Picture of SCR Tester from a Flashlight
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.
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Step 1: What is an SCR?

Picture of 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

Picture of 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

Picture of 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.
Toxalry1 year ago
I have a quick question I follwed your instructions but once i ge to the stp about removing the gate once removed the bulb turns off. And also if I touch the cathode lead to the anode it produces a bright blinding light. It should not do this. Help?
cberes Toxalry1 year ago
That is normal for the SCR tester. It is as if you are closing the switch on the flashlight.
That's a great experiment! i have a 3W 160 lumens LED Flashlights, is it suitable to experiment?
botronics (author) 5 years ago
You can also test Mosfets and IGBTs if you add a 9 volt battery inseries. I have instructions at this link. You can test leds too. Its a multipurpose tester and even a flashlight!
I recently obtained a couple large SCRs that look just like the ones inyour instructable. They're labeled "GE C55B", but I couldn'tfind any information or datasheet for them. Do you have a similar model,and if so, do you have or know where to obtain datasheets for these?

Thanks for any help you can provide!
botronics (author)  inventorjack5 years ago
I don't know about the GE SCR, but here is a link for he datasheet on the big one shown. You can get the Powerex scrsfrom Galco.
Phil B5 years ago
Thank you for a well-done, useful Instructable; even though I do notwork with large SCR's. 
botronics (author)  Phil B5 years ago
Thank you.  Here is a youtube video of it working.