v_various said:
The only problem anyone had was one lady who lost her entire collection a while ago due to a broken thermostat. Her snakes cooked.
Spyder Robotics discusses that on their site and explains how to add a cheap second thermostat as a fail safe.
Question:
What happens if the thermostat malfunctions and the power remains on? Does the thermostat have a fail safe that will shut it down?
Answer:
This is a interesting concern that I have been asked before. I have heard reports of other manufactures having problems with their thermostats failing in this mode. The component in proportional thermostats that controls the power is called a Triac. These devices produce a lot of heat and require a heatsink in most uses. They have amperage ratings just like a fuse. In my research I found many manufactures used a triac that was rated at or barely over the maximum wattage rating for the thermostat. Knowing that this component takes the most abuse I opted to use a triac in my designs that was well overrated for its use.
For example:
A 500 watt load on 120vac is pulling 4.2 amps.
A 700 watt load on 120vac is pulling 5.8 amps
Many of the 500 watt thermostats out there use 4-8 amp triacs. The Herpstat products use 15 amp triacs. This means they "in theory" can handle up to 1800 watts. However, things like board trace size and heat generation limits these devices so the Herpstats are fused at lower ratings which protect the device. The cost difference between the high quality triacs I use and the triacs in other thermostats is about $0.50. Personally I thought it was money well spent.
So the short answer is there is no secondary failsafe if the device fails in this mode. However; in the two years I have manufactured these I have NEVER had a report of a herpstat failing in this mode. In fact the Herpstat II has temperature sensitve fuses and a digital sensor inside the device that monitors internal temperature which shuts down the outputs if the internal heat exceeds the normal operating temperature. The Herpstat II is also vented and has a built in cooling fan. Remember, heat is the typical killer of these components. Also keep in mind that even with the highest quality components ALL electronics have the possibility of failure. Therefore I offer the following suggestions.
SUGGESTIONS:
As a secondary backup to ANY proportional thermostat use the following setup. Purchase a nonproportional thermostat with a wattage rating at or above the proportional thermostat's rating. Attach this thermostat to the wall outlet and set it for your desired failsafe temperature (NOT your normal target temperature). Then attach your proportional thermostat to the output of the "failsafe" thermostat and set it as usual. Place the probes of both thermostats in the same location. The idea is the proportional themostat controls the environment and should it fail and reach the failsafe temperature the "failsafe" thermostat will cut power. Of course at this point the environment will be regulated by the failsafe thermostat at its temperature. The environment would be warmer but assuming the failsafe is set to acceptable temperature the animals will remain safe.
Experiments:
I was able to use a inexpensive Zoo Med Repti Temp 500R with a Herpstat Proportional thermostat. I initially set the Repti Temp 500R up by itself and monitored the environment. I set it to regulate at a temperature slightly higher than I wanted the environment to be at. I monitored it closely to confirm its regulation. I then disconnected the heating element and attached it to the Herpstat output and attached the Herpstat's power cord to the Repti Temp's output.
Outcome:
It worked very well. I simulated a failure by removing the Herpstats probe from the enclosure. The enclosure continued to heat until the Repti Temp cut power. Once the temperatured lowered the Repti Temp powered up, the Herpstat powered up, and the Heat was regulated by the Repti Temp.
Conclusion:
For inexpensive backup of standard 500 watt proportional thermostats I would recommend the Zoo Med Repti Temp 500R. For higher wattage backup I would recommend a relay controlled thermostat such as the Ranco or Johnson Controls models. This worked well with the Herpstat products and should work for others.
Another good practice is to use a surge suppressor (such as the ones used to protect computer equipment) in between the thermostat and the wall outlet. This will protect the devices from current spikes that are common to household current.
Dion Brewington
Owner, Spyder Robotics
The information in the article is the opinion of the author and is to be used at your own risk. Spyder Robotics assumes no responsibility for the use of this information. Using this information could void the warranty on the thermostats. Check with the manufactures for details. Repti Temp 500R is a trademark of Zoo Med Labs
