Lincoln Electric SVM188-A service manual SCR Operation

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E-5	THEORY OF OPERATION	E-5
	FIGURE E.2 - SCR OPERATION

SCR OPERATION

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A silicon controlled rectifier (SCR) is a three-terminal device used to control rather large currents to a load. An SCR acts very much like a switch. When a gate signal is applied to the SCR, it is turned ON and there is current flow from anode to cathode. In the ON state the SCR acts like a closed switch. When the SCR is turned OFF, there is no current flow from anode to cathode. Thus the device acts like an open switch. As the name suggests, the SCR is a rectifier, so it passes current only during positive half cycles of the AC supply. The positive half cycle is the portion of the sine wave in which the anode of the SCR is more positive than the cathode.

When an AC supply voltage is applied to the SCR, the device spends a certain portion of the AC cycle time in the ON state and the remainder of the time in the OFF state. The amount of time spent in the ON state is con- trolled by the Gate.

An SCR is fired by a short burst of current into the gate. This gate pulse must be more positive than the cath- ode voltage. Since there is a standard PN junction between gate and cathode, the voltage between these terminals must be slightly greater than 0.6V . Once the SCR has fired, it is not necessary to continue the flow of gate current. As long as current continues to flow from anode to cathode, the SCR will remain on. When the anode to cathode current drops below a minimum value, called holding current, the SCR will shut off. This normally occurs as the AC supply voltage passes through zero into the negative portion of the sine wave. If the SCR is turned on early in the positive half cycle, the conduction time is longer, resulting in greater SCR output. If the gate firing occurs later in the cycle, the conduction time is less, resulting in lower SCR output.

DC-600

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Contents DC-600 ISAFETYi Can be dangerous SafetyElectric Shock can kill Fumes and GasesFor Electrically Powered equipmentSûreté Pour Soudage a L’Arc Précautions DE SûretéElectromagnetic Compatibility EMC Safety Master Table of Contents for ALL Sections Table of Contents Installation Section Technical Specifications DC-600 InstallationSafety Precautions Electric Shock can KillSelect Proper Location Fuse and Wire Sizes Electrical Input ConnectionsGround Connection Connection for Highest Rating Plate VOLTAGE, 50 or 60 HZ Reconnect ProcedureConnection for 440 Volts 50 or 60 HZ V 60HZ See Machine Rating Plate for Required Input Supply Voltage Output Connections ELECTRODE, Work and #21 LeadConnections Auxiliary Power Control Connections Terminal Strip 1 T.S.1Terminal Strip 2 T.S.2 DC-600 Table of Contents Operation Section Operation Safety PrecautionsDesign Features Advantages General DescriptionRecommended Processes and Equipment Output Control Output Voltage Current Meanings of Graphical Symbols on Case FrontOFF Input Power Welding CapabilityControls and Settings Meaning of Graphical Symbol for Ground CON- NectionOVERLOAD, OVERCURRENT, and Fault Protection Auxiliary Power in MS-RECEPTACLERemote Control of Machine Operation Welding Procedure RecommendationsOperating Steps Remote NA-3 Automatic Wire FeederLN-7 and Other Constant Wire Feeders LN-8 or LN-9 SEMI-AUTOMATIC Wire FeederDC-600 Table of Contents Accessories Section Field Installed Options AccessoriesWire Feeders and Tractors Factory or Field Installed OptionsFigure C.2 MULTI-PROCESS Switch Cable Connections Accessories Table of Contents Maintenance Section Maintenance Safety Precautions Electric Shock can KillRoutine and Periodic Maintenance 1TABLE of CONTENTS-THEORY of Operation Section E-1 Input Circuit and Main TRANS- Former Theory of OperationFiring CIRCUIT, Control CIR- Cuit and Rectification RectificationFiring Board Thermal Protection SCR Operation DC-600 1TABLE of Contents Troubleshooting and Repair F-1 HOW to USE Troubleshooting Guide 2TROUBLESHOOTING and REPAIRF-2PC Board Troubleshooting Procedures Troubleshooting and RepairControl Feeder. See Protective Devices Check all three phase input Output Control J1,Pin2. See Wiring Diagram Wiring Diagram PC Board Troubleshooting Guide Test DescriptionMaterials Needed PC Board Troubleshooting Guide Figure F.2 Control Board LEDs Control BoardDC-600 Internal Trigger Circuit Test Internal Trigger Circuit Test Procedure75 76 Internal Trigger Circuit TestDC-600 Firing Board Test Firing Board Test Control Board Test Figure F.7 Control Board Control Board TestMain Transformer Voltage Test Figure F.8 Input Contactor and Reconnect Panel Main Transformer Voltage TestFigure F.9 Transformer Secondaries Main Transformer Voltage Test Control Transformer Test Control Transformer Test ProcedureInput Contactor Test Voltage Test Input Contactor TestSCR Bridge Test SCR Bridge Test Anode Cathode Remove Insulating PaintActive Test Scope Settings Maximum Output Setting no Load Minimum Output Setting no Load 35TROUBLESHOOTING and REPAIRF-35 2V/Div Volts DC-600 Input Contactor CR1 CLEANING/REPLACEMENT Input Contactor CLEANING/REPLACEMENT Procedure Cleaning ProcedureContactor Replacement Procedure SCR Rectifier Bridge Removal and Replacement Procedure SCR Rectifier Bridge 42TROUBLESHOOTING and REPAIRF-42Removal and Replacement Procedure Removal ProcedureInsulator Lock Washer Plain Washer Output Snubber Lead Replacement ProcedureDC-600 Main Transformer and Choke Removal and Replacement Main Transformer and Choke Removal and ReplacementRemoval Procedure Weld Maximum Open Circuit Voltages Input Idle Amps and WattsDC-600 Table of Contents Diagram Section Electrical Diagrams Wiring Diagram 230/460/575 Volt Machines G3506-1 Schematic Complete Machine G3575 Schematic Control PC Board G3408-1 Control PC Board Assembly G3409-1 Schematic Firing PC Board G3741-1