A welding machine is an electrical device which is able to generate a very high current at relatively low DC voltages. This high current output can be suitably used for creating the intended welding arcs and welding joints. The welding joint is created by fusing the welding rod on the joint area through a high current shorting generated by the welding machine.
A small welding machine can be built using a few ordinary 5 amp transformers and a few high current bridge rectifiers. I have explained how to do it.
As shown in the following figure, we have used 4 nos of 25-0-25V 5 amp transformer in parallel to generate a reasonably good 20 amp current for the welding purpose.
This is recommended for small welding joints only.
The secondary of the transformers can be seen connected with high current bridge rectifiers in parallel.
The bridge rectifier convert the AC to DC and additionally boost the 25+25 = 50 V into a higher peak level of 50 x 1.41 = 70 V
So overall the output of this small welding machine circuit is able to generate 70 V at 20 amp which is equal to 70 x 20 = 1400 watts of power, enough to create strong welding arcs over small joints.
The bridge rectifiers must be rated at 10 amp each.
Either you can use 10 amp diodes to build the bridge rectifiers or you can ready made 10 amp bridge rectifiers modules for the assembly.
Using Capacitive Method
Caution: The following capacitive welding circuit is extremely dangerous to touch since the entire circuit is not isolated from mains AC and therefore it strictly not recommended for welding purpose.
Read it only for educational purpose and for gaining knowledge about a capacitive high voltage high current generator circuit.
A homemade small scale welding machine circuit is what most of the new hobbyists and mechanical engineers would be looking at for solving their occasional work bench metal welding jobs.
A mini welding machine without using complex circuitry could probably be built using a capacitive power supply as shown in the following diagram:
CAUTION: The mini welding machine circuit shown below is not isolated from mains and has the potentials to kill a person within seconds, therefore extreme caution is advised while handling this equipment in the powered condition.
The idea shown above is an ordinary capacitive power supply circuit incorporating extreme capacitors in terms of their values.
Circuit Operation
At the input side we can see a formidable 500uF/400V capacitor, while on the output side also a similar rated capacitor can be seen positioned for reinforcing current.
The most basic parameter essential in a welding system is a high current, so that an extremely high temperature can be formed at the short-circuit junction, over the metal joint in question.
This high current generation can be achieved either by using a high watt transformer or an SMPS version of the same which I have explained in the first paragraph.
A transformer could be too bulky and heavy, while the SMPS circuit too complex for the newcomers, the only alternative way to achieve the high current welding through a relatively simpler design is perhaps by employing the high current capacitive power supply as shown above.
The 500uF/400V capacitor can be expected to generate bursts of current upto as high as 36 amps @ 220V, and reinforced with the complementing output filter capacitor this current can be expected to do some serious welding actions.
You can verify the above mentioned specs by using the following two calculator software:
The shown push button enables the user to achieve the welding job through shorts bursts, and not through a continuous arcing, which can be dangerous, and anyway is not recommended in welding operations.
The input 500uF/400V capacitor looks massive and it might not be readily available in the market, therefore this can be built by using 500 numbers of 1uF/400V PPC capacitors wired in parallel, this could occupy some space, but still the method is easily achievable.
Use Non-Polar Capacitors
This capacitor needs to preferably a non-polar capacitor, however since a diode is positioned in series means an electrolyte capacitor could also serve the purpose without issues.
The second capacitor at the output side can be an electrolytic type for sure.
For more current, the values of the caps could be increased to higher limits, that's the only parameter that needs to be focused on.
Advantage and Disadvantage
The advantage of this circuit is that it is small, cheap, and can be quickly built and used. The disadvantage is that it is very dangerous since there may be AC voltage at the output, therefore you will have to handle the whole system wearing rubber hand gloves.