Dimmer as backup to Thermostat?

[Gloryhound]

After this question really bothering me last night I had to sit down and do all the math and draw myself many pictures plugging in various resistances. After doing all this I have come to the conclusion that the rheostat in series after the proportional controller would work! I still do not believe it should be done though as it has the potential to become more probmatic than just using the proportional and will slow down the reaction time of the proportional controller possibly causing the temps to vary almost twice as much as it would with just the proportional controller. Here is the math that makes it work.

All numbers are made up just to use as an example.

UTH = Resistance of 100 Ohms

Rheostat = variable resistance of 0 to 200 Ohms, but for the primary example we will use a setting of 100 Ohms to start out.

Max Voltage that can be applied = 120 VAC

That means with just the UTH you would have a current of 1.2 Amps going through the UTH.
Amps = Voltage / Resistance
1.2 Amps = 120 VAC (Max AC voltage that can be applied) / 100 Ohms (UTH Resistance)
Thus max watts on UTH would be 144 Watts.
Watts = Volts X Amps
144 Watts = 120 Volts X 1.2 Amps


Add in the Rheostat set at 100 Ohms and the resistance of the circuit doubles to a total of 200 Ohms. Max current through the circuit gets cut in half to 0.6 Amps. Also the Max Voltage the UTH will see would be cut in half as each of the resistances would drop 1/2 the applied max current through it.
Amps = Volts / Total resistance of circuit
0.6 Amps = 120 Volts / 200 Ohms
Watts = Volts X Amps
72 Watts = 120 Volts X 0.6 Amps

Now in a series circuit the voltage drops across each of the resistances proportionally to the amount of the resistance. In this example the resistance are equal, so 60 Volts would be dropped across each load. Also in a series circuit the Current is a constant so each load will see the full 0.6 Amps of current flowing through it. This means each load will have to dissipate 36 Watts of energy.
Watts = Volts X Amps
36 Watts = 60 Volts (voltage used by each individual load) X 0.6 Amps

Now for the more advanced let us set the rheostat at 150 Ohms.
Amps = total applied Voltage (120 VAC) / total resistance of the circuit (100 Ohm for UTH + 150 Ohms for rheostat = 250 Ohms)
0.48 Amps = 120 VAC / 250 Ohms
Total Watts Used = Applied VAC X Amps
57.6 Watts = 120 VAC X 0.48 Amps

Now the voltage is not equally divided between the two loads since they have different resistances so we have to figure out what Voltage each load drops by using Proportions.

120 VAC / 250 Ohms = Y / 150 Ohms (Resistance of Rheostat)
0.48 Amps = Y / 150 Ohms
0.48 Amps X 150 Ohms = Y (We are back to basic Ohms Law)
72 VAC = Y
Now 72 VAC X 0.48 Amps = 34.56 Watts

Now lets do the UTH
120 VAC / 250 Ohms = Y / 100 Ohms (Resistance of UTH)
0.48 Amps = Y / 100 Ohms
0.48 Amps X 100 Ohms = Y
48 Volts = Y
Then 48 VAC X 0.48 Amps = 23.04 Watts

Lets double check.
Do the voltages dropped across each load equal the total applied voltage?
72 VAC (for Rheostat) + 48 VAC (For UTH) = 120 VAC
Does the total Watts used equal the Watts used by each load?
34.56 Watts + 23.04 Watts = 57.6

OK! I confess, I am an electroncis geek!

[lord jackel]

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.

[Gloryhound]

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.

[ctrlfreq]

After doing all this I have come to the conclusion that the rheostat in series after the proportional controller would work!

I'm not sure the question was really whether or not it "could" be done, but whether it "should" be done given the potential risk to equipment, heating elements, and animals being heated.

Sure, you can super-glue yourself to the seat of your car so you won't go flying around in the case of an accident, but why would you when almost all cars have such nice seat-belts already, and the super-glue will most likely ruin your clothes

[lord jackel]

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.

But using this same logic doesn't this mean that you really don't need a back-up at all since even if the thermostat is stuck open it cannot push enough current to cause an issue?

A Rheostat by design lowers the amount of power getting through it...so if it blocks 10% then is blocks 10% of 1000watts or 10watts - the difference is still huge. This is the same reason why the actual temp in the tub fluctuates with the ambient temp of the room when using a rheostat cause it cannot compensate for increase or decreases in temp.

[Gloryhound]

But using this same logic doesn't this mean that you really don't need a back-up at all since even if the thermostat is stuck open it cannot push enough current to cause an issue?

A Rheostat by design lowers the amount of power getting through it...so if it blocks 10% then is blocks 10% of 1000watts or 10watts - the difference is still huge. This is the same reason why the actual temp in the tub fluctuates with the ambient temp of the room when using a rheostat cause it cannot compensate for increase or decreases in temp.

Absolutely correct and why I stated that while the circuit would work with the rheostat, I still do not recommend it! You would to clamp a UTH at a certain wattage you would have to calculate its resistance through the use of the Rated Voltage and Wattage of the UTH. Then calculate a resistance to put in series with that that would create a situation in which only a given voltage that would keep the wattage from going past a given point. At this point you would use a fixed type resister in series with the UTH to accomplish this. Still not the way I would do it, but it is a way to accomplish a means! But to figure out what wattage you want the heater to run at you would need to get into transfering Watts to BTU's and calculating volume of the enclosure and a whole bunch of other variables such as temp of room the enclosure is in, which in most houses will not stay constant throughout the year. Back to constantly making adjustments on your rheostat. Not fun!

[lord jackel]

Absolutely correct and why I stated that while the circuit would work with the rheostat, I still do not recommend it! You would to clamp a UTH at a certain wattage you would have to calculate its resistance through the use of the Rated Voltage and Wattage of the UTH. Then calculate a resistance to put in series with that that would create a situation in which only a given voltage that would keep the wattage from going past a given point. At this point you would use a fixed type resister in series with the UTH to accomplish this. Still not the way I would do it, but it is a way to accomplish a means! But to figure out what wattage you want the heater to run at you would need to get into transfering Watts to BTU's and calculating volume of the enclosure and a whole bunch of other variables such as temp of room the enclosure is in, which in most houses will not stay constant throughout the year. Back to constantly making adjustments on your rheostat. Not fun!

Oh good...I thought we were dancing around the same answer from different points of view (or to different ways of asking the question).

So in short for everyone following this -
Would the circuit work? - YES.
Should it be used as a backup - NO

[Gloryhound]

Oh good...I thought we were dancing around the same answer from different points of view (or to different ways of asking the question).

So in short for everyone following this -
Would the circuit work? - YES.
Should it be used as a backup - NO

I second this motion! All in favor say "aye"!

[td30]

Oh good...I thought we were dancing around the same answer from different points of view (or to different ways of asking the question).

So in short for everyone following this -
Would the circuit work? - YES.
Should it be used as a backup - NO

I second this motion! All in favor say "aye"!

Thanks for helping to figure this out. I guess I never thought it would be the best way to do it, but it made sense in my head and I was hoping it would work. So needless to say, I won't risk damaging my new t-stat (or snake) to test it.