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Home brew lab hotplate

Discussion in 'Electronic Basics' started by Melodolic, Aug 5, 2006.

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  1. Melodolic

    Melodolic Guest

    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

    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?
  2. Arnold

    Arnold Guest

    Pour the liter into a plastic photo developing pan.
    Plastic pans with steep enough sides
    and a pouring spout on one corner work very well.

    Float the pan on a bathtub full of hot water.

    Don't fill the bathtub too deeply with hot water.
    Just enough so the pan with the etchant floats
    slightly on it and won't capsize.

    Pour a liter of plain water in the pan first
    to make sure it will float in the tub and to practice
    doing this without spilling anything.

    You can agitate the pan if you want to, but usually I
    just go off and have a cup of coffee and when I
    come back it's all done.

    ** Just be careful not to spill any of the etchant
    or you'll badly stain the bathtub. **
  3. Melodolic

    Melodolic Guest


    Thanks, but the idea is to build a hotplate for peanuts. Heating the etchant
    is the use that led me to consider it, but I'd like to have one in any case.
    It strikes me that a hotplate is something useful and I'm more likely to
    come up with ways of doing things if there's actually one lying around.
    (Melting wax, for example...)
  4. Ken Moffett

    Ken Moffett Guest

    How about a real lab stirrer/hotplate. I have one that's going to the dump.
    Your's for the shipping. (Always looking for a good home for junk I don't
    need.) Email me direct.

  5. Melodolic

    Melodolic Guest

    (Email bounced.) Thanks for the offer, but your ISP leaves me thinking
    you're in the US - I'm in the UK, so the shipping might be pricey. Still
    worth considering, though - could you tell me what make and model it is?

  6. Chris

    Chris Guest

    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
  7. Then why not an actual hotplate?

    At the very least, an electric frying pan surely has exactly what you

    Note that if you want to melt wax, certainly when I made candles
    I put them in a tin can, and immersed the can in a cooking pan that
    had boiling water. I could have used the double boiler insert, but
    who wants to mess up something you cook food in with wax?

    In essence, that is what the other poster is suggesting, a double

  8. default

    default Guest

    Sounds like you're doing this the hard way You need a non-metalic
    tank for the etching. That limits you to anodized aluminum or
    enameled iron - or glass or plastic.

    Easy to build a wood box and coat the inside with polyester/epoxy
    resin or fiberglass it. Fiber/resin works better -won't crack in the
    seams/corners with temperature changes.

    Get an aquarium pump to agitate the etchent - you need that or the
    results won't be uniform.

    An immersion heater works better than a flat iron - an aquarium heater
    will be inexpensive and work well - or if you are destitute or like
    doing it the hard way - take a test tube and incorporate a heater-
    resistor and thermal switch. The tricky part is making it liquid proof
    so it won't suck in liquid.

    I use a wooden box, aquarium pump and long "air stone" with an
    aquarium heater. cost ~20 dUS

    I'm also using sodium something or other instead of ferric oxide -
    lots cleaner, cheap, faster acting, and doesn't stain everything.
    Bought a lifetime supply - so I don't remember the chemical name.

    Built a light table with some unfiltered UV lights to expose the
    boards too.
  9. Melodolic

    Melodolic Guest

    I'm looking to make something that can go underneath it to maintain the
    working temperature.
  10. default

    default Guest

    Good design for an etching system.

    Why underneath? That's a plastic container. heat the liquid with a
    liquid proof resistor (easy with epoxy and silicon tubing) down at the
    bottom to avoid stratification, and put a simple thermostat on the
    surface or mid-level.

    With plastic, a thermal insulator, temperature control is iffy - or
    you need to insulate the heating element and liquid bath together -
    you don't want too much heat becasue the insulating properties of the
    plastic will slow the response - plastic may sag before the bath is at
    temperature, or it may take longer to come up to temp (if you play it

    A thermostat is just a thermistor (liquid proofed), comparator and

    Some flat irons go 700+ watts - overkill for etching.
  11. Ken Moffett

    Ken Moffett Guest

    Yes, I'm in the US. And if you want "cheap", shipping won't work here.
    Thias is a large heater/stirrer that will handle five flasks at one time.

