I design and test high power equipment that uses large SCRs (silicon controlled rectifiers). Sometimes one will go out. I use 6 in a three phase bridge configuration and if one goes out, its difficult to find the bad one without taking them all out. You can make a "in circuit" tester out of a flashlight.

Step 1: What Is an SCR?

An SCR is a Silicon Controlled Rectifier. They are used in power supplies, welders, inverters and other equipment for controlling power. The one below can carry 350 amps of DC current with a rating of 2400 volts. Think of them as a diode that can be turned on with a small current to the gate. Once turned on, they stay turned on till the current flow is interrupted or below the minimum current. Large SCRs like this one requires 150 milliamps at 3 volts to turn on. The SCR behaves like a solid state latching relay. The large base at the left is the Anode and attaches to a heat sink. The "pigtail" end at the right is the Cathode and the white wire is the Gate. The additional skinny red wire is connected to the cathode and is used with the gate when connected to a triggering circuit.

Step 2: The Circuit

To test the SCR, you need to connect a power source to the device. The anode connects to positive and the Cathode connects to negative. The flashlight's light bulb is in series and limits the current through the SCR to about 400 milliamps. A flashlight can be modified to make the tester. You can also use the modified flashlight as a continuity tester, test diode polarities and smaller SCRs.

Step 3: Drill a Hole

You will need to bring out three wires at the end of the flashlight. Start by drilling a hole just big enough for your wires. I used a Yellow, Red and White 18 gauge wire.

Step 4: Feed Wires

Feed three wires about 18 to 24 inches long through the hole at the end of the flashlight. Make them different colors like I did and attach alligator clips to the ends. You can also use test lead wires with alligator clips already attached. Just cut one end off. You can find test leads with alligator clips at Radio Shack.

Solder a washer or copper disc (like mine) on the Cathode clip. This will be the negative end and will be shoved down the barrow of the flashlight against the spring.

Solder the other two wires to the contact ring at the bulb head. These wires will be the positive Gate and Anode clips

Step 5: All-to-gether Now

Carefully adjust the lengths of the wires by pulling on the other ends. Trim the wires and attach alligator clips. Leave a little slack for screwing in the reflector.

Now you can load up the batteries and screw in the reflector.

Step 6: Your Done!

Test the device by bringing together the red or white lead (positive) to the yellow lead (negative) together. The bulb should light. Now go find an SCR to test.

Step 7: Test a SCR, Connect Anode.

Clip the red positive lead to the SCR's Anode.

Step 8: Test SCR, Connect Cathode.

Now connect the yellow (negative) lead to the Cathode. The flashlight bulb should remain off. If it comes on, you have a shorted SCR.

Step 9: Trigger the Gate

Touch the white (positive) clip to the gate lead. The bulb should light up and remain lit even when you remove the connection to the gate. If it does not light, the SCR is bad, you have not met the SCRs minimum gate current or your batteries are weak.

Step 10: In Circuit Testing, Negative Halve of Bridge.

Remember to turn off all power to the equipment and lock out, tag out (LOTO) before testing.

You can test SCRs in a 3 phase bridge without removing them. Below is a typical 3 phase bridge in a power supply. Find the negative and positive bridge output bus bars.

Start by testing 1/2 of the bridge. The three SCRs on the right is the negative halve of the bridge.

Clip the red positive lead of the tester to the negative bridge bus bar (see image notes). Then clip the yellow negative clip to one of the secondary transformer connections. If the bulb remains out, good for you, the bridge is not shorted. Touch a gate wire with the white test wire and test one by one. turn off the SCR by disconnecting the red or yellow wire and move to the next SCR.
If you find any shorts at all, you will need to disconnect all the pig tail connections (cathodes) to isolate each one. Then test each one separately.

Step 11: In Circuit Testing, the Positive Halve.

Now test the other halve of the bridge. Connect the yellow negative clip to the positive bridge output bus bar at the left. Connect the red positive clip to the transformer secondary input to the bridge. Use the white gate wire to trigger each gate of the three SCRs on the left side of the bridge. After you turn on the SCR, disconnect the red or yellow wire to turn off the SCR and move to the next one.

Remember, If you find any shorts at all, you will need to disconnect all the pig tail connections (cathodes).

This is just an example with a three phase bridge in a power supply. Other equipment such as inverters, welders and regulators can use this tester.