Connections to Hob Spark Generator

[RodF]

[After a further search this query may be more appropriate to this Forum]

The spark generator PCB in our hob is an APCO 16440 from 1998 and has an isolation transformer between it and the mains and an earth connection, plus the four EHT leads to the burner electrodes and four wires to the corresponding knob microswitches with the common back to the transformer secondary.

The replacement generator module is enclosed in white plastic with blade terminals and came with no connection instructions and looks similar to many on the internet spares pages. Whilst the direct mains input is marked L and N (so presumably does not require the isolation transformer) and the microswitches are presumably SW(common), 1, 2, 3 and 4; the EHT output is puzzling as there are just four blade terminals. No where on the module is there an earth connection? How is the return path for the spark provided before ignition, even though the current might be microamps?

Any advice/clarification would be appreciated.
RodF

 

[Taylortwocities]

You'll be better off on the Appliances forum.
I've asked the mods to move you.

 

[RodF]

Thanks for the suggestion, but I am already on the Appliances Forum - it was after further searching I found gas hob electrics on this one!

Regards
RodF

 

[jj4091]

Only guessing, but I would have thought it is done electronically, rather than through an isolating transformer, as the old one was & that all the connections are made internally. Personally I would connect it up as you suspect it should be & try it. That is presuming you are 100% sure you have the correct unit for your model.

 

[RodF]

I haven't checked the isolating transformer output as yet, but the Stoves manual shows the secondary winding connected to the board with the individual microswitches for each burner in series to the board inputs to trigger the 4 electrodes.
I was told this replacement was correct for the hob, although I provided the hob type and model number. I believe that was has happened is the internal design was changed at some point and the APCO model 16440 PCB was replaced with a new design, such as the GSG 888 generator, or Derlite Rangemaster P033459, found on the internet. It was assumed, I expect, that I had the later design hob, although I had no reason to think the PCB was replaced with an enclosed module.

One AGA manual I've found on the internet shows the generator with an earth lead, another AGA manual shows it without one!

The EHT output side of the generator shows wires and resistors going back inside the module from what appears to be the ends of the EHT transformer windings (visible through the clear potting compound). I am not sure what is the purpose of these, as even if they eventually connect to the L or N mains leads that is surely not the path for the return current? It is very odd?

I have just made another search for "gas generator eht circuit" on the Forum and found this comment on Plumbing & Central Heating Forum by "agile" on 28 Sept 2005 for a Potterton Netaheat 16-22;
The spark electrode also acts as a flame detection element. The earthy side of the EHT transformer is not taken directly to earth. It has a small AC voltage applied to it. This earthy end is connected via some high value resisters of a meg or so to a DC detection circuit which serves to detect a flame and enable the second stage of the gas valve and to turn off the spark generator.

A flame contains ionised gas and this conducts the applied AC to earth. However because the gas is leaving the burner at quite a high velocity the effect is to conduct more for one polarity than the other and results in a DC offset to the applied AC voltage. Its called flame rectification! This DC is detected as described in the previous paragraph.

Whilst I can believe the gas ionisation allows a small current to flow to earth, is this a usual design for a gas hob, and in particular how can it work before the gas has ignited?

RodF

 

[RodF]

Repair now sorted!
The isolation transformer, which I assumed was 1:1 ratio was, in fact, providing about 278VAC on the secondary, which I thought might be a little high for the replacement DerLite generator module as I had no idea if the internal circuit had protection from fast high voltage spikes on the mains. The PCB in my gas boiler has such "spike" protection across the mains, provided by a Siemens varistor S10K250.

I have found how the puzzling design with no return path for the EHT spark is done. Looking at the EHT end with the four miniature blade terminals I noticed that at each end of the pulse transformer there were "pointed" blades above the potting compound, spaced about 1/10" from the adjacent EHT terminal. I wondered if these would pass a spark as the spacing was less than on the each burner ceramic electrode.
Connecting the module in accordance with the labels on the top it generates sparks on pressing any of the four microswitches and the two bypass electrodes also sparked. They provide a complete circuit and the four burner electrodes are just picking off the pulse energy, so no earth is required by the circuit and they appear without any gas ionisation path required!

The original generator PCB produced a fast series of sparks, 3 or 4 per sec. This new module produces a healthy spark about once per second.
It's a shame these items do not come with clear instructions, especially when they replace a completely different looking original.

RodF