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E-6E-6

TROUBLESHOOTING & REPAIR

FIGURE E.5 SCR OPERATION

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SCR OPERATION

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 sup- ply. 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 controlled 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 cathode voltage. Since there is a standard PN junc- tion 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 contin- ue the flow of gate current. As long as current contin- ues 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 volt- age passes through zero into the negative portion of the sine wave. If the SCR is turned on early in the pos- itive 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.

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Lincoln Electric SVM155-A service manual Figure E.5 SCR Operation
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SVM155-A specifications

The Lincoln Electric SVM155-A is a state-of-the-art welding machine that excels in versatility and performance, making it ideal for both professional welders and hobbyists. This multi-process welder supports MIG, TIG, and stick welding processes, allowing users to tackle a wide range of applications with a single device. Its user-friendly design and advanced technology make it an exceptional choice for those looking to enhance their welding capabilities.

One of the main features of the SVM155-A is its robust power output, which can handle welding materials ranging from thin sheets to heavier stock. The welder offers a high duty cycle, ensuring that users can work efficiently without the need for prolonged breaks. This is especially important in industrial settings where productivity is key. Additionally, the SVM155-A is equipped with a digital display that allows for easy monitoring and precise control of the welding parameters, ensuring consistency in weld quality.

The European-designed INVERTER technology embedded in the SVM155-A significantly contributes to its lightweight and compact design. Weighing in at only 19.4 lbs, this welder is portable and ideal for fieldwork or workshops where space is limited. The inverter technology not only reduces the size and weight but also improves energy efficiency, leading to lower operational costs over time.

The SVM155-A also features advanced safety mechanisms, including over-current protection to safeguard the machine from potential damage during excessive use. It also includes a thermal overload shut-off, which prevents overheating, ensuring a longer lifespan for the welder.

Another standout characteristic of the SVM155-A is its exceptional arc stability and ease of use. The machine is designed to provide a smooth and consistent arc, reducing spatter and ensuring clean welds. Moreover, the welder is compatible with a wide range of materials, including stainless steel, aluminum, and carbon steel, further enhancing its versatility.

In conclusion, the Lincoln Electric SVM155-A is a powerful, durable, and multifunctional welding machine that incorporates advanced technologies to meet the needs of modern welding applications. Its portability, safety features, and ability to deliver high-quality welds make it an excellent investment for anyone looking to improve their welding skills or expand their capabilities in various welding projects.
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