Solenoids and thermocouples

[Jackrae]

The thermocouple per-say doesn't need an earth but by virtue of it being connected to the structure of the fire/boiler it's outer skin is earthed. When the remote end of the t/c tubing is connected to the gas valve there is both mechanical and electrical connection between the tubing outer and the valve, hence completing a closed elctrical circuit.
On the question of reliability of the return spring driving the pilot valve (see my previous photos) I've measured the spring force acting on the plunger and it's in the region of 5 Newtons (0,5kg)
I'm currently (excuse the pun) building a test rig to determine voltage and current capability of a t/c- results in a week's time (maybe)

 

[JohnW2]

The thermocouple per-say doesn't need an earth but by virtue of it being connected to the structure of the fire/boiler it's outer skin is earthed. When the remote end of the t/c tubing is connected to the gas valve there is both mechanical and electrical connection between the tubing outer and the valve, hence completing a closed elctrical circuit.

Exactly, that's what I said. We're talking about what goes on 'within the boiler' - and provided only that both sides of the thermocouple are connected to the mechanism that hold the valve open, it will surely just 'work', regards of what external 'earth' (or anything else!) is connected to the boiler, won't it

On the question of reliability of the return spring driving the pilot valve (see my previous photos) I've measured the spring force acting on the plunger and it's in the region of 5 Newtons (0,5kg)

That's the sort of ballpark I would have guessed - and, as I implied, there is surely no way that magnetic flux derived from a thermocouple could possibly win in a fight against such a spring, is there?

I'm currently (excuse the pun) building a test rig to determine voltage and current capability of a t/c- results in a week's time (maybe)

I'll be interested to hear your results. As Harry and I have both reported, the voltage seems to be 25 mV or so, but I haven't personally ever bothered to try to determine it's current-providing capability.

Kind Regards, John

 

[bernardgreen]

Soviet era Kerosene Lamp Generator gives new meaning to “Let’s fire up the radio!”

Many thanks to SWLing Post contributor, David Day, who recently shares the newspaper clipping above. I was not aware of this Cold War era kerosene lamp generator, but it’s an ingenious use of…

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[Harry Bloomfield]

It presumably doesn't. As I said, my thermocouple-using boiler works fine from a floating generator - although, as I've said it's actually all-but-impossible, with copper plumbing, to have a boiler which is not connected to earth, even if the 'generator supply' doesn't provide an earth. Even if all the 'water plumbing' is plastic, gas pipework can't be,and that will usually be main-bonded to the MEWTT (aka 'earth').

For thermocouples, there is no need for an earth, because the outer copper of the thermocouple provides the return path.

For ionisation sensors, the return path is via the earth, so the boiler needs to be earthed and if a generator is used, there needs to be a return path of some sort between boiler earth and one of the poles of the generator.

 

[JohnW2]

For thermocouples, there is no need for an earth, because the outer copper of the thermocouple provides the return path.

Exactly.

For ionisation sensors, the return path is via the earth ...

What do you mean by that? We're only talking about what goes on entirely within the boiler, so if there is a return path for the ionisation sensor within the boiler (i.e. between sensor and whatever it's controlling), why on earth (sorry!) does it matter what parts of the boiler are, and are not, connected to anything ('earth' or otherwise) outside of the boiler?

You surely aren't suggesting that, for example, with a TN-S installation the ionisation sensor's electrical path goes all the way back to the DNOs transformer (and then back to the boiler via the DNOs neutral), are you?

Kind Regards, John

 

[Harry Bloomfield]

What do you mean by that? We're only talking about what goes on entirely within the boiler, so if there is a return path for the ionisation sensor within the boiler (i.e. between sensor and whatever it's controlling), why on earth (sorry!) does it matter what parts of the boiler are, and are not, connected to anything ('earth' or otherwise) outside of the boiler?

You surely aren't suggesting that, for example, with a TN-S installation the ionisation sensor's electrical path goes all the way back to the DNOs transformer (and then back to the boiler via the DNOs neutral), are you?

You right, forget that idea!