  12. Guest

    I usually just heat my etchant bottle in a container of hot water from
    the tap...let it set for 4-5 minutes then pour the etchant into a
    shallow pan and float the board on top.... works fast enough for
    me...10 min or less

    heating a lot of etchant for some really big boards ??? why not just go
    out and buy a hot plate....skillet head...~:>
  13. Rich Grise

    Rich Grise Guest

    Why not just go buy a hotplate?

    Good Luck!
  14. Don Bruder

    Don Bruder Guest

    Where's the "bodge-karma" in that, though?!?!?

    There's always the dual-purpose option: Take the top off your computer,
    pull the heatsink off your Pentium, and put the pan of etchant in its
    place. :) <-- Closed-captioned for the humor impaired.
  15. Melodolic

    Melodolic Guest

    Sounds rather bigger and more sophisticated than I need or have space for.
    Ah well, not to worry. :)
  16. Melodolic

    Melodolic Guest

    I'm not sure about their minimum temperature - I kinda assumed that getting
    stuff to simmer requires something close to 100C, given that simmering is
    gentle boiling. I need a minimum of no more than 50C for keeping etchant at
    the right temperature.

    A pal of mine who makes waxy stuff for a living uses stainless steel pans
    (well, buckets!) on a gas flame. I'm pretty sure some of his temperatures
    are around 130C or so, so a double boiler might not get hot enough.
  17. Melodolic

    Melodolic Guest

    Something I haven't done is monitor the temperature over time - worth a go,
    I think.

    The plan is to rebox it to make it into a proper little hotplate, including
    an aluminium plate that fits on top of the sole of the iron, mainly to give
    a neat rectangular shape rather than the bendy triangle of the iron itself.
    The plate will be bigger than the iron and of the order of 10mm thick, so I
    could machine a channel in the underside, which a vertical lip in the case
    fits into. A gasket or seal is a possibility if I can find something that
    can survive the max temperature (silicone?).

    I don't know how hot a coffee maker can get without risking the element
    burning out.

    Couldn't something similar be done using the iron? Its thermostat could be
    set to max, and some sort of circuit could feed it, say, a PWM signal (a-la
    motor speed control) which determines the power going into the element. That
    would eliminate the cycling thermostat issue.

    S'okay, done a few mains boxes before.

    I'll take your word for it (for now ;-).

    I was thinking of a Pyrex beaker, possibly used at a higher temperature, but
    I'm not sure how quickly the plate would cool down to 50C.

    Noted. I'm using the Sodium Persulphate stuff.
  18. Melodolic

    Melodolic Guest

    Thanks - I'm rather pleased with it. Designed on the hoof while I was out
    buying bits for a completely different concept (mechanical things to
    oscillate an etching tray).

    Doesn't give me a hotplate for other things, though. Still, how high would
    such a liquid proof resistor be? I had considered using an aquarium heater,
    but they were all a bit large in diameter to be sure of getting it low in
    the etching bath, and I figured something like a 4x6" hotplate would give a
    more even spread of heat.

    Yup. The plastic looks to be polypropylene, which I think is okay at etchant
    temperatures. The idea is to preheat the liquid in a beaker or something,
    pour it into the tank, and then sit everything on the hotplate running at a
    temperature which maintains the etchant at 50C (eg, maybe setting the plate
    to 70C would keep the tank and etchant at 50C).

    As I say above, it's more a case of finding what hotplate temperature will
    hold the tank at 50C - for a couple of hours, say (the purpose is to prevent
    cooling during an etching session).

    But not for other things like melting wax. :)
  19. jasen

    jasen Guest

    the thermostat can be dissasembled and wound back, but a hot water
    thermostat might be better suited.

  20. Rich Grise

    Rich Grise Guest

    Why not just spring $19.99 for "a proper little hotplate"?
    (If this wraps, you'll have to copy/paste it.)

    You wouldn't have to turn this one upside-down and figure out a way to
    secure it.

    Have Fun!
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