Does an aquarium heater element run at 120V or is it stepped down to a lower voltage? - The Planted Tank Forum
 
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post #1 of 15 (permalink) Old 02-17-2010, 02:53 PM Thread Starter
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Does an aquarium heater element run at 120V or is it stepped down to a lower voltage?

I'm playing around with the idea of modifying a heater to fit in a HOB filter used for a nano tank, largely b/c of the lack of available 25W options. I found a small adjustable heater (~3.5" long) that may work, but it is only available configured for 220-240V.

I know a heating element is just a resistive component, so wondering if anyone knows whether that resistor is typically hooked up to line voltage (ie 120V in the US and 220-240 elsewhere) or if there is a step-down transformer inside the heater to take it down to something like 12V which in turn feeds into the heating element.

The reason I'm asking this is that if the resistor is hooked up to line voltage I believe this is probably a straightforward conversion (ie I can buy a 50W heater @ 220-240V, replace the plug with a US version and it will work albeit at a lower wattage). However if there is a step-down transformer involved this becomes a more expensive / involved conversion because I have to find a new step-down.

Assuming the heater element/resistor is run at line voltage is the right conversion that a heater element that consumes 50W at 220-240W will consume 25W at 120V?

As you can tell, it has been almost 20 years since I had to do any calculations with Ohm's law that mattered. As usual, thanks in advance.

Last edited by tuonor; 02-17-2010 at 08:55 PM.
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post #2 of 15 (permalink) Old 02-17-2010, 04:08 PM
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The glass ones that look like they're in a test tube are usually just a resistive heating element made of nichrome wire, a simple mechanically-adjusted thermostatic switch using a bimetal strip, and a neon lamp. They are driven directly from line voltage.

I did see one that appeared to have something akin to a light dimmer in it, but at max power the full line voltage would still be across the heater wire at 100% duty cycle

I can't speak for all heaters, but economy would favor this simple design.

Your power calculations are correct. If I remember correctly, nichrome wire resistance changes with temperature so it won't be exactly 1/2, but it will be close.

Whatever you do, make sure to seal it well against water leakage, and avoid the possibility of the heater running dry!
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post #3 of 15 (permalink) Old 02-17-2010, 04:20 PM
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You also need to keep in mind that if you're dealing with glass, you need to make sure that your new glass shield is made of Pyrex or equivalent. Normal glass will shatter if used as a heat shield.
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post #4 of 15 (permalink) Old 02-17-2010, 04:29 PM
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P = V^2/R

Cutting the voltage in half cuts the power consumption by 4. A 220 volt, 25 watt heater would consume about 6 watts at 110 volts.

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post #5 of 15 (permalink) Old 02-17-2010, 05:11 PM
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Quote:
Originally Posted by DarkCobra View Post
The glass ones that look like they're in a test tube are usually just a resistive heating element made of nichrome wire, a simple mechanically-adjusted thermostatic switch using a bimetal strip, and a neon lamp. They are driven directly from line voltage.
Yes, which is why they tend to be failure-prone: the same element used for temperature sensing (the bimetal strip) is also used to turn current on/off to the high-power heater element. Better (more expensive) heaters use a solid-state sensor for measuring temperature, and a dedicated relay for switching the element on and off.

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P = V^2/R

Cutting the voltage in half cuts the power consumption by 4. A 220 volt, 25 watt heater would consume about 6 watts at 110 volts.
For a simple resistive load, that's true. However, the heater element's resistance is probably temperature-dependent; e.g., it will have a higher resistance when it is hotter (at 240V). So it may consume a litte bit more than 6W at 120V.
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post #6 of 15 (permalink) Old 02-17-2010, 05:38 PM Thread Starter
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Thanks for all the quick replies. It sounds like this is not as feasible as I thought, b/c I'd be taking a 50W heater and turning it into a 12.5W heater instead of a 25W one.

BrentD and DarkCobra, I'd just be cutting the heater cord near the original plug so new glass / sealing is not an issue. I'm prob not qualified to take the mod to this level.

