On Saturday, October 01, 2011 6:44 PM, "PICdude" wrote:
> I have a few chassis-mount resistors mounted to an aluminum block =20
> specifically for the purpose of heating the aluminum block (3D printer =20
> extruder head).  Can't I run much more power (than rated) through the =20
> resistors, as long as I don't exceed the max operating temp?
>=20
> Specifically, the resistors are 5-ohm 10W, and if I power it from 12V =20
> (lead-acid battery) I get 2.4A per resistor, and 28.8W.  I would PWM =20
> it to reduce the average power, but with feedback.  So it would run at =20
> 28.8W until it comes up to temp (I need <150 deg-C), then reduce =20
> average power.  The resistors are rated at 250 deg-C max, so I'd be =20
> well within that.

Hi Neil,

I have some experience using cheap ceramic resistors as heaters in a
commercial product. To sum up my experience, you will experience a
higher-than-desirable failure rate if you significantly overdrive them,
even if the temperature is low, the duration is short, and feedback
limits the temperature. I was using 5 watt resistors. 6.4 watts under
worst-case seemed to be the sweet spot for me. More than that and I
could expect some failures.

So 28.8 watts from a 10 watt resistor is about 15 watts more than I
would allow in a volume product.

You could probably hand-select resistors or use "qualilty" brands and do
OK. But don't expect cheap ones to all survive.

Cheerful regards,

Bob

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