EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: chinoy on June 02, 2024, 05:58:02 pm
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So an old circuit has these two transistors in SOT23 SMD package size.
As per the info I have the old transistors were 100v 50 MA
Amplification does not matter. i.e. hfe values do not matter.
Speed does not matter.
Id like to pick a PNP / NPN pair in SMD SOT 23 which can support the highest voltage and highest mili amps.
What would be a good option.
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Why not replace them with the same types? Those are super-common types.
Your existing documentation is somewhat wrong, in that the part number changes depending on the package. BC5xx (THT) ≡ BC8xx (SMD) for many parts, for others, the number even changes more.
In this case, BC557 ≡ BC857 and BC548 ≡ BC848.
Or are you attempting to find “upgraded” parts that can handle higher voltages and currents? In that case you need to share the full schematic and all the info you have, and what you want to achieve.
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Most of the bigger stores have parametric search engines for the stock they have. A quick check suggests MMBT3904 (40V 200mA) costs less then 3ct (when buying 100).
When sorting by Collector emitter voltage ZXTN08400BFFTA is on top of the list. It goes up to 400V and 500mA, but costs 35ct (quantity 100).
When sorting by Collector current, ZXTN19020CFFTA is on top. It goes up to 7A, but only 20V and costs 45ct (100 up).
In another store I sort of randomly picked FMMT493TA (100V 1A 500mW SOT23) for 13ct (25+)
There are thousands of choices for a BJT in SOT-23 package, and when you go to more "exotic variants" (higher current and voltage, or other extreme) they become more expensive. I suggest you try the search and sort functions of those stores. With smaller (local?) stores their offerings are much more limited, and without search options, but often you can scroll though the whole list, and often they have some balance in what they offer.
Another option is to buy an "transistor sample book" from China (amazon, ebay, aliexpress, etc). A quick search shows you can get a book with 60 different transistors, 25 of each (total 1500) for around EUR20. Those books are not all the same. Some may have a more differentiated content then others, so get the datasheets and make some comparisons before you buy such a book.
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Yes, the ubiquitous 3904/ 3906 are great general purpose transistors. Available as through hole and SMD.
Keep a small stock of medium power types, eg ZTX651, ZTX751. Not so cheap but very handy and very avaialble. How often will you really need to go above 40-60v?
I use Zetex FMMT555 and FMMT4555 types for >60V. $0.10 in quantity.
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You can consider MMBT5551 (NPN) or MMBT5401 (PNP)