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Primer

Most people know about 24V HVAC control systems, as that's what's commonly found on furnaces, heat pumps, central air conditioners, and other such systems. If you have electric unit or radiant heat, you may have seen line-voltage thermostats as well that run directly on mains voltage.

However, there is a third type of control system for gas valves, the so-called "millivolt" system. This system uses a device called a thermopile made of many thermocouple junctions in series to produce a control voltage on the order of 500 to 750mV when heated by the pilot flame. This is enough to operate a gas valve through the contacts of a manual switch or mechanical thermostat. This system has the advantages of being intrinsically safe (the system won't attempt to run at all if the pilot's not lit) and self-powered (it needs no external AC supply to work), so it's most commonly found on gas fireplaces.

An example thermopile is the Honeywell Q313 series; we can assume that its specs are representative.

My problem

However, if you want to test this type of system, a problem arises. Running a gas valve with all the ports open to outside air won't hurt it, as far as I know. However, I don't know about your workbench, but mine is not rated for the temperatures needed to get such a thermopile to generate its rated output!

So, in order to mess around with making a timer, fancy-thermostat, etal that works with such a system, or to test theories about suitable switches, you need a thermopile simulator. In the past, I built one according to the below schematic (with Roffset = 54.9kΩ, a 20kΩ single turn pot for Radj, and Rseries selectable between 1.5Ω and 3.6Ω) and it appeared to produce reasonable results, but I never was able to fully test it with an actual gas valve, and do not have the constructed unit at hand any longer.

schematic

simulate this circuit – Schematic created using CircuitLab

The question

Is this a reasonable/reasonably accurate simulation of a gas valve control thermopile under steady-state operating conditions, assuming that my open-circuit voltage and series resistance are chosen to match the specifications of an HVAC thermopile (for the Honeywell linked above, I'd be setting up the LT3080 as before, but with an Rseries of 2.87Ω)? By "reasonably accurate", I mean that if I took 100 random, known-good millivolt gas valves, and attempted to operate them "on the bench" with this rig, 99 of them would operate normally for say, at least 10 on-off cycles of operation.

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  • \$\begingroup\$ Power dissipation could be pretty high in the regulator. Why do you want to tamper with safety devices? \$\endgroup\$ – Spehro Pefhany Dec 7 '16 at 2:59
  • \$\begingroup\$ @SpehroPefhany -- power dissipation is something I can live with here (the original board had a big bare copper pad smack in the middle for that). The original intent was for testing an ultra-low-power programmable thermostat's ability to harvest energy from the thermopile; that project's no longer on my radar, though, but I actually came up with a usage recently to simulate a common failure mode of manually controlled millivolt gas valves where ordinary wall switches sulfidate and develop excess voltage drop, leading to the switch no longer being able to turn the gas valve on. \$\endgroup\$ – ThreePhaseEel Dec 7 '16 at 3:04
  • \$\begingroup\$ Is your workbench rated for a spirit burner, or are naked flames not allowed? \$\endgroup\$ – Andrew Morton Jan 23 '17 at 13:36
  • \$\begingroup\$ @AndrewMorton -- naked flames are a no-no. I'd use a stovetop burner in the kitchen but I don't think abusing a smoothtop electric range in that way would be a good idea or yield equivalent data to a gas flame. \$\endgroup\$ – ThreePhaseEel Jan 23 '17 at 23:06
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To obtain a definitive answer, I recommend that you obtain the 100 gas valves and or their data sheets, find and average their optimum operating characteristics(requirements), and determine their standard deviation.
Design your simulator to provide the average optimum parameters and allow an adjustment equal to 3x the standard deviation. This guaranties that you will be able to adjust your simulator to meet the particular requirements of any given gas valve.

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