PDX-PLT, how much variance are we talking in terms of temperature impacting resistivity? Are we talking 5-10% or something more significant? Its I want to raise the temp 5-10 degrees in 5 gallons so my guess is 15W still won't do it.


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post #7 of 15 (permalink) Old 02-17-2010, 07:09 PM
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Oops. Hoppy's right, the wattage will be cut 75% if voltage is reduced 50%.

There are different types of Nichrome wire. I looked up Nichrome Cr C, the rise in resistance is:

0% at 68F
1.7% at 200F
3.5% at 400F
5.2% at 600F

I doubt the wires get this hot, so it's probably ok to ignore this effect.
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post #8 of 15 (permalink) Old 02-17-2010, 09:14 PM Thread Starter
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Hmmm...back to the drawing board.

Thanks again guys for all the insights.


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post #9 of 15 (permalink) Old 02-22-2010, 02:20 AM
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Quote:
Originally Posted by Hoppy View Post
P = V^2/R

Cutting the voltage in half cuts the power consumption by 4. A 220 volt, 25 watt heater would consume about 6 watts at 110 volts.
Also keep in mind, if there is a transformer involved (I doubt it), it may over heat running on 60hz as it was designed for 50Hz. Resistive circuits don't care about frequency, but inductive circuits do.

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post #10 of 15 (permalink) Old 02-23-2010, 12:32 PM Thread Starter
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Came up with a potential alternative, although not sure how sketchy this is from a safety perspective.

My understanding is most travel step-up converters are not built for continuous use unless you get a behemoth like this.

However, I found this one, which is rated for 40W continuous use and is actually UL listed (which I've discovered is a rarity among these sketchy imported travel devices).

Do you guys think it'd be feasible to run a 25W (ie 25W @ 240V heater) off this converter (feasible as in I'm not risking an electrical fire)? My thinking is that an aquarium heater is really not continuous duty b/c of the thermostat. I have a kill-a-watt meter that I could use to check the start-up power draw to make sure I'm not getting a temporary spike north of its 40W capacity.


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post #11 of 15 (permalink) Old 02-23-2010, 12:55 PM
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why not jsut order a 25 watt 120v heater?

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post #12 of 15 (permalink) Old 02-23-2010, 12:57 PM
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A 25 watt heater is listed here for $15

http://www.fosterandsmithaquatics.co...8&pcatid=11368

15 watt could wasily use 2 of them:

http://www.fosterandsmithaquatics.co...7&pcatid=12087

Another 25 watter

http://www.fosterandsmithaquatics.co...2&pcatid=21322

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post #13 of 15 (permalink) Old 02-23-2010, 01:08 PM
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While I thoroughly enjoy job security.... please do not attempt to do this. You do this and step it down to 120 V and then I have to use 300 J to get your heart going again. Burnt flesh smells funny, too.
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post #14 of 15 (permalink) Old 02-23-2010, 01:09 PM
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While I thoroughly enjoy job security.... please do not attempt to do this. You do this and step it down to 120 V and then I have to use 300 J to get your heart going again. Burnt flesh smells funny, too.

Can I watch.

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post #15 of 15 (permalink) Old 02-23-2010, 02:19 PM Thread Starter
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why not jsut order a 25 watt 120v heater?

Craig
Its because the 25W 240V heater is 4" long while all the 120V ones are 5.5-9" long. And for this particular application, I have about 4.7" to work with. I know it sounds more complicated than it should be but this would allow the equivalent functionality of running an in-line heater but with a HOB.

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While I thoroughly enjoy job security.... please do not attempt to do this. You do this and step it down to 120 V and then I have to use 300 J to get your heart going again. Burnt flesh smells funny, too.
lol. Let me make sure I understand your comment though -- what's the issue here, is it running one of these little converters too long and the coil overheats? Or is it that the step up from 120V to 240V creates an electrical shock issue.

I am probably taking too much comfort in the UL listing here and the assertion that the heater would already be wired (ie properly insulated) to operate at 240V. But I admit my knowledge of this stuff is dangerously cursory.

Again, thanks for the help guys.


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