The term OCV is an acronym for Open Circuit Voltage. It is the unloaded DC voltage that is present at the Weld output terminals of a Welder. This voltage normally averages between 60-90 VDC in STICK (SMAW) mode of operation. The one exception to this (where an AC voltage is present) occurs when dealing with a Transformer based STICK Welder (commonly called a Buzz box) which has no DC rectification of the transformers secondary voltage. In other modes of operation (Weld processes) such as MIG (GMAW), or TIG (GTAW), the OCV will be different to that specified in STICK mode. The Weld process that is chosen is based on the functionality of the Welder. Not all Welders are capable of multiple processes. For example, simpler Welders have STICK mode of operation only.
When Trouble Shooting an SCR (Silicon Controlled Rectifier) based Welder, an OCV measurement provides a valuable tool towards an effective repair. The first step in this process is to consult the Welders Service Manual. The Specifications page will show the Maximum OCV for the chosen Weld process. Record this voltage!
Next, consult the Schematic to reveal the number of SCR’s in the design, and the effective number of switched current paths. Record the number of current paths! In this case, a current path is a branch from the Transformers secondary winding, through an SCR (Anode to Cathode), and culminating into a current node prior to filtering by the output Inductor (Reactor). The current node usually has multiple branches feeding into it with one dedicated output.
Using a DVM (Digital Volt Meter) measure the OCV of the Welder. If it is close to the Maximum OCV then this indicates that all SCR’s are firing (triggered from an applied gating voltage), and would be an ideal situation. If not, divide the Maximum OCV by the number of switched current paths. This number represents the Voltage that each SCR or current path provides as part of the design. Subtract this value from the Maximum OCV. If the value (Voltage) equals the Welders measured voltage then one SCR or current path is faulty. If not, continue this process for the number of current paths that are being used.
An example of this is a Welder with a Maximum OCV of 60V produced by synchronously firing three SCR’s. The measured OCV from this Welder is only 40V.
60-(60/3) = 40. This would indicate a problem with one of the SCR’s or a triggering issue. As you can see, this process is capable of pointing the Repair Technician towards a speedy fault resolution.
Up until this point, the measurements that have been taken (namely, OCV) occurred without opening the Welders case. The following paragraphs refer to component level Functional Test procedures. To move forward with this, the components (SCR’s, IGBT’s or MOSFET’s) must be removed from the suspect Welder prior to carrying out these procedures. For safety reasons, under no circumstances should the components being tested be electrically connected to any other devices or Power Supplies apart from the Test Equipment being used to identify component defects. If you are not experienced in Electronic Repair, and don’t have a solid electrical background do not attempt any type of electrical repair on this type of equipment. Inside a Welder, there are High voltages present that can Kill. This warning is not to be taken lightly! In other words, if you are not a qualified Electronics Technician, Electrical/Electronics Engineer, or Electrician then treat this information as reference material only. Refer your repair to qualified personnel.
The SCR’s should then be individually tested out of circuit utilizing an SCR Tester or Component Analyzer. This type of Test Equipment is capable of triggering the Device-Under-Test. A DVM will not. Another important concern is ESD protection of the SCR during this testing phase. Select Test Equipment that grounds (or connects) all SCR terminals to the same potential while completing Functional Test. If there is no potential difference (voltage as a function of charge) between the SCR’s terminals, there is no potential for ESD damage!
When Trouble Shooting an Inverter based Welder, a low OCV usually indicates a Switching issue with the Welders IGBT’s (Insulated Gate Bipolar Transistor), or MOSFET’s (Metal Oxide Semiconductor Field Effect Transistor). This is normally caused by a faulty Gate Driver or Control Board. Both of these problems are solved through board replacement, or component level repair.
When the measured OCV is at 0 VDC (and the Welder isn’t blowing fuses or Circuit Breakers) the Switching Components should be tested for functionality (Out-Of Circuit).
An IGBT & MOSFET Tester or Component Analyzer should be used to perform these tests. As with the SCR Functional Test Procedure, a DVM can not Gate these devices. There are however some exceptions with MOSFET testing due to the high Gate input capacitance in some devices. Also, as previously stated: a DVM does not provide any level of ESD protection for the component while undergoing Functional Test. If a long lasting, quality repair is attempted, ESD protection through good Test Equipment selection should be a major concern.
David Willcocks is an Electronics professional, and Technical writer. For more information on Automated Test Equipment please visit http://www.imagineATE.com
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