Dimmer as backup to Thermostat?

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Originally Posted by td30

With the Thermostat at full output, it should be able to achieve the target temp (92) while at the same it will be capped at 95 deg by dimmer.

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Don't do it. If you want a backup, keep the rheostat on hand in case the thermostat fails, don't try putting it inline.

As for why, a thermostat is just an electrically controlled rheostat, and as such, it's internal circuitry is only certified to operate at a specific voltage and amperage. By under-volting that circuitry, you can potentially damage it, and leave it incapable of controlling the rheostat, leading to possible fire hazards and increased risk to the animals being heated. Beyond that, you would be trying to keep a steady temperature based on a balancing act, and a slight change on either side (ie, bump the dimmer by accident) could lead to dangerously high temperatures.

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Originally Posted by ChrisBowsman

Ok, I know it's only been a few minutes, but the more I think about it, it *should* work.

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It might technically "work", but it will completely invalidate the ability of the thermostat to thermo-regulate correctly, will most likely cause damage to the thermostat itself, and will put the animals at risk.

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Originally Posted by td30

Both of these devices (Dimmer and Tstat) are current limiting devices.

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Yes, but one has internal circuitry designed to operate within a specific power range to limit the current.

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Originally Posted by td30

And yes, the t-stat will increase power to compensate. Does it do this by reducing resistance?

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Depends on the model of t-stat, but most simply switch the power on and off until the temperature is in range. In either case you can damage the circuitry that does the thermo-regulation, whether switched or metered, by under powering it.

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Originally Posted by td30

The t-stat running at something like 98% open to achieve a temp of 92 deg. If it were to fail completely, then the dimmer would be getting 100% of the power

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Most will shut down in case of a failure, an important feature to note when purchasing a thermostat. Also, most thermostats don't meter the power amount, but rather are "set" for target temperature ranges, which are measured for by the probe.

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Originally Posted by ctrlfreq

Most will shut down in case of a failure, an important feature to note when purchasing a thermostat. Also, most thermostats don't meter the power amount, but rather are "set" for target temperature ranges, which are measured for by the probe.

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This is why everyone should own a Herpstat.

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Originally Posted by ctrlfreq

As for why, a thermostat is just an electrically controlled rheostat, and as such, it's internal circuitry is only certified to operate at a specific voltage and amperage. By under-volting that circuitry, you can potentially damage it, and leave it incapable of controlling the rheostat, leading to possible fire hazards and increased risk to the animals being heated. Beyond that, you would be trying to keep a steady temperature based on a balancing act, and a slight change on either side (ie, bump the dimmer by accident) could lead to dangerously high temperatures.

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If the rheostat was going BEFORE the thermostat, I'd agree with you. However, it is AFTER, so it won't affect the amount of anything the thermostat sees.

The t-stat will be set to provide enough juice to heat the UTH to 92 degrees. If the r-stat is set to allow the passage of enough to heat it to 95, there is no problem.

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Originally Posted by Skoalbasher

This is why everyone should own a Herpstat.

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The Helix DBS-1000 has the same features. I have two Helix DBS-1000 T-stats that work great, I also have 2 Herpstat Pros and they work great also. I also use 3 Rancos on my incubators and have used them on tubs with no issues. All three of these are quality products in my opinion. I plan on trying Big Apples on off T-stat and proportional eventually, but I already see a problem in they do not have a digital temp read out showing what the probe is reading.

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Originally Posted by ChrisBowsman

If the rheostat was going BEFORE the thermostat, I'd agree with you. However, it is AFTER, so it won't affect the amount of anything the thermostat sees.

The t-stat will be set to provide enough juice to heat the UTH to 92 degrees. If the r-stat is set to allow the passage of enough to heat it to 95, there is no problem.

