Flame Rectification

In 2017, fires claimed the lives of over 3,390 people in the United States. Please use extreme caution when working around fire or accelerants.

For those that are not aware, flame rectification is the process by which the presence of a flame partially rectifies an AC power source. This slight AC rectification creates a measurable DC current in the low micro-amp range and is how a normal short is distinguished from the type of short a flame produces. A normal short will cause equal current draw in both directions, but a flame is conductive in only one direction like a diode. In fact, diodes are used in place of an actual flame to test furnace controllers.

ALL that being said and knowing the risks involved, who wants to contribute to an Arduino friendly solution for an effective proof of flame circuit?

Many capable individuals around the world tinker in brewing, fabricating, HVACR, and other prototype applications and are looking to automate their systems using the Arduino. One area where the community is pretty much vacant is in the flame detection department. I don't really consider the optical flame sensors that detect 760-1100nm IR light to be something you could use in an actual application that doesn't involve Bics and candles... So let's explore more pro options!

I'll start with my first impressions and thoughts about some of what is needed in achieving proof of flame. The project I am working on is reviving a 36 year old Suburban propane forced air heater that uses 12vdc for the main power and control voltage.

The stock flame sensing rod I am using is combined with a spark ignitor and has three separate rods, a flames sensor, ground, and spark ignitor rod. An AC source is needed that is typically over 90vac to feed into the conductive sensing rod that will be partially engulfed in the flame when lit. The popular El Wire inverters are perfect for this application as they produce 120vac from a 12vdc source and are very small and low powered.

For the spark ignitor, I purchased a 12VDC Manual Input 15kV Spark ignitor that produces a healthy arc on my sensing rod.

I have a bank of four relays that is operated by the Arduino that controls the gas valve, spark ignitor, and blower motor.

I'm using an Arduino Nano at the moment since it is nice and small and fits in the space I have. The program I wrote is a simple state machine that drives the furnace through its various states based on input from the thermostat and safety sensors. Everything is working like a charm right now but the only piece of the puzzle that I'm missing is proper proof of flame. I refuse to use that janky little IR flame sensor module that is so popular right now and also my project does not allow for mounting a sensor like that near the flame window.

The crux of this whole post is how to measure the microamp DC current through the AC sourced flame rod in order to prove that a flame is present and not a simple short. My first thought was to do a half wave rectification and separate the +120v from the -120v in order to do a comparison to see if partial rectification has occurred. Looking at some of the patent circuits online I see that there are a few different techniques for achieving this. Does anyone have any recommendations on how to do this simply and reliably?

Here is an example project to do the same thing: Pilot Light Flame Sensor for Burning Man Art
You may find something you can use there. The author is, incidentally, the guy behind the "teensy" series microcontrollers.

Edit:
You might be able to generate the "AC" more simply by using an H bridge motor driver chip (say L293D) as in this example: Ringer circuit for vintage telephone bells. - Exhibition / Gallery - Arduino Forum

Excellent link! This guy really went the extra mile on the burner controller board for the propane torch, even down to the labels on the wire terminals. Exactly what I was looking for.

The reason I chose the El wire inverter as the AC source is that they cost less than $2 and I wanted to solve as many hurdles as possible using existing modules to make it more available to others. I really like the idea of using an H-bridge and this is probably the best option for a custom circuit.

I haven't looked at Paul's project yet, but it occurs to me that you could probably do this by having two sets of probes polarized in opposite directions. Subtract one signal from the other and calibrate the difference for flame vs no flame or short.

Probably an easier way to do it, but that's my first take on it.