Assistance identifying TO92 in GFCI outlet - Page 1
Forum section: electronics / repair

author: FliesLikeABrick
author_rank: Newbie
timestamp: April 01, 2018, 05:38:27 pm
content: Hello all,I have an older GFCI outlet which failed and has been replaced. For fun/education I have disassembled it to discover the root cause of failure. A TO92 has the side blown out of it, but I cannot identify this component. My Google abilities are failing to find a datasheet or confirm what this is.It's a TO-92, so I suspect a transistorThe last line next to the Motorola logo is "741" which makes me ponder if it's an LM741 ... but I can't imagine that coming in a 3-pin packageImage attached and athttps://photos.app.goo.gl/hpyCdNyq2hQXE97N2The TO92 with the blue face. Markings:X28963-1(M)741

author: technogeeky
author_rank: Frequent Contributor
timestamp: April 01, 2018, 07:17:20 pm
content: I didn't find anything conclusive. My intuition says it must be an SCR or triac or thyristor or something of that nature.I foundthis 1975 Motorola master selection guide. From the part designations, it would seem that "28963-1" could be a reference to a SCR (in the sense that the SCRs are the only parts with such long identifier, and hyphen in it).Furthermore, while looking for that IC I foundthis patentwhich uses the RA9031 in its description. The only suitable part that could be a three pin device in this description is an SCR.

author: glarsson
author_rank: Frequent Contributor
timestamp: April 01, 2018, 07:27:03 pm
content: 741 on TO92 is a date code. Week 41 of a year ending in 7 (..., 1987, 1997, 2007, 2017).748 on DIL8 is a date code. Week 48 of a year ending in 7, probably same year as the TO92.

author: amc184
author_rank: Regular Contributor
timestamp: April 01, 2018, 10:46:22 pm
content: Maybe a BCX28?

author: floobydust
author_rank: Super Contributor
timestamp: April 02, 2018, 04:20:54 am
content: GFI's use a small SCR to energize the circuit-breaker, like MCR100-6.If the GFI circuit-breaker gets stuck, the SCR and coil stay energized and things burn up. Normally, the circuit-breaker trips and removes power from the GFI circuit.But if this does not happen, the coil melts and open circuits, but here the SCR popped.Because this has a safety function and GFCI's don't last forever, I would not repair it- I would bin it.More info on the IC datasheets: LM1851, RV4145A,FAN4147 Ground Fault Interrupter

author: FliesLikeABrick
author_rank: Newbie
timestamp: April 02, 2018, 04:33:10 pm
content: Quote from: floobydust on April 02, 2018, 04:20:54 amThanks, agreed - I do not intend to repair it as much as to take these steps for their educational purpose. I will look into SCRs and their role in this application.

author: SeanB
author_rank: Super Contributor
timestamp: April 02, 2018, 06:01:20 pm
content: In this application you are looking for a sensitive gate SCR, as they require a lower trigger current to turn on, and the device is probably only a 200mA 400V rated device, with a gate requirement of 1V5 0.5mA to trigger it. Older units used a Gate Controlled Switch, which is a SCR with all 4 internal junctions brought out, and they typically were used with the anode gate triggering the trip relay instead of the cathode gate like almost all SCR units. They typically used a BRY39 GCS device to do the detection and triggering all in one package.