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Oh the difficulty of sourcing P-MOSFET in TO-92

Yann Guidon / YGDESYann Guidon / YGDES wrote 08/29/2016 at 02:42 • 2 min read • Like

If you follow Yet Another (Discrete) Clock, you know this project uses quite a lot of MOSFET in the easy-to-solder TO-92 package. A bit less than 500, including about 300 N-FET (BS170 or 2N7000) and the rest are P-FET (BS250).

The N-FET are OK: I could source 300×2N7000 for around 10$, 3 cents per piece is a nice deal and a decent price for the project.

The P-FET are another story :I could find cheap reels in SOT23 but the prices are much higher for the TO-92 package and there seems to be a sort of shortage in the usual cheap markets. Earlier this year, an order never made it to my home (among other postal problems) and a 200pc lot was found to have a reverse pinout (https://hackaday.io/project/9376-yet-another-discrete-clock/log/37927-it-was-too-easy).

This post is about another escalation in the BS250 horror story. I found a decently priced lot of 100 on eBay (due to Brexit) and this time ordered only one lot (I was not feeling rich, and at 20GBP, or about 24€ for 100, the unit price is almost 10× higher than the N-channel). Since the precedent lot (2×100) was functional (despite the reversed pinout) and was delivered, I bought from the same seller. Delivery was pretty fast (maybe due to summer vacations and less business) so I was ready to leave another good feedback.

Just to be sure, I tested a few parts from the bag. I don't remember why but you never know, and I wondered what was the pinout this time. Guess what ? I found a MOSET but in N-channel, not P. WTF ? It's marked "BS250" !

I take another then another and start to find a "mixed bag"

After I sent a complaint to the seller, I received the following:

« Hi,client.So sorry for your trouble.Could you let me know how many pieces are not working as excepted,I will check with our supplier for this issue.Regards, Namedeleted »

Wait, you send me worse-than-counterfeit parts then you want me to do your job, for which I paid you ?

Alright, since you wanted it, let's waste 2 hours testing all the parts by hand.

I could identify:

Only 1/4 of the order can qualify as conform.

Ah, China...


Pictures to come...

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K.C. Lee wrote 08/29/2016 at 05:13 point

2N7000 are the only TO-92 MOSFET in the aliexpress stores I shop.  Everything else are huge parts or SMT.

BSS84 are the cheapest P-MOSFET in SOT23 on cut tape - unlisted manufacturer.  They are $1.99 for a 100.  I bought some from here previously, but haven't tested them. 

http://www.aliexpress.com/store/product/Free-shipping-100pcs-SMD-mosfet-transistor-SOT-23-BSS84/506373_32360564835.html

You didn't use the tripod style footprint, otherwise the SOT23 could have fitted some what.

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Yann Guidon / YGDES wrote 08/29/2016 at 05:18 point

I got a couple of reels of BSS84 and the complementary, but SMD is not beginners-friendly :-/

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Ted Yapo wrote 08/29/2016 at 04:50 point

That sucks.  I can only imagine how this happens - ebay seller gets a random bag-o-parts swept up off the factory floor, pulls one out - it happens to be a BS250 - and that's what gets listed.

"P-channel" BJT's like the 2N3906 are really cheap...maybe you should consider bipolars :P

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Yann Guidon / YGDES wrote 08/29/2016 at 05:15 point

no bipolars, I stick to CMOS :-)

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