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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
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
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.
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