Solenoids and thermocouples

[JohnW2]

I should have resisted the temptation to fit a more modern boiler, that old boiler was bomb proof and so easy to fix when it did go wrong.

About 35 years on, I am still 'resisting' - but conscious of the fact that there will presumably be a day when the cast iron water jacket (or whatever it's called) eventually 'rusts through' (as happened with my previous ancient boiler, 40+ years ago) and thereby becomes effectively unfixable!

Kind Regards, John

 

[bernardgreen]

John

The attraction force is NOT used,.... only the holding force is used to keep the armature attached to the yoke.

The armature is moved into contact with the yoke by the mechanical action of the ignite button being pressed.

 

[SUNRAY]

If the magnetic path is closed then the force that resists the separation of armature from yoke is many times greater than the force that attracted the armature to come into physical contact with the yoke for the same current.

Depending on materials and construction the force required to separate armature from yoke can be as much as 50 times the force of attraction across a 2 mm air gap.

Relay specifications will often have a series of currents, typically:
Non operation, ie will not operate at 40mA
Operate, ie it will operat at 50mA
Hold, ie it will not release at 10mA
Release, ie it will release at 5mA
Totally random figures chosen but not unrealistic. The gap between the higher and lower figures are controlled by the tension against the armature and the residual air gap when operated.

 

[JohnW2]

John ... The attraction force is NOT used,.... only the holding force is used to keep the armature attached to the yoke.

I realise that. However, I was saying that if the attraction force (if used) would be (as I would have expected) 'tiny', that I would not have expected that even "50 times" that tiny force would have been enough to overcome a strong spring. ... but, as I also said, maybe my intuition is wrong.

Kind Regards, John

 

[AdrianUK]

True, but they were always commonly called a 'solenoid valve'.

Indeed... but is that name coming from the second part of the 'gas valve unit' ..... the bigger, electrically driven valve, that releases the gas to the main burner under the control of the control system (often 24V AC) switched by the boiler thermostat etc etc?

 

[RandomGrinch]

John

The attraction force is NOT used,.... only the holding force is used to keep the armature attached to the yoke.

The armature is moved into contact with the yoke by the mechanical action of the ignite button being pressed.

...and the magnetic remanence can be large, even when the current is removed

 

[Jackrae]

As promised in response #31 I've done some tests on the two old gas valve tripping solenoids that I had in my junk box (see response #20 for photos). Although measured results were quite varied, the typical values were as follows :

Force required to overcome the solenoid plunger spring was 0.65Kg or 6.37N
Coil impedance of the solenoid armature (16 turns) was 17milli-ohms
Minimum plunger hold-in current was 600mA
Minimum plunger hold-in voltage was 14mV
Drop-out voltage was 12mV

The thread on the solenoid's thermocople connection point was aparently a non-standard or obsolete size, being around 9mm diameter with a thread pitch of around 0.8mm. This is in contrast to a modern thermocouple (Myson 308S048 / 232084) which has a connection thread of 9mm diameter and 1mm piitch.

The new Myson thermocouple's open-circuit output was in excess of 25mV when heated by a gas flame

I found this thermocouple, when heated with a gas flame, fully capable of holding in the solenoid plunger suggesting that it was therefore able to supply the 600mA minimum hold-in current.

 

[JohnW2]

Force required to overcome the solenoid plunger spring was 0.65Kg or 6.37N
Coil impedance of the solenoid armature (16 turns) was 17milli-ohms
Minimum plunger hold-in current was 600mA
Minimum plunger hold-in voltage was 14mV
Drop-out voltage was 12mV

Intriguing! I must say that my intuition was telling me that, no matter how low the impedance of the 'load', a thermocouple would not be able to supply anything like 600mA - thereby proving yet again that we 'learn something every day'.

I have to say that, if such a tiny thermocouple can generate ~8.4 mW, this makes me ponder the question of whether 'thermocouples' might perhaps represent a more efficient way of generating electricity than the established alternative methods!

Kind Regards, John

 

[Harry Bloomfield]

I have to say that, if such a tiny thermocouple can generate ~8.4 mW, this makes me ponder the question of whether 'thermocouples' might perhaps represent a more efficient way of generating electricity than the established alternative methods!

You might need quite a few, to make your living room light up

 

[JohnW2]

You might need quite a few, to make your living room light up

Sure, but one could presumably get thousands of junctions in a pretty small space.

Kind Regards, John

 

[Jackrae]

The above two images shows a thermocouple fan sitting on top of my woodstove. The differential heat between the lower and upper sections of the fan generates sufficient power to run the fan motor.

 

[JohnW2]

The above two images shows a thermocouple fan sitting on top of my woodstove. The differential heat between the lower and upper sections of the fan generates sufficient power to run the fan motor.

Thanks. Interesting - what sort of power are we talking about, I wonder?

Kind Regards, John

 

[Jackrae]

I suspect the unit is fabricated from several 10s of thermocouples wired in series such that we have a multiplicity of connections such as :
A-B + B-A + A-B + B-A + A-B etc
The fabrication problem of course is ensuring the A-B couples are on the hot side whilst the corresponding B-A couples are on the cold side since, if A-B + B-A saw the same temperature, the nett result would be zero volts.

 

[bernardgreen]

The element in those fans is a single large semi-conductor junction, A difference in temperature produces power, ( Seebeck effect )

Applying a voltage to the junction creates a difference in temperature ( Peltier effect ) Solid state refrigeration,

( I may have got the names mixed up )

 

[SUNRAY]

At school di a couple of experiments with thermocouples, they consisted of a wire, half of which was copper plated. As an experiment I wrapped nickel (or could have been iron but nickel is most likely) wire round a comb using the teeth as spacers then copper plated half by suspending it in a copper sulphate solution and passing a current through the solution. I think it must have been something like 50 turns, waving it over a gas flame I saw enough to make the meter move on a 10V range, putting it in the icebox of the fridge then heating just one side I saw something approaching 2V however I didn't see any current on the 25mA range.