Problem description

I have a small formicarium for ant keeping, it is basically a tiny glass tank with inside dimensions of 20cm x 10cm x 2cm, filled with sand or soil. Ants I am keeping, like to dwell in the nature under rocks, which are heated by sun rays, and they are using its heat for faster pupae (final stage of ant cocoon) hatching. They are moving pupae to such vicinity of the stone so the correct temperature is achieved. I would like to replicate this in indoor conditions for my ants.

Solution suggestion

I intend to use old laptop power source, characteristics from its sheet:

  • 90W AC Adapter
  • Output DC 19,5V 4,62A

And basically just put resistor to its output connector big enough to provide power of about 10W. Then either put this resistor to e.g. some concrete mixture to create artificial stone or (if it will be too big) attach it to the outside glass to create one spot temperature increase. Close to this resistor there will be thermostat sensor which will cut of power to the laptop power source as soon as the temperature increase would suffice. It will be operated indoor and desired temperature increase will be 3-5 degrees of Celsius for the 40 cubical centimeters of soil/sand.

My Questions

  1. If I put five 2W 10R resistors just after each other I am effectively creating 10W 50R resistor, correct?
  2. If having 50R resistor, my math is telling me that with the 19,5V of the power source, it will be drawing 0,39A and will provide power of 7,6W. Is my math correct? If not please provide description of how to calculate this.
  3. I totally do not know how would I calculate if this power would suffice to provide desired temperature increase for that amount of soil/sand and artificial concrete stone. I intend to experiment with different resistor size, but if someone would be able to provide me with the info on this or point me to some articles on this topic it could be nice.
  4. Safety - it will be in wet environment most of the time. Do you have any tips on how to insulate this water proof, electrical proof but with low thermal insulation? It needed to be cheap and possible to apply in the small size since it will be quite small

My level of knowledge

I have high school education in electronics but I am not working in that field for 20 years. So my knowledge is fuzzy at least :)

What I (believe) cannot use and why

  1. Artifical heating stones from pet shops - they are too big for my use case. Maximum dimension 2cm
  2. Heating cables - they are too long and thus most of the heat would dissipate without use and I am not sure what to do with the rest of the heating cable length
  3. Light bulb - its too big and is heating entire formicarium and not only part of it
  4. Heating mats which work by spreading infrared light - they are too big and thus would heat larger area
  • 1
    \$\begingroup\$ If you heat a 2cm stone with 10 Watts it will reach high temperature, probably above 100°C. \$\endgroup\$
    – bobflux
    May 22, 2020 at 19:10
  • 1
    \$\begingroup\$ that's 4·10⁻⁴ m³ of sand volume, and sand has a density of 1.6·10³ kg/m³, so that's 6.4·10⁻¹ kg = 0.64 kg of sand. Sand is mostly quartz, so specific heat of 800 J/kg/K. So, per Kelvin you heat this, you need to put 800·0.64 J = 512 J into that. A Joule is one Watt for one second, you've got 10 of these, so that means you'll heat your sand by 1°C every 51s, if the enclosure is well-insulated. I wouldn't recommend leaving the heating on for more than an hour... you'll have finely roasted and water-free sand :) \$\endgroup\$ May 22, 2020 at 19:16
  • 1
    \$\begingroup\$ you could use a small 5 W incandescent night light \$\endgroup\$
    – jsotola
    May 22, 2020 at 22:25
  • \$\begingroup\$ you can buy kapton heaters off eBay that are kapton tape with copper heating elements inside and they typically have double stick adhesive on them... That combined with a low voltage or a pwm signal will make an excellent heater. \$\endgroup\$
    – MadHatter
    May 22, 2020 at 22:56
  • \$\begingroup\$ Mechanical thermostats (not the comfort heating type, but the type used in cookers etc) usually have quite a bit of hysteresis- like 5 degrees C. \$\endgroup\$ May 23, 2020 at 4:13

3 Answers 3


Perhaps start with a larger resistor, perhaps 220 or 270 ohms and see how hot that becomes; if you need more heat you can add extra resistors in parallel. As a very rough rule of thumb, 1W per square cm of exposed surface is more or less the max for semiconductors and 6W per square cm will start to glow a dull red. Generally a resistor running at full power will get to about 100C. At that temperature the dissipation is mostly conducted so the temperature rise is proportional to the power. So a resistor running at 20% or rated power will give you a temperature 20 degrees above ambient. If you encase the resistor or attach it to a heatsink of some kind then the temperature rise decreases in proportion to the surface area where heat can be lost.


Look for heating cables that are designed to be cut to fit. Places like McMaster-Carr will sell them by the foot.

Beware of the cables which automatically decrease and increase their resistance based on ambient temperature. They are deluxe, but you probably don't want that feature.

  • \$\begingroup\$ I am not able to find in my country any heat cable which could be cut to fit and which would be sold by very small quantity (one meter would be more probable in my country). And still I would need to trim it to use it on top of formicarium to something like a 4cm max which I am not sure how it would work. Since from comments to question it is now clear that 10W is way too much, I believe that the single point of heating will be better then cable and can be much smaller \$\endgroup\$
    – Mireczek
    May 23, 2020 at 8:22

One thing you might look at is a 'grain of wheat' bulb. These are very small incandescent bulbs that are used for things like model railroad lighting and as back-lighting in older hifi equipment.


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