Subject: [Shop-talk] Is there an electrical engineer in the house?

From: Scott <[email-masked]>
Date: Thu, 11 Apr 2013 22:58:56 -0400

I would like to build several warmer 'pots' for SWMBO.

They'll be made from 1/8 inch aluminum sheet, and they'll about 3x6x3 
inches each. They'll have a lid, so they're more like boxes, I suppose.

They'll be used to keep fluid warmed to about 100 degrees.

I'd like to use an electric heating element with a thermostat 
permanently set to that temperature, and it could go on the exterior 
bottom surface of the box.

The only thing I don't know how to do is the wire grid and thermostat, 
switches, etc. What should I use to attach the wire to the box, that 
sort of thing.

EE1101 was a long time ago, so I thought I'd ask you guys. Is this 
something that can be done? I don't mind if it takes a fair amount of 
work, I want these to be nice, and lasting. I just don't know where to 
start looking for the stuff, what to Google, etc.

Thanks.

Scott
_______________________________________________

[email-masked]
Archive: http://www.team.net/archive

---

From: "Randall" <[email-masked]>
Date: Fri, 12 Apr 2013 00:31:59 -0700

> Can anyone give me primer on how to do such a thing? What 
> gauge wire to use, should I weave it in a mat, and what sort 
> of switches and thermostats should I use?

What about using a pre-made element, like this:
http://goo.gl/i7CGk

If you really want to do it yourself, my approach might be to source some
nichrome wire and epoxy it to the bottom of the plate.  JB Weld would
probably do, or there are purpose-made epoxies available.  JB Weld says it
is good to 500F continuous and you aren't going that high (if you don't go
overboard on heater power).  Lay down a fairly thick layer first and let it
cure, to be sure the nichrome doesn't get too close to the aluminum (and
cause a short).  

So pick how much power you want.  I'm guessing that 60 watts should be
plenty (after all, an Easy-Bake oven runs on 60 watts) so you can calculate
resistance from ohms = volts * volts / watts.  60 watts would be about 240
ohms (using 120 volts as the design voltage).  Now figure out about how much
wire length you want, and go look for a wire that will give you something
close to that.  Say for the sake of argument that you wind up with 6' of
wire in your design (more wire will spread the heat out better, which SWMBO
will like); you want about 40 ohms/foot.

Using the chart I found at http://goo.gl/3Rpqg, 38 AWG is just what you
want.  Here's a nice piece on fleabay for $4
http://goo.gl/GGxvk

Brian already gave a link to a thermostat.  Switches are available in dozens
of styles, so pick the one that goes best with your design.  I might go for
a rotary switch mounted on the side, maybe with a flattish sort of knob, 
http://goo.gl/K9Oby
But a rocker switch might look nice, too.  I like this one, but you'd need
to build a power supply for the LED, which might be more trouble than it's
worth
http://goo.gl/3D5lN

Randall

[email-masked]