I have though heard numerous times, that for a modern boiler with ionisation flame sensing, that to use a generator - the earth needs to be connected to neutral.

 

[flameport]

Flame rectification works by applying AC voltage to the probe in the flame, and if the flame is present, a tiny DC current will flow between the probe and the metal case of the boiler.
That current is used to determine that the flame is present. No current = no flame, and if AC is detected that indicates a fault such as soot creating a conductive path between the probe and case.

For any current to flow, the metal case of the boiler must be referenced to the incoming AC supply, which in normal use it will be via the earth connection in the supply cable to the boiler.

If connected to an inverter with a floating or isolated output, there is no return path for the DC current, and flame rectification cannot work.
If L&N in the supply are reversed it won't work either, as the probe is then connected to the neutral and there is no or very little voltage between that and the case of the boiler.

with a TN-S installation the ionisation sensor's electrical path goes all the way back to the DNOs transformer

Correct.

(and then back to the boiler via the DNOs neutral

No. Neutral is unrelated.
Current is L-E only, and if the voltage between them is incorrect or missing, the boiler won't work.

The only other option would be to have the metal burner inside the boiler electrically isolated from the rest of it, and then have an internal connection to the burner as a virtual earth so that current could be sensed between that and the probe. Doing so would be highly impractical or even impossible, as it would require the use of high temperature insulating materials which had a leakage current in the order of nanoamps.

 

[JohnW2]

You right, forget that idea! I have though heard numerous times, that for a modern boiler with ionisation flame sensing, that to use a generator - the earth needs to be connected to neutral.

Yes, I've often heard that too - and, as I said way back to the OP, connecting the neutral output of his genny to the 'earth output' is the way to address that - but that hasn't actually got anything to do with 'earthing' - it will surely still work fine even if the genny is floating (relative to true earth) provided only that the neutral and 'earth' outputs of his genny are joined, won't it?

Edit: I've just read flameport's post, which seems to explain things, but won't have enough time to respond until later.

Kind Regards, John

 

[JohnW2]

Flame rectification works by ........... For any current to flow, the metal case of the boiler must be referenced to the incoming AC supply, which in normal use it will be via the earth connection in the supply cable to the boiler.

Thanks for the explanation.

However, we're back to what I recently wrote to Harry, isn't it - that this has not got anything to do with 'earthing', per se, but merely a requirement for the source to be earth-referenced - so, as I said way back, if one has a generator with it's neutral and 'earth' outputs connected, then the boiler will work as well as it would with a DNO supply, regardless of what is, or is not, connected to true earth?

Kind Regards, John

 

[flameport]

Yes. Whether it works with a generator/ inverter or not depends entirely on how it's configured.
If the inverter output is 230V AC between L&N, and the inverter N&E are connected it will work.
Whether it's connected to the mass of Earth or not is irrelevant.

 

[JohnW2]

Yes. Whether it works with a generator/ inverter or not depends entirely on how it's configured. If the inverter output is 230V AC between L&N, and the inverter N&E are connected it will work. Whether it's connected to the mass of Earth or not is irrelevant.

Thanks. That's what I've been saying all along.

Kind Regards, John

 

[SUNRAY]

Correct, the thin tube is like a single core Pyro cable. They only generate a tiny voltage/current, so the connections have to be good, where they connect to the coil.

Not all are a single pyro. Im my elderly non electric Chaf's the thick turns (I'll estimate 20-16 SWG) continue to the thermocouple where they are attatched with washers and nuts on studs.

 

[SUNRAY]

I haven't read the whole thread as I was getting a bit lost.
I wonder if the gas valve should be called an electromagnet rather than a solenoid?

 

[AdrianUK]

A piece from a 1970's Gloworm boiler manual ......

It compares the open-circuit & closed-circuit voltages. If the intersection falls within the shaded area then the thermocouple is good.

 

[AdrianUK]

I haven't read the whole thread as I was getting a bit lost.
I wonder if the gas valve should be called an electromagnet rather than a solenoid?

Yeap, thats really what it is. The tiny electro-magnetic 'powered' by the thermocouple keeps the pilot gas supply open.