino
GND – To GND on the Arduino

Test Weld results

Max Weld housing

“This blog or my self will not be held responsible for any mishap/accident caused during operation of the above PCB. The PCB handles Mains power hence avoid contact on the solder side of the PCB. If you are not confident on how to use this PCB, please seek advise from an expert.”

Assembled PCBs on sale




23 thoughts on “DIY Spot Welder – Max Weld

  1. Hello George, got notified by customs office of the arrival of the kit today and rushed to pay the kings share of the bounty. Now that I have it here I went on and wired the mains and the lads to the mot that does not have the secondary winding yet. Plugged it in and the display lit as well es the led the chirp was there also, all looked good until I pressed the button to fire the scr’s it flashed some sparks for a fraction of a milisecond and the board went dark since both 5A fuses did what there is to do when the current is too high. Disconnected the primary coil of the mot and replaced fuses, first the one on the input and found the mcu/display still in good working shape. Replaced the output fuse and now do not dare to fire it up with the unloaded mot. Maybe a bulb as resistive load? You sure have experienced the way the controller and scr behaves when a idle mot gets the pulses with no load, is it ok to blow the 5A fuses? Both? Can you lease advise how to proceed? I’d like to see the schematic without reverse engineering the board. Did I miss it or is it not available on your site?

    Thanks for the work.




      1. Was not aware of this “feature”. Scared me when the fuses lit up and all the lights on the board and display went dark with an idle mot. Did not pay attention to the timer setting before pressing the pedal switch since there was no secondary winding I naively assumed no harm to fire any number of pulses since no load on the transformer. By the looks of it the first magnetisation of the core sucks enough amps to melt the wire in both 5amp fuses. Then again nice that only the fuses needed replacement and other components as well as the copper traces on the board suffered no harm.

        Liked by 1 person

  2. Hi George. is it possible to buy only the pcb? this would be a good thing because I could update my card to this new version by limiting the expenses. Best regards.


    1. I have no problem with a 230 v version it will be closer to 240 here. I can either change it or but in a double pull breaker. I’m an industrial electrician we just hook up controls not design them. I have some understanding but not enough to build circuit board. Is this version cheaper or quicker delivery? What is the max amp draw at 240v and what is the continuous amp draw. I want to make sure I get the correct breakers. I also would like to know max weld power in watts. Ideally I’d want fuses in place to go before the breakers does. Is the power going to the mot ac or dc if ac I need a bridge rectifier how many amps volts is about 2 or less u said. I believe this can be underated some because such a short pulse. I will also need details on display specs and what I need to tie together. Please advise me of above info. Do u have a schematic drawing of complete layout? Not your board but a set up drawing for all different components. What is return policy if it doesn’t work like u say. If I fry it I will buy it np there.



      1. Heath i doubt if you have got the whole idea of a spot welder wrong or i may have misunderstood when you said “Is the power going to the mot ac or dc”.

        Max-Weld’s total amp draw would be less than 1A. Max-Weld is just a second person with a push button-switch in hand knowing when the zero cross of a sine wave happens and fires the MOT. So basically your power consumption questions all depends on the MOT your using. My MOTs power consumption would be different from yours all depending on how you wire them.

        The MOT is supplied with AC voltage through Max-Weld PCB. There wouldn’t be any need to wire a rectifier after the MOT.
        As far as wiring goes, the connector marked “Mains” connect the wall socket to this. Next you can connect a mains powered FAN to the “uBEC “socket or leave it unconnected. Next is the “LOAD” which is where you connect the MOT. That is all to it.

        You can return the PCB anytime within 30days and will be replaced with another or money-back as you choose. The return policy does not cover shipping or damage to the board or components rising from the end users mishandling.

        Hope i’ve answered your query. Do let me know if i can help.


