At 05:24 PM 7/23/01 -0400, you wrote:
>If you put in a FET you still need to juggle the resistors.  For a FET
>start by swapping the two resistors  You will need some resistor to GND
>to stop the solenoid from firing while the PIC boots.

Just a series resistor will do it. The gate charge will keep the MOSFET
reliably off for the few milliseconds while the PIC is starting, but it
doesn't hurt to put a 100K or so in there to take care of the case if the
PIC is unplugged. A logic-level MOSFET such as the IRLIZ44 will do this
with no heatsink (0.55W in a TO-220).

>I think your TIP41 is fine with better drive.

It's a bit marginal, but it could be made to work. Watch turn-off time
and the SOA limitations with an inductive load! Vce(sat) with a 6A
load is spec'd at 1.5V max. with 600mA drive. Using "typical" figures,
Vce(sat) is 0.4V at Vbe is about 1V at 5A/500mA drive. That's typical
2W dissipation (requires a heat sink) use, say, a 2N4403 to drive
the base current, and the PIC can sink the 25mA of base current the
for the drive transistor (forced beta of 20). The base resistor would
be around 7 ohms at about 2W, for a total loss of about 4W (typ).

P.S. That's a 120W solenoid. Either it is very large, or it is only
rated for intermittent duty.

Best regards,
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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
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
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