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[APD] Re: 24 volt transformer for BIY substrate heater

To those DIY gurus out there. I have an Azoo 300 watt substrate cable heater and would like to use a 24 volt transformer to heat it. I
found that the local electrical outlet have a 24 volt transformer that puts
out 40 amps (used in furnaces). So that will give out 960 watts, right ?
Will that be suitable for my 300 watt cable ? I also plan to use a 500 watt
azoo heater controller for the cable. OTOH, I have not found out if both
the primary and secondary coils are physically separated in this
transformer. If it is and if you think that it is suitable, I think this
will be a viable project. Thanks for your time and any inputs will be
greatly appreciated.

Electrical systems don't operate in a way that would allow you to do that. Electrical loads consume power based on their resistance (I'm going to simplify a bit to make the math easier and neglect power factor and some other things). For example, your 300 watt cable uses 300 watts when operated on 120 volts. Volts multiplied by amps gives us watts, so from your 300 watt cable we get 300/120 = 2.5 amps. Ohm's law gives us the resistance by volts/amps, so 120/2.5 = 48 ohms. This is a convenient number for the next step, which is to determine the wattage the cable will be when fed with a 24 volt power supply. At 24 volts, we get a current (in amps) of 24 volts / 48 ohms (again from ohm's law) of 0.5 amps. To find the wattage you multiply the current by the voltage, which is 0.5 * 24, which gives 12 watts. That's not a very potent heating cable.

To get 960 watts from your transformer, you need to have a load that draws 40 amps when operating at 24 volts. Ohm's law gives 24/40 = 0.6 ohms, which is the resistance necessary at 24 volts to draw 40 amps and thus dissipate 960 watts. You can not easily modify a heating cable to operate at a lower voltage.

If you are concerned about safety, you have several options. You could get a normal Dupla cable (if they're still available), or you could build your own heating cable. If you build your own cable I recommend the use of Teflon-insulated wire (which can take high temperatures and is unaffected by water), with silicone-insulated lead-out wires to go between the heating part of the cable and the power supply. There is a lot of info on the Krib about this, and in the list archives (much of it written by me :-), and I can offer you some pointers if you like too. You could also modify your Azoo cable to make it "safer". You can get tubular copper braid from electrical suppliers (it's often used for shielding) which you could put over your Azoo cable and then connect to the electrical system's ground. This would result in a failed cable shorting out immediately to the "shield" and then tripping a circuit breaker or better yet a GFCI running your tank. The shield would carry the current instead of your tank (again simplifying a bit), and would protect your tank inhabitants and possibly you too if things happened at a bad time. I think the Azoo cables are already grounded though, in which case internally they are probably very similar to what I just described. You would also want to put heat shrink tubing or some other inert material over the copper braid to protect the copper from corrosion and the tank inhabitants from copper compounds forming in the water.

And don't let anyone tell you that an isolation transformer will make your high-voltage cable heater safer. IT WILL NOT, IT WILL MAKE IT MORE DANGEROUS. An isolation transformer makes what is known as a separately derived supply, which simply means a power source that does not have an electrical path back to the "main" source. Using such a device will PREVENT a GFCI from tripping if something should fail, but will most likely not prevent a possibly hazardous condition still being present in your system. The National Electrical Code actually specifically disallows the use of isolation transformers in many underwater systems for exactly this reason. While not part of the original question I thought this worth mentioning here.


***************************** Waveform Technology UNIX Systems Administrator

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