On Tuesday, May 24, 2011, Olin Lathrop <olin_piclist@embedinc.com> wrote:
>> - Coil Power: 1200 mW
>> - Nominal Voltage: 12Vdc
>> - Coil Resistance: 120ohm
>
> If that's all you need, you can use much smaller packages. =A0All you nee=
d is
> a transistor that can withstand 12V and carry 100mA. =A0Just about anythi=
ng
> can do that. =A0Don't forget the diode to catch the coil kickback when tu=
rning
> off the relay else whatever transistor you choose will get fried.
>
> Something like a IRLML2502 should do nicely, but it doesn't need to be a
> FET. =A0Especially considering your extreme cost sensitivity, you could u=
se a
> MMBT4401, emitter to ground, base to PIC pin with resistor in series. =A0=
Let's
> say you count on a minimum gain of 50. =A0That means you need a minimum o=
f 2mA
> base current. =A0To be pessimistic, let's say the B-E drop is 700mV. =A0T=
hat
> leaves 2.6V accross the base resistor. =A02.6V / 2mA =3D 1.3kOhms, so a 1=
kOhm
> resistor sounds about right. =A0The combination of MMBT4401 and 1KOhm res=
istor
> will cost a small fraction of a suitable FET.

Would a VN2222 N channel MOSFET be suitable? Or an equivalent in surface mo=
unt?

I checked the datasheet and everything seems suitable, but there may
be something I missed.

Just checked your IRLML2502. Loving the current drain capability at
Vgs =3D 3.3V. I guess I'll go with that.

My only concern is that the SOT23 part will be a bit hard to solder by hand=
..

My cost sensitivity isn't really toward $1 dollar components. I don't
mind spending for those. I do, however, mind the $20 more per board
for a few sq inches. That's unjustifiably unacceptable. But I found a
good fab house so it shouldn't be that bad.

--=20
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