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I am sensing a flame using a flame rod. I am using 12V AC at the input and at the output flame sensing circuit which is made of opamp, some resistors and capacitors, I get 1.6-2.4V when there is a flame, and 3.8-4V when there is a short circuit, and 5V when there is no flame. My flame sensing circuit seems to work fine but when there is a flame and short circuit happens at the same time my circuit only detects short and gives output of 3.8-4V. But I want my circuit to give output of 1.6-2.4V and see a flame instead of a short when both occur together. Is there any way I can achieve this? I would really appreciate your opinions on this.

Thank you

I am using this circuit below. enter image description here

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  • \$\begingroup\$ I don't know much about flame rods, but from this article ( wardburner.com/flamerectification.html ) it looks like you are going to have trouble because the mechanism used to detect the flame is something directly affected by a short circuit. You might want to consider preventing short circuits, or playing it safe if a short occurs. \$\endgroup\$ – Cort Ammon May 5 '16 at 21:47
  • \$\begingroup\$ Show us your circuit. \$\endgroup\$ – Dave Tweed May 5 '16 at 22:11
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    \$\begingroup\$ @Sam: Welcome to EE.SE. We love circuit schematics much more than circuit descriptions. There's a schematic button on the editor toolbar. Add one in. Add links to any reference material. Explain what a flame rod is. I've never heard of it. Why do you get short circuits? \$\endgroup\$ – Transistor May 5 '16 at 22:26
  • \$\begingroup\$ I usually get short due to foggy weather or rain. If there is a flame I want the circuit to detect a flame and ignore short if there is any short. \$\endgroup\$ – Sam May 6 '16 at 14:18
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    \$\begingroup\$ What you ask makes absolutely NO sense at all! If the flame rod is shorted to ground, it is not possible for it to detect a flame! \$\endgroup\$ – Dwayne Reid Apr 26 '17 at 0:40
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STOP tampering with safety equipment!

Flame detectors are designed to be fail-safe and incorporate redundancy to ensure failures lead to the gas valve closing.

If you continue with gas supply in case of a short and the flame is not present the gas will eventually find a flame source and your house will likely be exploded into kindling.

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  • \$\begingroup\$ I want the system to turn off when there is no flame. Only thing I want is my circuit to see whether there is a flame or not regardless of a short. \$\endgroup\$ – Sam May 9 '16 at 20:37
  • \$\begingroup\$ Here is the result of such a gas flow when it finds a spark (happened last month- Feb 2018, long after the original posting): youtube.com/watch?v=4WowQw68TII A strip mall with more than a dozen businesses was totally destroyed, by the explosion and ensuing blaze. \$\endgroup\$ – Spehro Pefhany Mar 14 '18 at 22:55
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It may be possible. But it would appear to make the flame detector inoperable. This could be a very dangerous situation. Not knowing if there was a flame or not. And is probably the reason that grounding the flame detector shuts down the furnace in this video.

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  • \$\begingroup\$ Yes that's the problem, my system shuts down when there is a short. But is there any way I can make it run as long as there is a flame? \$\endgroup\$ – Sam May 6 '16 at 14:20
  • \$\begingroup\$ What I am trying to do is to make the system work when there is a flame even when there is a short. But I want the system to shutdown only when there is just a short and no flame. \$\endgroup\$ – Sam May 6 '16 at 14:28
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When I get a difficult problem, I start by writing down the requirement details and then jot down all the possible solutions I can think of, simple, complex wild or crazy. The IC makers data books and online op-amp applications are a big help. I'm an Electrical Engineer with many years experience, but I still go back and study the basic circuits in the text books.
The voltage divider circuits can be rearranged, with the flame sensor, anywhere in the series, shifted on the high side or the low side. this will give you different voltages levels as inputs. You can use one or more comparators (like the LM311) to sense a particular voltage level, or a high low-band. You can use more components, two flame sensors, maybe a thermocouple ckt, shielding one section from moisture, then combine multiple signals for the output.

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To decrease the likelihood of a short circuit you could increase electrode to ground air gap distance. If there is a short you cannot detect a flame if the flame 'diode' is effectively being bypassed

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