Thermopile power supply in water heater control

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bob prohaska 9 years ago

I recently bought a gas water heater and made a point of getting one that didn't require external electrical power to operate.

To my astonishment, it nonetheless came with what behaves like a digital controller, complete with a green LED status light that must require close to 3 volts to turn on.

The electronics are operated from a standard Honeywell thermopile generator producing 750 mV open circuit with a 3 ohm source impedance. I expected to find an explanation of how it's done via a quick Web search, but have so far come up with nothing very informative: Only that one can get MOSFET transistors "programmed", evidently by something like charge trapping, to have arbitrary gate threshold voltage, down to zero. That seems rather exotic for a water heater 8-)

Can somebody point me to an application note explaining how this is done?

Thanks for reading,

bob prohaska

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Phil Allison 9 years ago

** It might use one of these, or similar.

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All fitted into a 2mm square, SMD package.

..... Phil

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Phil Allison 9 years ago

** Or this one, specially for thermopiles.

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..... Phil

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Phil Hobbs 9 years ago

I have one like that. There's a battery in the control box.

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net

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Tim Wescott 9 years ago

I get 45mW from the thermopile -- that's enough to light up the LEDs in interior light, and have enough left over to run a thrifty microprocessor.

Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!

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John Larkin 9 years ago

That sounds like the Honeywell Frankenstat. It is powered by the thermocouple, probably with some sort of switching regulator voltage booster.

They occasionally screw up, and there are Youtube videos about how to prevent it, and how to reset them after they throw an error.

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John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com

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bob prohaska 9 years ago

That's plausible, it mentions Honeywell by name.

What search terms did you use? I tried thermopile in various combinations and never came close....

Thank you!

bob prohaska

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bob prohaska 9 years ago

Hmm, I hope not. Nothing said in the (very skimpy) instructions about batteries, but there could be a battery. The entire heater is warranted only for six years, so a lithium coin cell as a backup would have a long enough life.

Time will tell......

bob prohaska

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Phil Allison 9 years ago

** The first LT device I linked came from " 700mV boost converter" and the LT3108 is mentioned at the top of the first LT page .

Couldn't have been easier.

..... Phil

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bob prohaska 9 years ago

Mine looks essentially the same, apart from color scheme. No battery, which is good, but it sounds like reliability is an issue.

Once the warranty expires I'll likely have to become an expert.....

Thanks for writing,

bob prohaska

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bob prohaska 9 years ago

8-) "boost converter" never occurred to me......

thanks, bob

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Phil Allison 9 years ago

** Shame on you - there simply had to be one of 'em in some form.

I didn't know specific thermopile input ones existed.

Using a 1:100 input tranny IS a neat idea.

.... Phil

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Helmut Wabnig 9 years ago

Google "energy harvesting" and/or "Joule thief" just for entertainment.

w.

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Thermopile power supply in water heater control

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