How to Check for a Shorted SCR

Jonra Springs

A silicon-controlled rectifier (SCR) is an electronic component made of two diodes that amplifies and controls the voltage running through a device. Machines, such as metal inert gas (MIG) welders contain stud-mounted or hockey puck SCRs. The most common type is a stud-mounted SCR. The component runs current in one direction, so SCR testing for a short is a matter of checking to see if current makes it from one end terminal to the other.

Welders test expensive SCRs before replacing them.

Step 1

Shut off and unplug the machine containing the hockey puck or stud-mounted SCR. Remove the flexible leads of a stud-mounted type by loosening the screws and pulling the cathode and gate lead terminals off. Remove the anode lead stud with an adjustable wrench. Loosen the terminal screws on the gate and cathode leads of a hockey puck type as well. Loosen the heat sink from around the puck and pull it out of the recess.

Step 2

Connect the positive lead of an ohmmeter to the anode of either type component for SCR testing. The anode is the round metal plate on a hockey puck type and the threaded post of the stud-mounted SCR.

Step 3

Connect the negative lead from an ohmmeter to the cathode lead of a silicone-controlled rectifier which is at the opposite end of the stud on stud-mounting types and the terminal on the red wire of hockey puck SCRs.

Step 4

Set the ohmmeter on a low setting. A good SCR will show very low resistance. A shorted SCR shows infinite resistance or no current flow from cathode to anode.