Quote Originally Posted by lord jackel View Post
I still disagree...as to how it relates to the use as a backup. Firstly how to you know precisely what the rheostat is set at (it is only a rotary dimmer and has NO WAY to monitor any temp or wattage change coming to it). It is still limiting the output coming through it as a percentage of the total power being applied to it.

So you apply 100 watts from the thermostat and dial in the dimmer so it achieves a 92 degree temp (you could even argue that you increase the dimmer slightly so as to give some unknown amount of overhead - but again you can't be sure what that is as there is no way to be sure).

So you set it up as you describe...but then the thermostat breaks and locks open applying the full 500 watts (what most thermostats max out at) to the system so the dimmer will only limit this 500 watts to what?? It will only limit it as a percentage so even it the dimmer was turned down to 50% you would still be applying 250 watts to the heat tape...which is plenty to cook your snakes.

Again a dimmer won't work as a back-up to a thermostat.
Don't get me wrong either I don't like the idea of doing this as a back up either and believe if you really felt you needed the security of a back up an on off type Thermostat is the best way to go!

But as for the math and science it should work. Also you have to remember the resistances are not changing and the proportional T-stat has a max voltage output. Even though the Proportiona T-stat is capable of supplying 400 Watts of power it can only supply 120 VAC or some other voltage lower depending on manufacturer. The resistance of the heat mat is based on a given voltage (Normally 120 VAC, 110 VAC, and 220 VAC if in europe.) So say it is rated for 20 Watts Amps = Watts/Voltage
Amps = 20 Watts / 120 Volts = 0.166666666 Amps.
Resistance = Volts / Amps or 120 Volts / 0.166666666 = 720 Ohms. The value of resistance is not going to change regardless of the actual applied voltage. So the Proportion T-stat will not be able to push its full rated watts out through this singel load. You have to have multiple heat strips in parallel to get close to the max watt rating of the controller.