I AM NOT AN ELECTRICAL ENGINEER, AND I AM NOT TO BE HELD RESPONSIBLE FOR ANY INJURIES, DAMAGE OR WHATEVER MIGHT BE THE RESULT OF USING INFORMATION STATED IN THIS INSTRUCTABLE. Having said that, I do like to think I know what I am doing. If there are any questions related to this instructable feel free to ask, in either English or Dutch.
Step 1: Parts Etc.
- 4x 2Ohm 10W resistor .About 2 dollars per 5, from ebay. Other values and more or less resistors will do, as long as the Wattage is high and the resistance is low. Note that 10W doesn't mean they use 10W, it means they can handle up to 10W of power dissipation before it burns out. If you feed it so that it dissipates 10W it will get VERY hot, so a no go for this build. For reference in some other build I have a single 10W 2Ohm resistor running at 3.5W which is too hot to touch.
- A way of attaching it all together, I used some screw terminals.Soldering is also possible, but make sure you won't short it all out. (SAFETY IS IMPORTANT).
- An adapter to power it all. I STRONGLY recommend using an old phone charger, since they are designed to have a limit on the amperage output. Next to that they usually have an output of 5 Volts and between 0.5 and 1 Ampere, so that would be between 2.5W and 5W which is quite low. Low we want, so that's good. My setup runs at about 0.4A 5V, so about 2W. Not getting super hot, so I might switch to a 0.6A 5V source later.
If there is no current limit it could potentially deliver a large amount of Amps with the risk of melting things, blowing fuses etcetera. So other mains adapters might work, but be sure to test the power consumption(power=volts*amps), the temperature and preferably both.
Step 2: Testing
Sidenote: In general a parallel setup would give a higher heat dissipation due to the voltage usually being the constant factor. If the current is limited, putting the resistors in series gives the most heat. Next to that some mains adapters have a build in protection against shorting out, which can be turned on when using to low resistance. This would prevent current from flowing through the resistors rendering the heating pad useless. If I would put my 4 2Ohm resistors in parallel that would equal 0.5Ohm which is quite low and might trigger this protection.
Step 3: Use It!
Put a plant pot on it and plug it in mains (perhaps via a timer switch). Wait for plants.
Edit: Running continuously for about 3 days now, no overheating but heat is being generated for sure! Soil temperatures fluctuate between 25 and 35 degrees, depending on the amount of sunlight on the pot, room temperature etcetera. I also get the idea that having a bit of water in the bottom of the pot helps transporting heat into the soil (I have a clay pot with some clay pellets in it, on top of that is a plastic plant starter pot with the soil).
For making it look better you can always but something around the resistors, for example make an epoxy cast, or put some fabric around it. Be creative! (or lazy and leave it as it is, like I did).
Step 4: Appendix: Calculation Example
Current limited in series:
P=UI=I²nR --> P=0.4²*4*2=1.28W
Current limited in parallel:
P=UI=I²R/n --> P=0.4²*2/4=0.08W
Voltage limited in series: (note that I don't have a constant voltage, but just as an example)
P=UI=U²/nR --> P=5²/4*2=3.125
Voltage limited in parallel:
P=UI=U²n/R --> P=5²*4/2=50W <-- NOTE HOW HIGH, though I don't think you will truly reach this, it will probably be somewhere between 10 and 50W.
You can tweak with this by putting say two resistors in parallel, and two resistors in parallel behind that (thus 2 in series and 2 in parallel, total of 4 resistors).