View Full Version : Thermostat thingy

10-26-2001, 08:17 PM
Whats that little thing-a-magig underneath the cap of your basic round manual thermostat? Is like usually a copper arrow on a scale with different settings ranging from .1 to 12. Whats the point of it?

I feel my masculinity depends on knowing these types of inane home/electronic items.

10-26-2001, 08:25 PM
I think what you're describing is the heat anticipator (http://www.howstuffworks.com/home-thermostat2.htm).

(This site (http://www.howstuffworks.com/) has saved my masculinity countless times)

10-26-2001, 11:13 PM
hmm, go figure

JJ Richard
10-26-2001, 11:26 PM
Looking at the article (http://www.howstuffworks.com/home-thermostat.htm/printable) on HSW, a question popped into my mind concerning those mechanical thermostats. How is hysteresis achieved in that design?

On digital thermostats, hysteresis is obviously achieved by programming, but what about the mechanical devices?

Seems to me that once the temp coil (un)winds enough to reach the gravity "threshold" (to pull the mercury in one direction), that there would be a point where the mercury is "on the fence". Assuming that the temp coil moves very slowly, wouldn't this result in the mercury repeatedly "making-and-breaking" contact prior to fully flowing to the opposite side of the ampule (chatter)?

Is there a time delay built in between the time the mercury initially switches states and the corresponding result? If there is a time delay, where is the circuitry for it located? (I don't see any caps or dicretes on the PCB.)

If there is no active time delay circuitry, then I must assume that the physical temp coil movement is fast enough to avoid chatter. But then that raises another question...

What accounts for the upper/lower temperature trip points? If the temp coil moves quickly enough to avoid contact chatter, then it seems as though it would constantly be cycling the heating/cooling operation at too rapid a rate. In other words, it would turn on and say heat the home to 70, but as soon as the temp drops to say 69, it would cycle back on again, since the physical movement of the temp coil is so fast.

What gives?

FYI, holding down the ALT key, pressing 167, and then releasing the ALT key makes the symbol...

JJ Richard
10-26-2001, 11:40 PM

Meant "...discretes..."

10-27-2001, 08:49 AM
Something is confusing me about their description of the heat anticipator. They are saying:The farther the wiper is positioned (moving clockwise) from the yellow wire, the more of the resistive wire the current has to pass through. Like any resistor, this one generates heat when current passes through it. The farther around the loop the wiper is placed, the more heat is generated by the resistor. This is implying that MORE resistive wire in the loop will generate more heat. This is at odds with what I remember from my electronics classes (admittedly almost 10yrs ago)

One of the valid equations for power generation (heat) is:
P=V 2/R

If you increase the length of the wire (R) without a corresponding increase in voltage, the total power generated is reduced, right?

I must be missing something here, since their description of how it is wired appears to be correct, based on the picture. That is, moving the wiper to a higher number appears to include a greater length of the resistive wire in the circuit.

10-27-2001, 01:12 PM
If you increase the length of the wire (R) without a corresponding increase in voltage, the total power generated is reduced, right?
Yes, but keep in the mind the voltage is the voltage drop across the load, not the total voltage in the system.

Since the anticipation resistor is in series with the main load (the gas valve or relay in the furnace) and is relatively small in comparison with that load, the current through the resistor is basically constant. This means the voltage drop across the resistor does increase as the resistance increases, and so you get a little more power dissipated, and a little more heat generated.


10-27-2001, 05:17 PM
I thought this guy was Omniscient? ;-)