Spehro Pefhany wrote:

> Really worst case, and that is junction temperature, vs.
> rated 175'C. Remember typically the FET will be more like
> 0.025 ohms or so with a drive from 5V.

Yeah, but I prefer to measure ten or so before
I really trust the makers specs. Being close to Asia
here brings a certain distrust of Asian specs, like I
bought some 65w autotransformers last month, to convert
240vac to 110vac. One I sold to a customer kept going
into thermal shutdown, I tested it into a resistive
load and at 52w it went into thermal shutdown after
about 1 hour at room temp. Dishonest crooks, some of
those Asian companies. :o(

Having said that, some of the better companies have
a higher respect for their specs, we use Motorola
diodes, many test at double the official performance.
Testing is the key. :o)


> >Anything over 55'C total temp at the transistor will cause
> >failures, probably within months as the solder weakens and
> >goes through enough heat/cool cycles to get circle fractures.
>
> Yes, thermal cycling can kill the die eutectic bond.

Yep, and just as often kill the circuit board solder joints,
especially with rigid-mounted vertical heatsinks, where the
expansion/contraction of a TO220 part causes vertical stress
on the solder joints. Very common.


> >I suggest 0.3W max from an un-heatsunk TO220 pack.
>
> That is extremely conservative by most people's standards,
> I think my 600mW for industrial environments (up to 60'C
> behind the panel) is pretty conservative for 24/7 operation,
> and it's been shown to be very much ok in some high volume
> products..

Yeah, I have stuff here running TO220s at 0.6w, when mounted
away from the board and in decent air convection boxes this
is ok without a heatsink. Even 1w can be done if you don't
mind it getting hot and smelly.


> For the MOSFET driver, your suggested pull-up resistor is
> usually a bad idea because it will cause the solenoid to
> come on if the PIC output is high impedance for any reason
> (eg. during startup), or if the PIC is unplugged.
> That's why I suggest a pull-DOWN resistor. The MOSFET can
> also be damaged if it drifts into the linear region.

VERY good point! I hope the newbies are watching before
they start posting "why does my solenoid operate during PIC
bootup"... :o)


> >Sorry to be picky, but newbies especially should be VERY
> >conservative with all their ratings. The expert engineers
> >can start to push the limits... (but then a year later i'll
> >be fixing their product!) ;o)
>
> Not a chance!


Hey Spehro, i'd buy your products without a second thought. :o)
Just some "engineers" worry me, as do some parts manufacturers
with dishonest specs. Sure it's ok being in the right, but try
explaining to the customer that it's NOT your fault, it's
the crooked Asian company that messed up... I had to do exactly
that last month.
:o)
-Roman

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