Replacing old SCR's

[David Hess]

Thank you. I also was looking at this document, which suggests the turn off capability of an SCR can be improved in various ways, including adding a heatsink (not possibly in this case), or a low R_GK resistor (see page 19). Are any of these possibilities in the circuit I'm working with?

SCRs normally include the gate shunt resistor internally as part of the construction which serves to remove charge over the entire gate area.  An external resistor only connects to the gate at one spot and is less effective.  Sensitive gate SCRs lack this internal shunt resistance.

An SCR can be made from a PNP and NPN transistor and this allows access to the internal nodes allowing the performance to be tailored.  I have never tried this but I think a pair of schottky diodes could be included to Baker clamp the transistors considerably improving turn-off speed.

[akimpowerscr]

How many fast thyristors equivalent to the E0102YA do you need ?

There are 4 more BR103 Fast Thyristors available at 4.99 € / pc on ebay, why don't you buy them?
(see link in my previous post)

If you are looking for a long term supply solution, you will not find it because of fast low current thyristors, it is not manufactured any more for a long time.

[desertrout]

I did buy 5 of the BR103's from eBay for my immediate and future needs (should be arriving soon so I can verify they'll work), but in the future others and I will probably need a solution for when these old fast SCR's are simply no longer available, and that's what I'm curious about. David Hess's idea of using a couple of bipolar transistors and some resistors to 'make' an SCR is intriguing, or something like this which does something similar, using a modern replacement for the E1022YA then modifies the circuit with an NPN transistor, a Schottky and some passives: https://hackaday.io/project/168100-fat-macintosh-repair/details ... But it would require an oscilloscope for specifics so this is all academic at this point.

[akimpowerscr]

You can also select 2N3050, trying them in circuit to see if the power supply works or not.

No need of an oscilloscope to do this.

[desertrout]

You can also select 2N3050, trying them in circuit to see if the power supply works or not.

No need of an oscilloscope to do this.

Perhaps an ignorant question, but what do you mean by 2N3050? The only info I see is for the 2N3055 transistors I've seen in amplifiers...?

[akimpowerscr]

Sorry, 2N5060   

[desertrout]

Ah lol that makes more sense

[akimpowerscr]

This is the kind of mistake that happens when you do two things at the same time, responding on the forum and having a conversation on the phone.

Responding to a forum takes a lot of time, it's volunteering, it is difficult to devote yourself to it exclusively and full time, some topic starters often forget it.

[desertrout]

I, for one, appreciate it. Cheers.

[desertrout]

Just to tie a bow on this, I've tested the BR103 in circuit and it works. Thanks everyone. Now to design a fast SCR replacement with modern components....