Sunko 788H battery tab spot welder repair and improvements - Page 1
Forum section: electronics / repair

author: garyinlyons
author_rank: Newbie
timestamp: February 23, 2018, 11:45:46 pm
content: I bought a Sunko 788H in early 2016 and the first one just blew fuses so I exchanged it for another and it would blow the circuit breaker quite often. It is a 120VAC version and suffers from severe inrush current as the high current transformer has two windings in parallel and initially looks like a dead short until the flux builds up and increases the primary impedance. Finally after making a 15 cell NIMH battery pack the TRIAC blew up. I replaced it with higher current capability BTA41 41 amp unit and it blew up two of these when turning on power. I bit the bullet and added in rush current limiting and other mods as described in the attached PDF I generated. It works better than ever now and just purrs like a kitten! The PDF shows what I did. I cobbled together the modifications from parts I have in my lab. I put the TRIAC on the back of the PCB on a bigger heat sink and added a cooling fan, new knobs that actually fit the pot shafts, replaced the fuse holder and it is all mo betta. In my testing I used a variac to an isolation transformer so I could hook my expensive equipment up to this dangerous beast and made sure no potentials nor currents could flow between my equipment ground and this unknown design. I wish you could get the schematics but I have found nothing on the net. There are many ways to do what I did. I just used what I had. If you attempt this modification be very careful!!! Let me know if any one wants more info.

author: Armadillo
author_rank: Super Contributor
timestamp: February 24, 2018, 12:03:38 am
content: Thanks for the circuits;Mine has an output of 500A max printed on the box.interestingly, I don't have anything blew up as yet luckily.But that fan is outrageously obstructing. Can it be flatter type.I would study the improvement circuit to implement but not the fan. Thanks.

author: garyinlyons
author_rank: Newbie
timestamp: February 24, 2018, 12:41:15 am
content: The fan can be a box fan like on a desktop or tower PC. I used the fan as it just fits over the air vent and I did not have to buy one. You could use a mains AC voltage fan and avoid the wall wart also. Is your unit a 220Vac or 120Vac? The 220Vac model has the primary windings in series and the 120Vac has the two primary windings in parallel and half the D.C. resistance of one winding and higher in rush current.

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 24, 2018, 04:02:56 am
content: Hey how about a complete replacement of the pcb that I had designed.https://georgehobby.wordpress.com/2018/01/27/maxweldv2/Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869s

author: garyinlyons
author_rank: Newbie
timestamp: February 24, 2018, 02:26:19 pm
content: Nice looking design! I basically did my fix for fun andno costexcept the TRIACS. One of the problems with the original circuit board is that at power up it turned on the TRIAC from what appears as transients in the gate control circuitry. They used a logic level controlled TRIAC to control the main TRIAC(about as far as I got dissecting the design). This turn on transient seems to be why the unit blew the replacement TRIACS. The in rush time delay could also be implemented with a time delay relay. I also played with the idea of making a spot welder using lower voltage DC and a number of power MOSFETS in parallel. Use a high current half bridge gate driver like a IXYS IXD_604PI 4A or a Analog Devices ADUM32223ARZ 4A unit(the total gate charge is pretty large and to get fast switching times you need high current gate drive) and use a bank of gell cells or Lion 18650's(3500maH 18650's with 10 A discharge are easy to get). I much prefer switching D.C. than A.C. When you consider the frequency of 60Hz - 16.666mS per cycle and high peak voltages I am way more comfortable and can, I think have more precision control with low voltage high current D.C. I am thinking of using an Allegro Hall Effect Current sensor(ACS756SCA-100B-PFF-T) for feedback to servo the setpoint current and a Arduino DUE or some similar controller for timing pulse width, number of pulses, current setpoint and user interface. Thanks for the link to your design.