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**To**:**Mortimer Snerd <n9720235 at cc_wwu.edu>****Subject**:**Re: Substrate heating cables****From**:**Erik Olson <erik at thekrib_com>**- Date: Thu, 31 Dec 1998 09:17:14 -0800 (PST)
- cc: aquatic-plants at actwin_com

On Wed, 30 Dec 1998, Mortimer Snerd wrote: > Finally, I need to get a transformer for the cables, and am a little > confused. I was under the impression that the cables would run hotter > if you put more amps through them. I am now wondering if this is the > case. Do the cables develop only so much heat, and the rest of the > power put out by the transformer is unused, or will the transformer only > put out as much power as the cables need to develop a particular > wattage? Or (as seems to happen with frightening frequency :-) am I > completely wrong? Justin, I'm sure you'll get a handful of responses to this one. Just like a fuse, the current rating on a transformer designates the maximum current it can deliver before it fizzles out. The amount of current it actually supplies is determined by the transformer voltage and the resistance of the load, in this case the heating cables: The cables have a fixed Resistance R, which means for a given Voltage V applied across them you will have a current I = V/R (This is Ohm's law of V = I*R). The amount of heat delivered (Power P) = I * V = V^2/R. In practical terms, the wattage goes as the applied voltage squared. To get more power, you need a higher voltage transformer. BUT there is a caveat: it's probably not a good idea to go above the rated voltage for a given cable; there's no telling what it will do (melt the insulation? burn out? etc), so a Dupla 150 watt cable shouldn't be used to give 200 watts by hooking it up to a 30-volt transformer. I have, however, used lower voltages in order to reduce the wattage on a 100 watt cable to about 50 watts. So let me do a practical example which might help illustrate. You have a 150 watt Dupla heater designed to work at 24 volts. First, we find out how much current that will draw: P = I * V -> I = P / V = 150 watts / 24 volts = about 6 amps. So a 7 amp transformer would work just fine here. It won't fizzle. A 4 amp transformer cannot handle this load, and may burn out. Additional fun: Since we know the power goes as the voltage squared for a given heater, it follows that if you hook the same heater up to a 12-volt transformer, you will get 1/4 power or 37 watts. At 18-volts, you get a bit more than half the power. The corresponding current draws will be proportional to the voltages, and will be 3 amps for the 12 volt transformer and 4.5 amps for the 18-volt transformer. This has been left as an exercise to the reader. :) - Erik -- Erik Olson erik at thekrib dot com

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