      2. You did answer some but not all. Is the 230v version have a different price and any difference in availability?

        I currently weld tabs with a 200 ccs 12 v battery with only 2 electronic controls a on off switch and a momentary switch that controls an old time vehicle solinoid. This leaves a lot of room for human error but can be done with practice. So I see your board eleminating potential human mistakes and replaceing them with electronic controls. I assume your 2 arc adjust input power to make main load power what u want it to be which also works with the pulsed times. They could compensate for each other to a certain degree. But u said only 2 variables the 2 times. I am looking for power adjustment and time adjustment. Now 16 amp max on 120 and 240 are going to be exactly the same close but not. Of course u can move 240 through your board with less amps so u should have less heat and less where and tear heat bad. This is why u do all the controls in higher voltage ac instead is on low voltage dc not to mention the price difference it would cost to build.

        I don’t know much about hertz and waves but I do know v*a= watts. Power to weld does not change because of mot assuming the effiency is the same between them. The variables are more about temp, tab thickness, battery, material of tabs battery wall thickness, amount of pressure on the electrodes, size of weld and resistance from transformer to electrodes. It very possible I am way behind on circuit tech but that how it is to me anyhow.

        So price and availability. Adjustable power to the load pulse or not? How is your board protected fuse circuit breaker. Do u lose efficiency by not rectifying dc power to the electrodes on secondary side. This is assuming mot is the last device before the load. I trust u if my circuit Tec is not right but I see it that way, because i don’t know it at all. But that last paragraph has my questions in it along with 2 at the beginning.

        The next variable I would would look at controlling for me. Would be electrode pressure with a pressure or limit switch pressure switch.

        Thanks Heath


      3. Max Weld works on both 115 and 230v just place the jumpers and your good to go.

        I think you’ve got the whole ac spot welding completely wrong. ac spot welding works only on the ac current and nothing to do with dc. Here the load is a Microwave Oven Transformer.

        Let me explain how it works. Max Weld is powered by the mains connected through the Mains terminal on the PCB. The primary of the MOT is connected to LOAD on the PCB and the secondary of the MOT is the one that holds the Electrodes used for spot welding. For comparison, Your 200 ccs 12 v battery with only 2 electronic controls a on off switch and a momentary switch that controls an old time vehicle solenoid is taken care by the Max Weld PCB. The MOT is the thing that produces the Amps. Max Weld sends a dual pulse that controls the time delay to switch ON/OFF the MOT, here from 0 (disabling the second weld all together) to 1000ms. On an average the weld current would be around 200A to 300A probably even lower when spot welding Nickel Strips to 18650. These figures can easily double depending on the resistance of the Nickel Strip and the time.

        Here are some test
        @10ms 2.266A 543W
        @200ms 9.7A 2,328W

        10A is the max i can go with my DMM beyond that would blow the fuse on the meter.

        Max weld does not incorporate power control just because the variable time is low enough to control the power output at the same time.

        Hope this helps.


      4. Hi heath,

        A quick correction on the readings.

        Earlier i calculated the watts with 240VAC which was wrong. The voltage at 40ms is 1.2VAC and the amp drawn by the MOT is 1.7A so doing the math it would be 2.4Watts


  3. I am very interested in this product. What is the lead time? How many watts are recommended from the mot. How many mots and preferred volts. What kind of initial set up support is there. Warranty and how many expected cycles per life? How much is the LCD?


    1. Hey Heath thank you for showing interest on Max Weld v2.0

      Max Weld is rated for 7.6kW, that is, the SCR are rated for 16A each. Recommended Watts for the MOT is 1000W but can work with even lower as well. Use the thickest cable available to wire the secondary and most of them should produce a max weld current of 800A with an open voltage of 1.2v which would be ideal for spot welding. Max Weld is configured before they are shipped and the only setup that would be required is the pre-weld pulse (W1) and the servo travel, that is if you plan to use a servo other than that there is nothing more to it. The W1 pulse set at 15ms which should be ok to begin with. Read the blog how to do an initial setup of the product where i have explained how to find best the time.

      The life cycle depends on several factors such as ambient temperature and usage. To my tests they have lasted for more 1000 welds but they last much more than that. Max Weld will last longer as long as the SCR do not over heat and keeping a FAN in the enclosure would allow adequate air flow to cool the product during operation.

      Depending from where you buy the LCD, they would cost as low as USD2.17 or more.


      1. The old version actually had a limitation on the possibility of changing the welding times. Thanks to your support we managed to find a good compromise.


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