> -----Original Message-----
> From: piclist-bounces@mit.edu On Behalf Of PICdude
> Sent: Sunday, October 02, 2011 9:31 AM
>
> How do these things fail?  This is not a volume or commercial product,
> but a one-off 3D printer head.  Resistors are these ones from Yageo...
> http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=3D8
> 10F5R0E-ND

When thinking about using a power resistor as a heater don't forget the
power derating for temperature. The datasheet linked from the DigiKey page
has the derating specs a bit out of place (just above electrical instead of
below power). The spec is linear 100% @ 25C to 0% @ 275C, so @ 150C the
power handling is only 50% (5 Watts for the 10 Watt version).

Paul Hutch

>
> So I have some options now...
> (a) Use an 18V source (perhaps add a 6V battery in series with the 12V
> battery, so each resistor would generate 10.8W of heat),
> (b) Build a PWM switcher now and put all 4 resistors in parallel, then
> adjust for 10W to each resistor,
> (c) Swap the resistors to 15-ohm units (same physical size and 10W),
> which in parallel will be 9.6W per resistor,
> (d) Run 28.8W through each resistor and see if I can get it to work
> for some time before it fails.
>
> (a) is complicated cause I'll have to buy a 6V battery to experiment.
> (b) will take time to get components for a PWM switcher, but probably
> makes most sense.  (c) is pointless, cause I might as well try (d)
> first and if they fail I'd then do (c).
>
> Think I'm leaning to see if I can dig up a power mosfet to build a
> PWM-switcher temporarily using a function generator.
>
> Cheers,
> -Neil.

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