My data sheet for the 2N7002 is dated 1995. It's 23 years old. And it's still a really good part. Capacitances are low, it can switch over an amp, and it's really fast. There's nothing newer that can do some of the stuff it can.
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John Larkin Highland Technology, Inc
lunatic fringe electronics
It's my default low-power MOSFET. My Philips datasheet is from
1995 too but the peak current rating is given as 800mA.
For use at medium currents I stock AO3400 (N) and AO3401 (P). I normally choose my general-purpose stock parts after carefully considering numerous alternatives but not in the case of these two types. I bought them because they're cheap and have ratings in the appropriate range and low Rds(on). The gate charge and capacitances aren't bad but nothing special. They are available at AliExpress for less than 1.5 US cents each in small quantities with free shipping. They have served me well.
Idea for using P fets for load switching a USB charger; if USB power is connected to the charging circuit while the load is switched on it shuts down the load by flipping the flop and locks out the ON switch:
The 1A or so capability of the BSS84 and 2N7002 are peak ratings. For continuous loads as for charging, FETs with higher ratings and lower Rds would be more suitable. Like the AO3401 I mentioned earlier or something similar.
The input needs to be pulled down in any case, so if the trigger voltage is higher than the supply there will have to be a divider there of some value. The diode is probably just paranoia if it's being fed from a logic output rather than directly from the USB supply
Fraction of the capacitance. Lower voltage, obviously, that's how you get the higher performance.
It seems RF MOSFETs are completely kaput and no one cares. There are a few dual gates still remaining, with better specs. If you don't mind that. I suppose if you randomly needed a discrete NAND gate that could be handy.
BJTs are altogether better (and the other RF transistor types), so many of them survive. Except the PNPs, it seems...
Yeah! At least in the US, as soon as I design something into a board, it either goes end of life, or you just can't get the parts from any distributor. A major pain.
Hmm not just the US, reordering susumu 0.1% R's. obsolete, I find a new 0805 part number. 1/2 the power rating, now I've got to think/review... grumble.
15V into 1 k ohm, they were 1/4 W R's, now 1/8 W should I worry? I don't really care if the R's loose their value at 15V max overload, only that they return to normal at ~5V.
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