DX-YQ050 Pulse Spot Welder - What purpose does SCR serve? (and other questions) - Page 1
Forum section: electronics / repair

author: M1010
author_rank: Newbie
timestamp: August 09, 2024, 08:41:22 pm
content: Hello. Long-time lurker, first-time poster.A while back, I'd acquired a pulsed arc spot welder in need of repair. In the process of repairing it (and because of some early safety concerns), I made a schematic and wiring diagram. It's finally working, but I could use some help.From a safety standpoint:A) The fuse is fusing a portion of the neutral side of the plug (110VAC USA plug). Makes no sense to me, and I plan to move it to the "hot" side of the plug unless there's a good reason not to.B) The ground (aka earth) wire was not stripped nor connected to anything. This will be connected to the chassis.C) The fuse was missing (no label to indicate type & size). Based on the minimum chassis wiring of 24AWG going to transformer T1, I installed a 3Amp fuse. I'm reasonably happy with this, unless someone has a good reason it should be something else.From a functional standpoint, can make sense of most of it (which I've summarized below) but I don't understand the section with the SCR.My understanding of it's workings:With power supplied, rectifier D2 is providing 167VDC to Linear regulator Q4 and Transformer T2 (rectifier D1 is supplying 26.4VDC - not sure why it is included in the design).When the pedal switch at J2 is pressed, relay K1 is picked up (via Q12, and a moment later (~5ms) Q11 turns Q12 back to off) and closes N.O. contact K1 which turns on MOSFet Q6 (and also provides power to "current adjust" circuit). This allows Transformer T2 primary current to suddenly rise. This turns on Q1 and pulls Q6's gate to ground ceasing conduction of T2 primary through Q6. The secondary of T2 is tied to the welding electrodes and causes the spot weld to take place.To achieve "current adjust", one leg of the output of T2 is also tied to the "current adjustment" portion of the circuit (via diode D9). Current adjustment appears to be accomplished by turning the two MOSFets on in "box 2" (ref connectors JU5 and JU6) which shunts some of the output of T2 back to the negative side of rectifier D2. Q9 clearly controls these two mosfets.As mentioned earlier, when K1 contact closed, power is provided to the current adjustment circuit. R14 turns on NPN transistor Q9 which causes the mosfets in "box 2" to shunt the secondary of T2 to the negative of D2 through R8 and R10. R8 and R10 provide a small resistance which provides the voltage to trigger SCR Q3. SCR now might stay on via R14 (if it provides minimum current - exact identity of SCR unknown) and will control the base of Q9 (thus, controlling Q9 and the "shunting" mosfets in "box 2"). Approximately 0.17ms to 19ms (depending on setting of current adjustment pot) after K1 contact closed, voltage will be applied to the gate of Q10 causing it to conduct and pull the gate of Q9 to the negative of D2 stopping the shunting of weld current and allowing the remainder of the output pulse from T2 to perform the weld.Please note, many devices had the numbers sanded off. In those cases, the device was removed and tested and roughly identified with an "MTester" component tester.[Side Note: I suspect the first weld may be slightly shorter than subsequent welds. This is due to C12 being at 0V on the first weld, but on subsequent welds, it would only be discharged to roughy 0.6v via diode D6. This would reduce the time to reach the zener breakdown of D7 on subsequent welds. Thus the "shunting" would be stopped sooner on subsequent welds.]This is where I need help.1. I can't quite figure out what the purpose of the SCR is (it seems Q10 is responsible for the timing and control of Q9)?2. What is the purpose of rectifier D1? (i.e. what does providing 26.4VDC accomplish?)3. What is the purpose of diode D9 in the welding circuit?