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Using resistors as simple heating elements.

Thread starter #1
Hi, does anyone have any experience using resistors to make a simple heating element?

I wish to make an aquarium heat stone. This is basically a heating element encased in plaster, shaped to look like a stone. I am probably looking at something rectangular, 100mm x 50mm, or circular, or maybe 90mm diameter, ideally not more than 25mm in thickness. When fully warm I would like it to be around 20degC.

I will be using a low voltage supply, between 12v and 20v depending on what a/c adaptors I can find, so there shouldn't be a problem with voltage?

Is it safe to embed a resistor (or bunch or resistors) directly into a lump of plaster to act as a heating element? Would it be safer to embed a resistor in potting compound, then set this in plaster instead?

What style of resistor packaging would be most suitable? Either as a heat transfer medium, or for durability, ie Wirewound, Ceramic, Aluminium?

I don't yet know what wattage will give off the required heat, and how much I would loose in the transfer, but I am thinking around the 5W mark using wirewound ones for the initial testing (as I have a couple lying about :) . I think its safer to run it below it's maximum rating, say 4.5W don't you think?

Thanks for reading!


Active Member
well if your resistor is 10 ohms and you feed it with 9V it certainly would heat up.. and burn! Well I recommend that you improvise, I've used small tungsten light bulbs for that purpose, it heats up to 40+ degrees celcius with a 3V double AA batteries.
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i have used resistors as heating elements before. the problem that i have had is that carbon film resistors tend to smell really bad when they heat up. i would suggest using a low resistance power resistor, something like 20 to 30 ohms and no less then 5 watts. you can make a temperature controller using an oscillator, something similar to a 555 timer (actually a 555 timer would work perfectly)


Well-Known Member
got any stainless steel safety tie wire? I turns out to be a great resistor and you can choose your resistance by the length you use. I think mine is .032 ". It's not a stable resistor, but it should do your job just fine.
Is it safe to assume this is not submersed in water?
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Ceramic resistors would be fine, small high wattage resistors are easy to find, as eric said you'd need a potting compound between the ceramic heater and the plaster outter casing, generating the heat isn't the problem however, you need to also detect the heat level the resistors is giving off so that it can be safely varried to avoid cooking the fish. The biggest problem with this is between the heating element itself the potting compound and the small rock that you want to use there's not a lot of chance for thermal conductivity it's going to act prety well as an insulator, and as eric said you need to heat the rock to a certain temperature, but if the heat can't get out you'll never keep the whole tank warm. It'd take a bit of experimenting with to see what you'd need to end up with. This is why most aquariam heaters are simply glass isolated heating elements.


Well-Known Member
My concern is waterproofing the thing.
Hi DragonTamer,

I built a reference oven to calibrate temperature control circuits using ceramic 5W resistors. Using resistors for heating purposes you should use resistors with a high thermal resistance for optimum heating.

4W resistors have thermal resistance of 65KW-1, 5W types still have 50KW-1. Higher wattage resistors (e.g. 17W) have a value of 15KW-1.

Attached is the datasheet of YAGEO (VITROHM) ceramic resistors. The suggested type is KH208-8.

4W resistors are seldom to find - so the next in the line (5W) might be used.

Depending on the volume of the aquarium you'll need a lot of resistors for proper heating using a maximum of 80% of the rated power. Stacked resistors have little effect on heating, hence placing them close to the surface of the rock will result in maximum heating. (The unused part of the rock could be a brick which stores heat)

So be prepared to build a mountain instead of a rock. :)

Since fishes are "heat seeking" creatures make sure that the water circulates well around the rock. Otherwise you'll have them all seating on the rock. :)

For water tight encapsulation (and good radiation capability) google for "Voss Chemie" and there look for "Giessharz".





Well-Known Member
A 15 gallon aquarium generally needs a 50 watt heater. Unless your using the heater in a fishbowl, 5 watts will be inadequate.


Well-Known Member
He may just be making a heated rock for a terrarium. 20C is low for a typical fishtank (usually 25-27C is typical).
He did say aquarium, but maybe your right.
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New Member
Forum Supporter
I think its safer to run it below it's maximum rating, say 4.5W don't you think?
It is hard to say exactly how much power your 5W resistor will be able to dissipate when it is under water and encased in plaster. The wattage spec on a resistor is usually at a certain ambient temperature in free air, with a certain lead length, etc. The resistor will have a different wattage rating when it is at 25C than when it is at say 150C. It will really depend on the thermal resistance of the plaster. You might have more success with some resistance wire wound on a larger open form which gets potted in your plaster "rock".


Well-Known Member
You have to de-rate the resistor power massively when they are encapsulated. 5 watt resistors run at 4 watt will melt the solder on their legs!

I would run the resistors at about 20% of their rated wattage, and just use more of them. It won't matter how much you de-rate them the "stone" will still give off X watts heat for X watts of input power.
You have to de-rate the resistor power massively when they are encapsulated. 5 watt resistors run at 4 watt will melt the solder on their legs!
He could use crimping material (as used in irons or toasters) instead of soldering the wires.

Electro shops generally have those on stock.
Thread starter #17
Wow I didn't expect so many responses, thanks all :)

I will have to sit down and read through them properly tomorrow. One thing to clarify though : Yes I did state it was an aquarium heat stone, however it would be used in an insect enclosure. But It might still come into contact with a small amount of moisture, so I will still think about potting the resistors first.

Thanks so far.
For something like this, honestly I'd purchase a commercial unit, takes all the guesswork out of it, and they're saftey certified. I am absolutly positive if you look around you'll find what you're looking for.


Most Helpful Member
I would just use an aluminum plate with a heat sink mountable power resistor on it. Just pick the right resistance and wattage capacity for what type of power source you are using and size the aluminum plate accordingly to what temperature you want.


Well-Known Member
He could use crimping material (as used in irons or toasters) instead of soldering the wires.

Electro shops generally have those on stock.
Sure, but's it's still a poor choice to run the resistors too hot. There will be a much larger thermal expansion and greater difference in hot/cold areas of the cast material, both likely to crack it.

I've done low-temperature encapsulated heaters using resistors a few times and it's best to run the resistors way below (ie at a fraction of) their rated wattage and use more resistors. The idea is to get *most* of the device acting as a heater at a lower element temperature than to run a small percentage of the device as a heater at a high temp and large thermal gradients etc.

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