Cheap way to vary a 1,500 W resistive load

Discussion in 'The Projects Forum' started by Steve R in FL, Jan 9, 2015.

  1. Steve R in FL

    Thread Starter New Member

    Jan 9, 2015
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    My (flat) roof is leaking and I have bought 3 five lb buckets of roofing tar, it is very thick, I need to use the drill press with a paint mixing paddle to mix it properly, and because of the cold temperatures, it is even more viscous. I want to use a 1,500 W/120 V hot water heating element to bring the temperature up to about 145 F and keep it there while I am mixing and until it is applied on the roof. Can anyone think of a cheap and easy way to vary the current thru the element? Once it is hot enough, I will want to turn it down until it gets on the roof.

    The stuff is much much easier to work with when hot, and I feel it soaks into all the little nooks and crannies in the roof much better.

    Any ideas are appreciated!

    Thx
    'Steve
     
  2. ebeowulf17

    Active Member

    Aug 12, 2014
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    I would think triac or scr based dimmer circuit. Here's an example. Having said that, I must say I'm a little skeptical about a water heating element doing well in there... If the tar is that thick and not moving well, there won't be convection currents. I'd be worried about the heating element overheating and failing long before the whole batch softened up sufficiently. Most water heating elements expect the heat they produce to be carried away quickly. They're capable of melting themselves in a relatively short amount of time if they're not cooled by their surroundings.
     
  3. Steve R in FL

    Thread Starter New Member

    Jan 9, 2015
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    Hmm. That is a very good point. My thought is that I would have a paint paddle mounted on a drill press rotating continuously while it is heating up. Once it gets good and hot its viscosity drops off dramatically. Perhaps with the triac design I could start with very little power and gradually work it up as it thins.
     
  4. #12

    Expert

    Nov 30, 2010
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    I say: Heat a bucket of water and sink the bucket of tar in the water. Much safer for the heating element and much safer when considering that tar is flammable. Yes, it's slow, but catching on fire quickly is a lot more frustrating.

    Forget about "dimming" the heater. The time constants of the water and tar will give you plenty of time to use your human senses to control the process.
     
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  5. #12

    Expert

    Nov 30, 2010
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    Ha ha. Beat you to this one. :p

    Don't be a poor sport crutschow. Deleting your post makes me look like I'm talking to myself.
     
  6. Steve R in FL

    Thread Starter New Member

    Jan 9, 2015
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    Well my other option was to set it atop a propane fish cooker, but the heating element seemed safer.
     
  7. #12

    Expert

    Nov 30, 2010
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    Professional roofers use open flames, but I'm a coward. Besides, 10 pounds of tar is not a big job.
     
  8. Steve R in FL

    Thread Starter New Member

    Jan 9, 2015
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    Actually, its about 150 lbs of tar. (3) 5-gallon buckets worth (I think I misstated the volume in my original post). When its thick and cold it is a lot of effort spreading it around.
     
  9. #12

    Expert

    Nov 30, 2010
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    Sorry. I thought you said, "5 pound buckets" in post #1
     
  10. alan7s

    New Member

    Jan 9, 2015
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    1
    I see you're in FL. I've had success putting a 5 gallon pail in black plastic and setting in the sun in a sheltered corner - preferably on a blacktop
    driveway or black board. Even in cool temps. the heat can go up pretty well inside, and if you have any old styrofoam boards or fiberglass insulation
    hanging around, an insulated cardboard box adds speed. Sorry this isn't a "circuit" answer but don't want you ruining a good element.
     
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  11. #12

    Expert

    Nov 30, 2010
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    There is also some water based stuff called, "Cool Roof". Not as good as real roofing tar, but dead easy to mop on, and it's white which refuses to heat up as badly as black.
     
  12. Steve R in FL

    Thread Starter New Member

    Jan 9, 2015
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    I live in Northeast Florida, its not likely to get warm enough till mid March. The roof leaked on my Son's Xbox the other night, and it was'nt even that big of a rain storm.

    Still, I could just do a local patch and put off the rest of the roof till warmer days.
     
  13. tcmtech

    Well-Known Member

    Nov 4, 2013
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    If it's in the typical metal buckets you can put it on a portable hot plate and cook it up to tempurature without any problems.

    Just set the heat on a medium low setting and let it cook all day.
     
  14. Steve R in FL

    Thread Starter New Member

    Jan 9, 2015
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    They are in 5 gal HDPE buckets. US Plastics says the buckets are good to 160F though
     
  15. crutschow

    Expert

    Mar 14, 2008
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    Sorry, wasn't trying to be a poor sport. Saw that my post was basically a duplicate of your idea so deleted it right away. Though nobody would even see it. :oops:
     
  16. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Am I supposed to be embarrassed when I talk to myself? Umm, maybe I should be.
     
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  17. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Try something like this. They do not overheat easily but definitely unplug befor pulling out of the bucket.

    Electric charcoal grill starter for about $12 from walmart.

    Note: photo did not copy correctly
     
  18. wayneh

    Expert

    Sep 9, 2010
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    Years ago I was working with 5-gal buckets of starch, which had to be warmed. Starch settles, so it had to be agitated during warming to suspend all the starch particles. Any source of concentrated heat above 60°C (140°F) would cause the starch to gelatinize, which was a no-no.

    Anyway, we always used the hot water in the sink method. We had a sink drain "extender", so that the drain was still open once the water exceeded a level just below the sink lip. That allowed us to leave a trickle of fresh hot water entering the sink. It's hard to imagine using anything else.

    Once we got the starch warm, it was placed in a hot-water-jacketed kettle for more precise control, like you might make batches of fudge in.
     
  19. #12

    Expert

    Nov 30, 2010
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    Only when you do it in public. :D
     
  20. RichardO

    Well-Known Member

    May 4, 2013
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    Just carry a cell phone. You can talk to yourself all you want to and nobody will think it is at all strange...
     
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