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The rheostat is a resister which limits current by using power! This means it will require the proportional T-stat to push more power to achieve the proper temps. This could result in overloading the proportional T-stat as most are only rated for between 400 Watts to 500 Watts. That equals a little less/over 4 amps at 120 Volts AC. In order to create a limiting point you will have to create a situation that limits based on its max Wattage. Running at max wattage is taxing on SCR's and transistors as they run hotter. This reduces the life of the proportional T-stat and also will create problems with the overload circuitry shutting down and no heat being created until it is reset. The only way I would do it is use a On/Off T-stat before the proportional controller if a back up was needed!

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Originally Posted by ChrisBowsman

The t-stat will be set to provide enough juice to heat the UTH to 92 degrees. If the r-stat is set to allow the passage of enough to heat it to 95, there is no problem.

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First, if the thermostat is set to heat to 92 degrees, you will never see 95 degrees regardless of whether there is a rheostat inline or not. The thermostat is going to turn on and off based on the probe temperature. The risk is now the fact that by introducing another ~12 feet of wire (the length of the run to and from the inline rheostat), and it's associated resistance, the power draw against the thermostat is going to be greater than it needs to be, and which can shorten it's lifetime considerably.

In either case, the answer is simply to make sure the thermostat will shut down in the case of a problem, and to have a rheostat on-hand to replace it with temporarily if needed. Anything else will shorten the lifespan of the equipment, and potentially put the animals in unnecessary risk.

As a side note and stating a situation where backups are a good idea I would like to bring up how I wired my incubator. I see a lot of people using 1 T-stat or such on simple home made incubators. While if you are incubating normal ball pythons or such this is a readily acceptable way of doing it. Or if you are just playing around to see what you can do it works pretty good and is an inexpensive way of getting the job done. Now when you get into high end morphs that have the potential to sell for over a grand apiece is the risk to even one egg worth the 3 to 4 hundred dollars saved? I personally bought a professional incubator that was recommended by one of the more notable breeders that we purchased ball pythons from, Michael Cole of Ballroom Pythons South. He uses the dual wafer type T-stats as a primary and back up, but also backs all this up with a Ranco as the ultimate back up that shuts everything down now if 91 degrees is reached inside. I took this one step further and replaced both wafer T-stats with rancos and then added in the third as a full blown emergency shut down. The primary is set to 88 degrees the secondary is at 89 degrees and the master over temp one is set to 91 degrees. So if the 88 degree one fails the 89 degree one will take over as normally closed relay contacts are in use on the ranco if the 89 degree one fails the 91 degree one will start to cycle the whole unit including the fan which the 88 and 89 degree T-stats have no control over. I really like this triple back up idea on an incubator as it may run 3 to 5 days at a time without getting more attention than a glance, where as our snakes have their water changed daily meaning they are inspected daily and heat gunned daily! When you consider the cost of the whole set up at $650.00 and it can hold 9 clutches of eggs compared to the cost of loosing one Bumble Bee morph before it even hatches $650.00 becomes pocket change really quick. This is probably one of the only cases I would really consider using a back up for. At most on my snakes I may install a Hi/Lo temp alarm on them to warn me if the temps get out of range and let me know action is needed!

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Originally Posted by td30

Thanks for the reply, but I am still having trouble wrapping my head around this. Both of these devices (Dimmer and Tstat) are current limiting devices. Yes? No? I agree that the dimmer will decrease the power of whatever is input. And yes, the t-stat will increase power to compensate. Does it do this by reducing resistance? If so then will the t-stat at some point be 100% open as if it were not even there? This is what I an envisioning. The t-stat running at something like 98% open to achieve a temp of 92 deg. If it were to fail completely, then the dimmer would be getting 100% of the power (instead of 98%) as if the t-stat were not even there. In that case, the dimmer would still restrict the power to a safe level. I may just need a really dumbed down explanation.

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Ok...let me try again to explain why it won't work...A dimmer does not measure temp so it has no idea when the thermostat is acting up and sending too much power. Back to my example: if the thermostat is outputting 100 degrees. First how do you set the dimmer at 105?? (you can't..it is impossible cause there is no way to tell it is set at 105 degrees - as the other thermostat is only outputting 100 and the dimmer cannot tell temps). There is nothing you can do with a dimmer when used with a thermostat (either before or after the thermostat...they work differently)