Gas burner takes ages to stay alight ???

[DownWest]

Gosh Vic, I was quite worried that my brain was going ( I get that fairly often) I also was surprised that there were enough milliamps to run a holding electromagnet.
A

 

[Martin_J]

Like VicS, I was intrigued as to how a thermocouple could generate enough electricity to create enough magnetism to open a gate control valve.

Searching around the internet - it doesn't! Opening the valve has to be done manually but then there is enough magnetism created to keep it open. Very subtle difference.

An interesting little training document here that seems to cover (with pictures) what daveyw has described - including colour pictures of the FSDs!

www.radmidlands.co.uk/controls.doc

 

[saltylegs]

In fact you were right.
Some further investigation reveals that they are electric. There is a junction at the tip between the copper outer casing and the wire ( iron ?) inside. The button on the other end makes the contact with a solenoid valve.

Obviously did not look closely enough at the one I replaced.

Quite surprising that you can get enough power to operate a solenoid but there you go apparently you can!

the inner and outer parts of the thermocouple are made of 2 different metals and when heated this creates a very small dc voltage measured in millivolts this in turn energises the electro magnetic valve (normally called a junkers valve) in the gas tap
failure of system is usually is due to
1 tip not positioned in the hottest part of the flame (blue part)
2 loose conection at nut on to junkers valve
3 burnt out thermocouple
4 faulty junkers valve

 

[steve28]

check under the burner caps, they get a small amount of rust dust which makes the flame divert slightly causing the thermocouple tip to not heat up quickly.
Steve