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Old 06-06-2014, 10:16 PM   #15
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Just a clarification on how these automatic lighting direct spark ignition systems work. They are now the most common burner lighting systems used in not only refrigerators, but in water heaters and furnaces.

In older pilot light systems, the thermocouple was a device which generated a small amount of electric current when immersed in the pilot flame. In essence, it told the system that the pilot flame was burning, and it was safe to allow gas through the main valve for ignition by the pilot light. You manually held the system valve open (remember holding the red button down?) until the thermocouple got hot enough to generate enough power to magnetically keep it open by itself. If the pilot light went out for any reason, the thermocouple cooled down and a spring closed the system valve, so no gas could flow to the main burner. Inexpensive water heaters, both home and RV type use this system to this day. It is simple and reliable and inexpensive. It is also used in some RV propane refrigerators (but not many anymore) which don't use any 12 volt power while running.

Then Direct Spark Ignition systems came along. The first ones essentially still used a thermocouple in this way: The electronic circuit board created a spark, the main valve was opened the main burner lighted, and the thermocouple quickly heated up and told the main board that there was heat (a flame) and it was OK to keep the gas flowing. If the thermocouple did not heat up the gas flow was stopped. Sometimes this was repeated automatically for 2 or 3 times until a lockout was automatically engaged if no flame was detected by the thermocouple. The system was kind of a transition between the old pilot light/thermocouple idea, and the next generation outlined below.

This next variation, and the one most commonly used today in most DSI appliances is a bit different in how it operates. It turns out that when an electrode is used in a flame path to ignite the gas, once the gas is lighted the spark no longer can jump the path between the electrode and the burner through the flame. This has to do with the flame becoming like an insulator, preventing further sparks from being thrown across the path. So, what the designers of the electronics did was something like this: When the system is called on to work (usually by an electric or electronic thermostat) the circuit board produced a spark series, (you hear a clicking) then the main gas valve opens sending gas to the burner. Once the burner lights, the spark no longer can jump through the flame. The circuit board is designed to sense that there is no more spark jump, and gas is allowed to continue to flow. So, the absence of a spark tells the board to keep the gas flowing. If the flame goes out when it should not, the spark automatically starts again and re lights the gas. It can start to spark again as the flame resistance has disappeared. Various lock outs are incorporated for additional safety. Pretty neat, and no thermocouple system needed, just some fancy electronic control systems.

So, in essence, there are no more thermocouples in DSI systems today. The spark finger MUST be in the flame once the burner is lighted off so the spark will stop and the board says it is safe to continue to send gas into the system. And that is the reason that the posters above have found the spark finger location to be so critical for proper operation of the refrigerator. Same would be true for water heaters and furnaces built now.

Realize that I have simplified things that the circuit boards do for safety; timing, repeat cycles, lockouts and so on. But the essential thing is that once the flame is established, the spark can no longer jump the gap and that is how the flame is "proved" to the circuit board. It is no longer done with a thermocouple, but with more what I would call a "stop spark due to flame resistance" system. So, the placement of the spark electrode is now very critical in most all DSI systems used in refrigerators, water heaters, and furnaces.

Just one more variation to keep in mind for you do it yourselfers out there. The spark finger must be in the flame path for proper operation of most current DSI equipment. If the spark keeps going the board electronics is going to say... OOPS, no flame, and shut the gas off.

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Old 06-07-2014, 02:25 PM   #16
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Idroba thank you for your explanation! After finding "my cure" it all makes sense... why.

Someone at the Dometic factory needs to tweak their placement of this thermo couple, as it is placed in the same stamped out metal frame that the two part system used. Because of the placement the "improved" system sensing system, the fridge did not operate as it should have. I am a bit surprised that it worked much even when new in 2006.

Now I understand that standing in your underwear at 3AM turning the fridge on and off and eventually the flame would stay lit... eventually heating the sensors that controlled the propane.

Because of your well described process of operation, there will be many other do it yourselfers that can bend the thermo couple into the propane flame and live happily ever after, as I am with a working unit!

Thanks thanks thanks to all!

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Old 06-13-2014, 09:50 PM   #17
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Does anyone want to see how I bent the thermo couple to place it over the propane flame? If so I can take a photograph of where I applied the "adjustable monkey wrench" and just a slight bend into the flame.

I am now hoping to repair the Dometic A/C that toasted the Starter Capacitor and Motor Starter and in the process of receiving those parts shortly. Another learning process we all should be able to do, as well.

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