Re: Home brew lab hotplate


Melodolic wrote:
I'm thinking of making a small hotplate for the purpose of heating PCB
etchant, and possibly to keep the tank up to temperature while etching. At
the low end of the temperature range, it should be able to maintain 50degC
max. Being able to go hotter and boil stuff, etc, would be good.

Seems to me that the cheapest way to get a heater, plate and thermostat is a
domestic clothes iron. I got one for 5ukp (new!) and it looks easy enough to
chop up and mount upside down in a box of some sort. The thermostat control
is mechanical, so some gubbins is needed to link that to a front panel
control.

I've never used an iron as a hotplate, and I've never used a lab hotplate
thingy either. Would an iron make a suitable base for a home brew effort, or
am I missing something? How long would it likely take to heat a litre of
etchant from, say, 20 to 50degC?


--
Melodolic Spielberg

Hi, Mel. While you've got the basics for a hotplate in an iron, you've
also got a couple of hangups that make the iron less than desirable as
the basis for one.

An electric iron, at its most basic, is just a resistive heater
thermally bonded to a flat surface (the iron flat), just like a
hotplate. But the temperature control works over a much higher range.
Assuming some thermal resistance between your plate and the pan, you
just might be able to achieve control at the desired temp if you put
the temp control very low. But probably not. Even at the lowest
setting, it will probably overheat your etchant.

Second, the iron itself isn't made for insulating you from spills on
the iron surface with the iron upside-down. If you tip over a pan of
liquid onto the iron, you might have a very hazardous situation.

If I were interested in getting a "lowest-cost" hotplate (yes, real
ones are expensive), I'd return the iron and start with a small
coffeemaker. You could bypass the thermostat control, and then just
use a 500W lamp dimmer to control power. With a little trial and
error, that will get you pretty close. Either that, or play around
with an intermediate material between the Mr. Coffee hotplate and your
pan such that the etchant temp would be close to 50C with the
thermostat cycling.

Standard precautions apply here, of course. Any mods you make to
consumer electronics may cause hazardous conditions. If you're not
experienced in dealing with line voltage, don't do this. Any mods you
make should be done with safety in mind, particularly paying attention
to the hazards of accidental spills causing short circuits.

An interlude of really basic physics. It takes about 4185 joules or
watt-seconds to raise the temp of 1 liter of water 1 degree C, assuming
no losses. So, it would take 125550 watt-seconds to raise the liter of
water 30 degrees C. With a 500 watt coffeemaker hotplate, and perfect
thermal transmission and insulation, that would take 251 seconds, or a
little more than 4 minutes. (If it's important to you, verify any
newsgroup math, at least from me! ;-)

Of course, the amount of time for a real world liter of etchant to rise
from room temp to 50C will be almost entirely dependent on the shape of
pan you use, and its thermal insulation, if any. Most of the heat will
be lost. I used to have a basic etching setup with a Pyrex pan and a
hotplate that used about a liter of FeCl3, and it usually took about a
half an hour to come up to temp.

And watch for fumes. Most etchants which go over temp emit toxic
fumes, which can also corrode metal and ruin electronics over time.
Never walk away from this potential hazard, and always etch in a
well-ventilated area, with the vent going directly to the outside.

Good luck
Chris

.

Relevant Pages

  • Re: Home brew lab hotplate
    ... etchant, and possibly to keep the tank up to temperature while etching. ... the low end of the temperature range, it should be able to maintain 50degC ... I've never used an iron as a hotplate, and I've never used a lab hotplate ...
    (sci.electronics.basics)
  • Re: Home brew lab hotplate
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    (sci.electronics.basics)
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