OK, so I am soon building a vivarium (that's the dry-land version of an AQUArium) for my boa constrictor.

To make a long story shorter, I purchased two UPM HTM611-A digital programmable thermostats, designed for controlling line-level electric heating elements, such as 120/240VAC electric baseboard heaters. They use a TRIAC switch to control the current to the heater(s). The units are TWO WIRE only (wired inline), non-polarized, and the control circuitry in them taps its power off of the line-level feed to the heater element.

The electrical specs for these thermostats are:

MAX load: 3000W @ 240VAC or 1500W @ 120VAC
MIN load: 500W

My plans for heating the vivarium (and lighting, which also gives heat) include at least one 250W ceramic heating element, at least one 100W spotlight, and about 10W for a floor heater.
As you can see, there is less than 500W load here no matter which way I slice it, which is below the MIN LOAD spec for the thermostat.

I wrote the "technical support" address of the manufacturer, and some bonehead from what I suspect was "customer service" replied with an admittedly vague, not very concise, but friendly reply to my inquiry as to what significance the minimum load specification had to do with the operation or functionality of the thermostat.
He alluded to possibilities like "the display may fade out" or "the clock may lose time during a power outage" or "the thermostat may falter and not operate properly". He invited me to inquire further if i needed to, so I did, to clarify what he meant by "falter" and "fade" and ask whether or not there was any risk of actually damaging the device, or if there was simply the possibility that it may not operate properly.
He didn't seem to like my questions this time, and his reply was rather curt, to the effect that ".. the TRIAC switch circuit will burn out completely if under-loaded, and that using the device as you intend to will void the warranty.."

Now, my workaround plan IF DAMAGE TO THE DEVICE is at all possible, is to just use a minimum of TWO 250W ceramic heater elements in parallel, and to control them with one of the built-in proportional modes that the thermostat offers so that I don't get heater overkill.

My question "Why?" still remains: Can someone tell me the exact reasoning behind a minimum load requirement for a TRIAC switch in this situation (or any situation) and whether or not the possible result of under-loading the TRIAC circuit could actually damage the TRIAC circuit in this thermostat, or if simply the thing may just not work correctly?

My electrical/electronic knowledge is pretty good; it has been my hobby for a long time. But I haven't done a lot with triacs, and certainly not lately. Therefore I don't know about the operation of triacs in general to make an educated judgment call here.

I found some related information HERE which seems to imply improper operation with a too-low load on a triac, but if you read the article, you will see it applies mainly to LEDs and other low current applications, though the theory is the same.
In their case, they add a parallel load (like a resistor) to make up the difference when the load becomes too low, but as also mentioned in the article, that is a waste of energy.

Oh, and finally: I should mention that I did test out one of the units by temporarily connecting it inline with a 60 watt lamp, and it operated perfectly fine.

Thanks for reading, and for any insight!

Sasha

be careful. Some unscrupulous hacker will hack into your vivarium through the electrical line.


















ahahahaha sorry.. couldn't resist.

As this isn't a Linux question, I've moved it to the other General.

It might be a linux boa constrictor.

It might, but boas tend to be BSD fans

Triacs are slightly strange devices - they act like a switch (on/off, not a linear control device), but they latch on and turn off if the current goes below a minimum value.

Now I don't know the electrical details of the exact model of switch that you will use, but you can see that there is potential for difficulty here: if the load is below some minimum value, the triac will not be able to switch on correctly, or may switch back off immediately. If this kind of behaviour occurs, it is unlikely that the device will meet its specifications for power factor or electrical interfernce generation (and may have susceptibility to noise itself).

You should also note that fitting, say, heating elements with twice the rating will not result in a higher level of power consumption, it will only result in the elements taking roughly twice the current for half the time (if that is what is concerning you).

On wikipidia you will find some general information about TRIAC
TRIAC are often used in electro world , for example in light dimmers in you are computer and so on


all the best