Flame detection

In message , dated Thu, 7 Sep 2006, Peter Amey writes

Detecting the light of the flame with a phototransistor would be more reliable and permit a simpler circuit. Thermocouples give very little output (40 uV per kelvin), so you need at least two transistors, and erode quickly in the flame unless you use the specific type for flame-failure detectors. The sort of circuit you need doesn't have zero current drain in the 'off' mode.

There are two sorts of phototransistor, with very different sensitivities. You need the more sensitive type. I doubt if it will light the LED itself, since you need to put it some distance from the flame so as not to get too hot, which means you need another transistor and at least two resistors. 12 V DC is ideal, and the 'off' current drain would be very low.

I don't have time to draw you a schematic, but I expect others here have them already drawn.

These units usually have this function built in as part of the temperature control circuit and ignitor. Is your unit broken or just made without this feature?

Luhan

Made without - flame detection on the fridge consists of small window through which you can see if the flame is alight. The ignitor is manual push piezo unit. This arrangement has 3 disadvantages: 1. you have to grovel on the floor to light the fridge; 2. you have to have the door open during the operation (losing all that hard-won coldness) and 3. You often have to partly unload the fridge to see the little window.

I just fancied a little light to replace all of the above!

Peter

Your goal is understood.

There have been many people before you who have had to solve this problem.

Now getting this done with safety and cost in mind, is what others are trying to offer. Also understanding what you really have. ( I for one have never heard of a gas fridge !! )

So, people here are not trying to stop you from building this project. We all would hate to hear that your fridge caught fire because of some idea presented was done just wrong !

Finding a commercial product seems your best way to go.

Good Luck

donald

Ugh - you must have flame failure detect, or you could end up dead !! There is usually a button you must hold in until it is warm enough - flame failure turns the gas off.

Otherwise, it sounds like an application for a piece of fibre optic cable ...

Dave

In message , dated Thu, 7 Sep 2006, Donald writes

It's a very old technique - evaporation cooling. The working fluid is usually ammonia. Whereas modern coolants evaporate at room temperature without assistance, ammonia doesn't do so well, and need a very small amount of hat input, The flame is hardly bigger than a match flame.

It would be quite difficult for even for some of the clowns here to set an evaporator fridge on fire.

My optical solution has the advantage that nothing is placed in the flame and everything that is added is added at a safe distance.

Yes, of course, there is some sort of capilliary tube set up that shuts off the gas if the flame blows out but I can't see any way of getting an "output" from that.

Now that might just be a brilliant idea!

Peter

In message , dated Thu, 7 Sep 2006, Peter Amey writes

That is actually a thermocouple. The 'tube' is miniature MICS cable. DON'T even TRY to get an output from it. That WOULD be dangerous, like BANG!, or wake up dead.

I would like to see the flame involved. Just to satisfy my lack of understanding.

Would you be looking for IR or a blue-white gas light ??

donald

Do a search on absorption refrigeration...

Neither; these things use a 'no air' smoky yellow flame. Only a very small amount of heat is involved. It's easy to detect the visible light.

Is the flame to be manually lit (like a pilot light) or controlled by demand automatically?

Gas appliances have various safety mechanisms which cut off the pilot gas supply in the event that it is blown out. Many of these use a sealed tube in which the gas (air?) expands when heated and operates a piston (or bellows) to hold the gas supply on as long as the flame is burning. This needs no power source and has a reasonably fast response time (15 seconds to a minute).

You could arrange to have the piston pop up an indicator flag for a visual signal.

If you want a more rapid indication of the presence of a flame, you could aim a fiber optic tube at the flame (make the 'hot end' out of Pyrex?) and place the output where its convenient to observe.

Traditionally a UV sensitive detector - perhaps a transistor - would be used. However for this an LDR and LED might work fine.

My personal favorite!

Maybe a length of 1/4" clear plastic rod bent to shape?

Flame failure devices usually work on the basis of detecting the flame by passing a current through its ionised gases. Some designs in gas boilers and camping fridges use a common electrode for spark ignition and flame detection. Depending on how the spark electrode in your own fridge is designed you might be able to convert it to electronic ignition using and off-the-shelf ignition unit intended for a more modern fridge fitted in place of the piezo igniter. You can buy these (not cheaply) from caravan/camping/boating supply shops (probably cheaper online).

They incorporate flame detection and spark generation ccts and keep sparking the electrode until they detect a flame. They have the advantage that if the flame is blown out they'll try to reignite it straight away, and if they succeed in doing so before the flame failure device (operated by the thermocouple) cuts off the gas save you a possibly lenghthy period of fridge warming up & food going yucky while you're out enjoying the sunshine.

To test if your fridge's spark electrode might lend itself to operating as a flame sensor you could probably try sticking an ohmmeter onto it while the flame's alight, to see if it's 'seeing' any ionised gases from the flame.

Hello Peter,

Some thoughts: Optical recognition has been done with mixed results on other gear, for example pellet stoves. After some time the optical surface closest to the flame can soot over and give a false alarm. IOW signal a flame loss when in reality the flame is there.

Another option that is often more reliable is to sense the heat of a shroud around that area. Stoves often use "thermo disks", probably containing a simple bi-metal type switch. If the shroud doesn't get hot enough over ambient for a clean detection there are thermo switches with a "feeler" that is bent to be near the flame. Similar to pilot light sensing. Actually a pilot light sensor might be the ticket here because they can be bought off-the-shelf as spare parts.

I know the problem, I toured much of Europe 30 years ago with a hired campervan. The bloody fridge had the "flame viewing" window about 1 inch above the floor with a viewing angle of perhaps 15 degrees. The only way to view it was with a mirror and the lights out.

There's a number of systems in place, I had a heatpump with gas booster that used a TC sensor. It never failed in the 5 years I was living there. Or they use a gas-filled capillary tube that shuts off the gas with no flame.

More modern systems use an optical system, as other have suggested, and it's possibly the easiest to home-build. The best detectors are possibly the Hamamtsu ones. See:

Barry Lennox

On Thu, 7 Sep 2006 12:17:07 +0100, John Woodgate Gave us:

Even better, a resistor bolometer. A transistor sized package that actually detects the temperature. Any simple IR activated light switch will do usually as they are not typically clamped to too tight an IR band.