Fish bowl heater

Discussion in 'The Projects Forum' started by tjohnson, Feb 14, 2015.

  1. tjohnson

    Thread Starter Active Member

    Dec 23, 2014
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    I have a betta fish for a pet that I keep in a fish bowl. Bettas are tropical fish, so they like water temperatures of around 60-70 °F. In the winter here in the northeastern US, it gets so cold that the poor fish lays at the bottom of his bowl most of the time and isn't very active at all.

    I was thinking of making a simple heating coil by connecting several 1 Meg ohm resistors in series to a 3V power supply (2 AA batteries), but I have several questions about doing so. Would it be safe? I don't want to melt the bottom of the fish bowl or start a fire. How much heat would be generated? Enough to heat the water in a fish bowl? Would it be better to use 4 AA batteries or a 9V as the power supply?
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    It wont work with 1meg resistors, you will have to use values of 10 to 50 ohms,,can you not put the fish bowl over an incandecent lamp to create heat,say a 20 watt bulb, or use a light dimmer, better than batteries.
     
    Last edited: Feb 14, 2015
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  3. tjohnson

    Thread Starter Active Member

    Dec 23, 2014
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    I'm curious why it won't work with large resistors? An incandescent lamp sounds like a good idea. (I know I could buy a fish bowl heater, too, but I don't like the ones that I've seen.)
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Any way you heat it, you should have a thermostat to turn it off when it gets to a set point.

    You can calculate the watts of power by Voltage squared divided by resistance. In other words, a mega-ohm resistor will not give you much heat.
     
  5. tjohnson

    Thread Starter Active Member

    Dec 23, 2014
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    Thanks, I wasn't thinking about that. For some reason I was assuming that resistors with higher values dissipate more power. I know the fundamental formulas (V=IR, P=VI, etc.), but I need to give more consideration to their implications.
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    You need a bigger voltage for bigger resistance, its about Ohms Law, W=IxR, or W=V2/R,

    smaller resistance for heaters, lower voltage.
     
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  7. bertus

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    Apr 5, 2008
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  8. Reloadron

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    Jan 15, 2015
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    I would suggest you go to your local Petsmart and look at aquarium heaters on a very small scale. Combine what Bertus mentions with how these heaters are rated. You don't need much for a small fish bowl. The smallest I see is a 7.5 watt heater and is likely more than you need for a small bowl.

    Started this post and saw Bertus already mentioned what I planned to mention. :)

    Ron
     
  9. tjohnson

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    Dec 23, 2014
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  10. ErnieM

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    Apr 24, 2011
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    Whatever you do DO NOT SOLDER TOGETHER SOME PARTS AND PLACE IN THE WATER.

    Either the lead from the (warm) solder will go into solution in the water and kill the fish or the voltage off the wires will zap you.

    If the bowl is too small for a purchased heater the most you should do is place a glass (only!) bowl over a small light bulb, such as a 5 or 7 watt C7 large Christmas style bulb. Watch it close to see how hot it gets, check with a thermometer you can get at the pet store.
     
  11. tjohnson

    Thread Starter Active Member

    Dec 23, 2014
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    Thanks for the warning. I wasn't intending to place anything in the water, but rather underneath the bowl. The bowl is plastic, so I suppose a light bulb wouldn't work.
     
  12. wayneh

    Expert

    Sep 9, 2010
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    The lightbulb will work, it'll just be a little slower thru plastic than glass, but that's a good thing. By far your biggest risk here is cooking your fish.

    I think I'd start with a single christmas mini bulb from a spent string, powered with some AAs. If that doesn't give enough warming, move up in wattage. I can't imagine you'll need more than 5-10W.

    I'm picturing the bulb being almost completely enclosed for insulation, in a chamber below the bowl, with most of the light blocked so that it doesn't disturb the fish.

    [edit] It wasn't clear in my answer, so I thought I should add that I'm another one in favor of using a commercially available solution. Without thermostatic control, I don't see how to prevent an accident that will end badly for the fish.
     
    Last edited: Feb 14, 2015
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  13. mcgyvr

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  14. tjohnson

    Thread Starter Active Member

    Dec 23, 2014
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    @mcgyvr: Thanks for the correction. I wrote that from memory, so I must have remembered it wrong.
     
  15. crutschow

    Expert

    Mar 14, 2008
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    That's true for a given current through the resistor (P = I^2 x R)
    But it's the opposite for a given voltage across the resistance (P= V^2 / R.
    Thus higher wattage light bulbs have a lower resistance, for example, since they see a constant voltage.
     
  16. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    I kept a B happy for several years with a home-made heater, details & paper work are lost, but heater is a 6 in dia Al. disc with 30 - 220 ohm resistors , 2 in series parall, measured at 35 ohms, glued with silicone glue insolated where needed. Believe it ran on 12 V DC giving about 5 W. Feedback control was waterproof 20 k ohm thermistor imersed in bowl. About 1990.
     
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  17. Alec_t

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    Sep 17, 2013
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    Whatever you choose as a low-voltage heating element, a couple of AA cells or a 9V PP3 battery will have a very short life and be impractical. Your heater needs to be run from mains power (via an adapter if necessary).
     
  18. wayneh

    Expert

    Sep 9, 2010
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    That's the key - a control scheme. Almost anything could work for supplying heat, but few things would work without a feedback control.
     
  19. GopherT

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    Nov 23, 2012
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    Without a control scheme, get ready to smell poached beta.

    Even a few watt heater that is keeping things at 78F running all the time could poach your beta if you have a warm day before you get a chance to turn off your heater.
     
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