Transistor identification SR51 (Probably BSR51)

[Chris Wilson]

I have a device that uses some transistors marked as SR51 Googling brings me to 2 leaded diodes. These are plastic cased three wire semiconductors, probably a switching transistor driving the gate of an SCR. Anyone recognize them? Thanks.

[camallison]

What device are they in? That may help narrow down the search.

Colin

[Chris Wilson]

A Danish TIG welder, I have no schematic, but I think they fire the current control SCR's. One SCR is not receiving a gate trigger. Thanks.

[Herald1360]

Philips tended to mark transistors with the initial B missing- eg C557 for BC557.

There is a BSR51 NPN Darlington in a TO92 package.....

http://noel.feld.cvut.cz/hw/philips/acrobat/7369.pdf

[Chris Wilson]

That looks a very likely candidate, a quick in circuit test of an accessible one shows it as a Darlington, and the package is right. Plus some other devices are shown without the B prefix, such as some C547B devices which seems likely to be BC547B. Thanks a lot, I think you have it nailed!

[Herald1360]

Finding one might be the next problem, though probably anything similar would do.

Or maybe not....

http://www.bgmicro.com/bsr51npndarli...o-9280v1a.aspx

Shipping might be an issuue, though.

Or you could bite this bullet:

http://www.littlediode.com/component...483fed9dc7a8de

[dazzlevision]

Originally Posted by Herald1360

Philips tended to mark transistors with the initial B missing- eg C557 for BC557.

In addition, they normally marked "PH" on the device, to show it was made by Philips.

[Barry Lloyd]

Or if you don't want to or can't buy a BSR51 you could make a Darlington pair of two separate transistors of appropriate rating.

[Chris Wilson]

OK, thanks for all the info and suggestions. Without a schematic I am really struggling, but i have pulled all the semiconductors in the board area that looks likely to be the output trigger circuitry for the SCR's, including the BSR51'2, and my Peak tester shows them all good, including all the diodes in the region. I need to see what chips are likely to be a ramp generator, and if one chip works both SCR's or if I am looking for two separate chips. I would just send it to the makers, but it's too old and they don't want to know, but nor will they give me a schematic. Guess they hope people buy a new machine. I have seen some American machines of a similar age for which schematics do exist, I hope to glean some info from those. Yes, I am a bit out of my depth without a schematic One thing that makes me wonder how this circuit has failed is the two SCR's are fired via isolating transformers wound on toroids. I would assume they would protect the trigger components from most downstream whoopsies?

[camallison]

Not ther same package, but these should do the job:

http://wwww.ebay.co.uk/itm/like/1607...d=636760708316

Colin