On Fri, 07 Apr 2006 07:39:59 -0700, Richard H. wrote: ...
MMBT2222:
Cheers! Rich
On Fri, 07 Apr 2006 07:39:59 -0700, Richard H. wrote: ...
MMBT2222:
Cheers! Rich
Resistor Equipped Transistors are now quite common, and cheap. So they are a good digital interface device, where you go outside the uC pins abilities. See the Philips web site, and they also have dual RETs with PNP/NPN designed for level shifting, giving (eg) 12V hi side drive from a uC pin, in a single SOT23-6 style package.
-jg
Please do not rule out transistor arrays in sot23 packages. Half a dozen transistors in a 14 pin package (emitters tied to pins 7/14).
No, AFAIK. Look at distribution quantities-in-stock and prices or ask.
There are thousands of part numbers which will work, and dozens which are close to optimal depending where the product is being built and how many. For low to moderate quantities in North America you can't go far wrong using MMBT4401/4403 for NPN/PNP. Google for data sheets, there are many, many manufacturers.
'Digital' transistors (transistors with base and maybe E-B resistors built-in) and digital transistor arrays were developed originally in Japan by Rohm and are now multiple sourced & are very popular in consumer goods, but generally not quite as cheap and easily available as resistors or resistor networks and discrete parts in low quantities. Generally the transistors 'inside' are referenced to popular Japanese discrete transistor part numbers.
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
The 2N7002 may be a good fit. FET with low threshold voltage and low on resistance.
Regards Robert
Indeed, this looks very promising. In particular, ON Semi's version in a tiny 1.6mm SC-75 package for 4 cents.
After wallowing a while in Digikey's search engine, I tried a different approach... I jumped over to ON Semi's site and sorted by price (something Digikey won't do). Thinking I'd work my way up the price list, I stopped after the first entry, which was a 2.7 cent 2N7002. Good enough for my purposes, but the smaller version was worth the extra penny. :-)
Cheers! Richard
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Depending where you have stuff manufactured, the LED-specific constant current drivers are likely to be cost-effective against the loading and procurement cost of all those discretes, and the reliability issues surrounding so many parts. Part prices are only part of the picture.
They're damn easy to use too.
-Andrew M
Interesting that everyone focuses on transistors rather than more integrated solutions. An interface chip I use a lot is the NC7WZ17P6X dual buffer in a 6 pin SC70 package, about 2 mm square. It will drive
24 mA at LVCMOS levels and will handle up to +-7 volts on the input. With 5 volt Vcc it will drive up and down at 32 mA. I don't think you can touch the density with transistors and we are only paying $0.06 each or $0.03 per driver.I recently removed a bunch of SOT23 transistors from my design and replaced them with these, some to drive LEDs.
Thanks! Half the battle seems to be finding these bargains when you need them. The other half seems to be getting distributors to talk with a small guy (even if you're willing to buy a reel).
I agree with you on using arrays where possible, and this package certainly seems small. I'll give it a look. Small PCB size is important to this project, so discretes are attractive because they can be squeezed into small spaces (even though the overall density is worse).
So far, I've found a discrete 2N7002 MOSFET with great specs (overkill, actually) at $0.04 in a similar tiny package; at this price, I could afford to buy a reel if I can't find a disti that'll do cut tape.
We got a few hundreds spare 2N7002 (100mA). If you can't find what you need, we can send you some. Shipping is probably more than the cost anyway.
Thanks - I really appreciate the offer. What mfr & package type do you have surplus in?
Digikey has the "larger" SOT-23 in cut tape, but at least for ON's part# they don't stock the smaller SC-75 package. More digging is in order with alternate disti's and other manufacturers.
I had to chuckle that ON would sample 10 for free, but charges $11 shipping. I'd pay for more to make the S&H worthwhile, but they don't offer that option. More vendors should follow Microchip's model of offering all their parts direct; they can offer better selection than the disti's and charging a small premium will keep the disti's happy.
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Where do you get one? The last time I checked for a constant-current diode (AKA "field-effect diode") they were about five bucks a pop!
Thanks, Rich
Keep in mind that the NC7WZ17P6X has *two* devices in this small package and you can get cut tape from Digikey.
We have the SOT-23. I think they are Phillips.
We can send you 50 for $10, including shipping.
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