At 10:45 PM 7/23/01 -0300, you wrote:
>Hello,
>
>    now I'm thinking about using a small signal transistor (BC548) to give
>the TIP41 more current.

See my previous posting where I suggest a 2N4403

(driven from push-pull PIC output, Low = ON)

                   +5   (500mA)
                  |/ E
PIC-----[160R]----|   PNP (eg. 2N4403)
                  |\
                   |
                [7R5 3W]
                   |
                   x---------> base of TIP
                   |
                [100R]
                   |
                   0V


I would suggest avoiding the Darlington connection at these levels,
as the output drop is more like 1.2V than the 400mV of the above circuit,
increasing the power loss in the TIP to 6W from the 2W in the above
circuit. OTOH, it won't draw 1/2 amp from your 5V supply so this is a
factor. Personally, I think at this current level, MOSFETs are a no-
brainer, with rare exceptions.

BTW, Vth is not really the MOSFET parameter you want to look at, rather
at the Vgs that the Rds(on) is guaranteed at. Vth is normally spec'd with
a fairly low drain current. "Logic" MOSFETs are normally spec'd at
Vgs of 4.5V or even lower.

Best regards,
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