Michael A. Terrell wrote:

larry moe 'n curly wrote:

http://www.harborfreight.com/1500-watt-dual-temperature-heat-gun-572-1112-96289.html
$19.99 regular price. On sale for $9.99 at times.

I received their weekly e-mail right after I posted. It is on sale
for $7.99 with the coupon below:

http://www.harborfreightusa.com/html...7/images/2.jpg

Here is the user manual:

http://images.harborfreight.com/manu...6999/96289.pdf

The diode in series with the heating element is a 1N5408, rated
for 3 amps average. The heater draws 8A average on low.

So my question is, how long will this 3A diode last?

That 3 A rating is for continuous duty at
a high operating temperature, not intermittent. The peak current rating
is 200A. The case would likely melt before the diode would fail.

Doesn't the continuous amp rating for a diode apply for any load
lasting more than something like one 60 Hz cycle (or half-cycle?). I
normally run a heat gun a lot longer than that, maybe up to 2
minutes at a time.

Have you used it for that two minutes? That should answer your
question, and show that it takes more than two minutes to damage it.

It takes time to heat the junction in the diode. The heat is
produced only when forward biased, and is from the forward voltage drop,
times the current flow. The higher the current, the faster it heats.
Since a heat gun usually gets very intermittent duty in low heat mode,
it works.

If it can handle a couple 200 amp half cycles while charging an
electrolytic in a piece of electronic equipment, it can handle the extra
current for the heating element even though it is above the 3A
continuous rating.

It looks like the motor is run from a tap on the heating element.
There is no schematic in the manual. I only saw the one diode, across
the switch in mine. Also, how often will you use one on low heat? I
rarely do, usually only when I accidentally hit the wrong end of the
rocker switch.

BTW, if that diode fails, all it will do is make it run at full heat
in either 'ON' position.

I'll bet customers will either not notice that the gun always puts out
full power (diode shorts), or they won't care that it doesn't work at
low heat (diode opens). I checked a few different items, including
some cheapo PC power supplies, and couldn't find any where the diodes
were underrated so much

I bought this heat gun to desolder surface mount stuff, and it seems
that its low setting puts out temperatures closer to what hot air
soldering equipment does.