Muliple Peltier elements: parallel or series?

[Syd Rumpo]

On 20/07/2012 09:05, Martin Brown wrote:

I presume the boiler thermostat is a big bunch in series.

No, just one junction. It provides enough current to *hold* a
low-voltage solenoid valve open against a spring, but you need to
manually hold the valve open - usually a button - until the pilot flame
has heated the thermocouple. This may take a minute.

I had a boiler once where the pilot kept going out and needed
relighting. It was an 'instant heat' boiler which only turned on the
main burners when a hot water tap was turned on. Eventually I got
around to taking a look and discovered a 2" layer of dead wasps inside.
Seems they'd been attracted in from a nearby nest by the warmth of the
pilot flame, then incinerated as someone turned on a tap. Over and over
again for many weeks, occasionally snuffing out the pilot flame on their
way.

Cheers

 

[Martin Brown]

You are using the wrong junction for this purpose. There are far
better junctions made specifically for this, although the thermocouple
in a gas furnace will almost do it. About 3 or 4 in series will
light an LED plenty brightly, in fact you'll need a current limiting
resistor. I think they use copper and iron, nothing exotic at all,
and in a gas flame, I think you get 600 mV out of these. A candle
flame should be a bit less, so 4 junctions at 400 mV each should
light a red LED.

Seems to be entirely wrong order of magnitude here - at least in the UK
these gas thermocouples seem to provide about 30mV in a flame. Though
apparently at sufficient current to pull in some kind of solenoid.

http://ecc.emea.honeywell.com/downloads/MU1R9124.PDF

A pair or more would generate enough voltage to run a step up. This
might be more satisfactory for demo than flat plates over the flame.