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From the standpoint of heating element wear, control device load contact wear, I don't like the idea that an on/off thermostat turns the heating load full on/full off continually in order to keep the desired temperature within the set point range. A proportional stat is even worse regarding full on/full off continual switching since it controls the heating element with a series of rapid on/off cycles to maintain the desired temperature (contrary to alot of peoples beliefs a proportional stat is harder than an on/off stat on the heating element).

My idea for control of heat cable in a rack I am building is that since It will be in a temperature controled room, I will control the cable with a voltage regulating rheostat ($10). I will adjust the rheostat using the reading of a remote probe thermometer($10). I will also install a ranco on/off thermostat but only as a high limit protector. If the rheostat were ever to fail to a full on condition or were accidentally turned full on, the ranco would open the circuit when a temperature of say 10 degrees above the normal set temperature occured.

(contrary to alot of peoples beliefs a proportional stat is harder than an on/off stat on the heating element)

If what you say it true, then your idea sounds great to me. But I'm hardly the electric/heating person on this forum.

I have heard that one of the reasons why Bob @ Pro-Products is so anal about the proper wattage of their panels is for this reason - so I'm inclined to believe it.
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Current snakes:

0.2 Gonyosoma oxycephala - (Silver/Yellow)

3.3 Gonyosoma oxycephala - (Green)

2.1 Gonyosoma janseni - (Black)

3.3 Gonyosoma janseni - (Black & Tan)

I agree with chris in this is an ideal stand point with some electrical background I can say your math is right. In fact I have the same setup with my cages. I used Com. dimmers to adjust each piece of heat tape rather than the wiring them in series. The whole setup is protected by a trusty Ranco.
Good idea
Jeremy

For us low tech sorts ( probably a lot of us) showing how we might do this for the sake of additional safety of our families and herps? Thanks. Bob.

Wall-X-Ranco---Dimmer--X--Source
.........X...'-----------------------X--Probe
.........X.................................X
.........X.................................X "cage"

Set Ranco for above normal temps. "safety cutoff"
Adjust Dimmer with heat gun to ultimate temp.

This can be more complicated if you like but this is the basics.

Using the dimmer first rid of the on off of a thermostat and keeps a constant power imput instead.

Questions?
Jeremy

n/p

I was thinking. With most of the wiring ideas that are presented, a common theme is not to depend on a primary way of controling the heat but to have a secondary fall back.

With a given that you are in a room with a controlled temp. Could you do the same thing without using a Ranco and just use 2 com. dimmers, one opened up and taped/secured a litte past the primarys setting so that if you used a timer for nite time drop if the first dimmer failed at full on you would have the second secured dimmer as a fallback/safety without the expense/complication of a Thrmostat? and without the full on power wear on your heater, not to mention the benifit of an auto nite drop for those times when your busy.

Just a thought. Not very savy with electrical stuff.

Thanks, Bob

I completely forgot that you said you were going to have a temp controlled room. What temp are you going to keep it at? If you called up pro-products, they can sell you a model that will use such a specific wattage that it can probably be plugged in 24/7. However, you could put your thermostat on it as a high level control to ensure it happens.
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-Chris

The reason mainstream thought is thought of as a stream is because it's so shallow. -George Carlin

A fool doesn't learn. A smart man learns from his mistakes. A wise man learns from the mistakes of others. Which one are you?

My Website
N. American Rat/Corn snake care sheet I wrote
Information on substrates

Current snakes:
0.1 Licorice Stick Black Rat (Lola)
1.0 Black Rat (Frankie)
0.1 Texas Bairdi (Rosa)
0.1 Blue Beauty (Brunhilde)
1.0 Green Tree Python (Monty)

I think the appropriate expression would be cutting off your nose to spite your face. A rheostat/dimmer works the same way, essentailly, that a porportional thermostat does. A porporational thermostat just basically automatically will control the level of how far it is "dimmed."

The way a 110 volt alternating current (110 V AC) runs is basically that of a heartbeat. The way a dimmer works is it basically cuts the top and bottom of the line off. So yes, you are correct in saying that a porportional thermostat is just constantly turning the system on and off - it is cutting power only letting a certain amount through. To put it in graphical terms here's some pictures.

This is how an alternating current runs.

This is where your dimmer will cut it on and off at.

To answer your question, will this dimmer/thermostat setup work? Yes. Will it theoretically actually shorten the life of your heating unit? Yes, however no more than a porportional thermostat. I would also set the thermostat to the temp while the rheostat is in the full on position - to ensure that it's not too high. Some of those lower end thermos aren't all that accurate (I know, I use 3 of 'em.)

By the way, these graphics were taken from howstuffworks.com - this place will tell you how just about anything you're wondering about works. (I hang out there a lot, lol)

This is a link to explain in more detail how a dimmer works.

home.howstuffworks.com/dimmer-switch2.htm

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-Chris

The reason mainstream thought is thought of as a stream is because it's so shallow. -George Carlin

A fool doesn't learn. A smart man learns from his mistakes. A wise man learns from the mistakes of others. Which one are you?

My Website
N. American Rat/Corn snake care sheet I wrote
Information on substrates

Current snakes:
0.1 Licorice Stick Black Rat (Lola)
1.0 Black Rat (Frankie)
0.1 Texas Bairdi (Rosa)
0.1 Blue Beauty (Brunhilde)
1.0 Green Tree Python (Monty)

Electrical switching of a heating element causes wear only if the heating element is temperature-cycling by a large amount. Consider a light bulb controlled by an ON/OFF controller. When the bulb is ON, the filament in the bulb may be at 200 deg. When the bulb is switched OFF, the filament may cool to the cage temp. This heating-cooling of the filament causes the wear. The more heating-cooling cycles with a large temperature delta you have, the faster the wear.

In heat pads/cable, the wear is less for a few reasons. One: we aren't talking about a brittle filament that is losing electrons to convert energy to light. And Two: The temperature of the filament in a heat pad is far less than in a light bulb.

Proportional controllers do not cause any undue wear on the heaters. In fact, since they maintain a relatively constant heat compared to ON/OFF controllers, proportionals are the best way to go. Doesn't matter if they are switching power to the heater all day long. The heater temp isn't changing much all day long. No worse than keeping the heater ON all day, or dimmed all day.

A dimmer acts alot like a proportional controller without the ability to maintain a setpoint as the load or background temp changes. A dimmer is not better or worse for the heater compared to a proportional controller.

And, a problem occurs with putting a dimmer in-line with an ON/OFF thermostat when you set the dimmer on a very low setting. The hysteresis in the dimmer trigger will cause the bulb or heater to not come back on once it goes off until you up the dimmer setting a little. This is a real pain. Only happens at low settings, but it happens.

Bottom line: For non-light-emitting heaters, proportional control is the best for temp control, for heater life, and for peace of mind that your temps will be at where you set them. For light bulbs, you can use a dimmer in line with an ON/OFF T-stat, but size the wattage of the bulb so you don't have to turn down the dimmer too low, this way avoiding that annoying hysteresis problem mentioned above.
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Mark G

I doubt I would have the dimmer set low enough to have a hysteresis issue. I think any way you slice the element wear argument, the comparitive life expectancy is in theory measurable, but in reality negligable.

an on/off stat like the Ranco would provide less steady temp control than my set up and could fail on. It has been known to happen. Someone I know lost $27,000.oo worth of Ball Pythons when a proportional failed on.

one down side to my setup would be that if the room temperature was lowered substantially (Say my furnace quits working in the middle of the winter when I'm on vacation.) an automatic temp control device rather than a manual one would sure be nice. Another would be that a dimmer switch is more vunerable to accidental adjustment. Another would be that when a dimmer fails, it usually fails on. Down side to a proportional stat would be the cost. My setup would run me around $60.oo a Helix stat would run around $130.oo does anyone know if Helix guarantees that if their stat fails it will fail off? If not add another $50.oo for a Ranco stat for high limit cutout. The cost curve really adds up when you are talking say 20 or 30 rack control setups.

You're right, dimmers and proportional controllers that use triacs or SSRs can fail ON. I've only seen it happen when one guy short-circuited his Flexwatt, but I have heard of cases when a dimmer failed ON for no good reason.

I've done the dimmer ON/OFF thing. Yeah, the ON/OFF does protect against high temps, but that d@mn dimmer needs to be adjusted all the time every time the room temp changes. I gave up on it, bought 3 proportional controllers and never looked back. I still use the ON/OFFs as a fail-safe. So much wiring, so many probes, but worth the peace of mind when I'm away.

I've also done the stupid trick of forgetting to put the probe back where it should be...
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Mark G

Another thought I had (I'm sure I am not the first) was that if there were a need to heat a large number of similar snake racks, with the same temp requirements, in a temp controled room, the whole group could be controled and protected by one dual on/off stat setup and a multi-pole contactor instead of a large number of temp control devices. The probes would be installed in a middle rack and would control a multi-pole contactor which all the heat element loads would be run through. Seems to me that the racks would all be kept pretty close in temperature. Hit me with some down sides to this.

This is fine. You can even get some solid-state relays and use 2 Rancos (or similar devices) in series (one is the fail-safe that is set a few degrees above the other). The output of the Rancos (120VAC) can drive the SSRs. Use the type of SSR with the 120VAC input. You would only really need one for each rack.

You could also use a mechanical relay (or three) with 120VAC coil voltage. Mechanical relays are cheaper but you'll need to solder to them usually, or else get the relay carriers with the screw terminals. You'll have to cover them so you don't get shocked.

The new SSRs come with a nice plastic cover and are finger-safe against shock. You can get this stuff at www.digi-key.com or search for DigiKey or Digi-Key on a search engine.
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Mark G