Gas heater driver PCB (Spark generator)

[nali]

Hi

I've been handed this PCB from a Truma gas heater / boiler which is blowing the main 12V supply fuse. I can't find a schematic on the 'net, but it's essentially an interface to the sensors and solenoids, and provides the HV spark for ignition. There's no logic or MCU on the board at all.

There's not a lot I can do with it out of the heater. It doesn't consume any excessive current just connected to a supply. Probing around the discrete devices doesn't show any signs of a short circuit - except for the HV ignition txformer.

SO my real question after all this is - what sort of primary resistance might I expect from this type of txformer? I'd expect it to be low, but it's below the measurement capability of my DMM. With my PSU in CC mode I ran 1A through it and measured 20mV therefore 20 mOhm. Considering it's supplied by what looks like a DC-DC boost converter and fired by a SCR that seems a bit low to me.

The only references I can find on the board is "Truma B096" and the transformer has "Karre" on the plastic moulding.

Cheers

[Gyro]

Without photos or schematic it's difficult to offer specific information, but I can hopefully offer a few tips...

- The ignition transformer is normally a high turns ratio step-up wound on a ferrite rod with a lot of insulation (potted). It's not intended to give continuous spark, just 'ticks'. It's driven by capacitor discharge (via the SCR) rather than AC. The turns ratio is very high, so the primary will just be a few turns. It's DC resistance will be very low. It's very unlikely that the primary would go short (as long as you don't start sticking higher DC current through it  ) Insulation breakdown of the secondary would be more likely, but I can't see that blowing the fuse, you just wouldn't get a spark. Check that the SCR isn't shorted, but apart from that, it probably isn't a particular suspect.

- If it's blowing the 12V fuse, then I'd start at the input. You ought to be able to measure a short and track it down. If the supply is 12V AC, then check for so shorted rectifiers etc. Strange that you haven't located anything like this - try drawing out a partial schematic to understand it a bit more. Do you actually mean 12V? These boards are normally directly mains powered. [Edit: I just re-read your post. If it isn't drawing excessive current on it's own, then it might be an external wiring fault or part].

- The normal failure modes on these 'sort' of boards are dried out electrolytic caps and bad solder joints. It's unlikely the either of these would cause the fuse to blow, you'd just get false trips, screwed up timings etc.

- A note of caution: Messing with any internal part of a gas appliance without being Gas-Safe (?) registered is illegal in the UK. If you screw it up, you could do something like accidentally turning on the burner control valve and leaving it that way. That could cause a nasty explosion if the gas builds up and then gets ignited. Please consider yourself advised.

[nali]

Thanks for taking the time to answer my slightly vague post. I omitted to mention it's actually a CARAVAN multi-mode heater, not a domestic household boiler or heater. Hence the 12V supply.

Anyway, an update. I've since found out that the caravan in question has been refurbished and during this process the 12V supply to the heater has swapped polarity It'd have been nice to know that BEFORE asking me to "repair" it, that it magically stopped working after a re-wire 

Anyway, at least we know the crowbar diode is working fine...

[Gyro]

You're welcome. I did wonder where the 12V supply was coming from, that explains it. Yep, reversed supply will do it every time - it's nice when these little 'facts' trickle through to the guy who's actually trying to fix it! 

[floobydust]

I have repaired these heaters, they are 12V powered with propane as fuel.
I see igniters with a DC-DC converter and capacitive discharge, it's a smaller coil and lower cost than a 12V coil, to use say 200VDC pulses.

The flame-detect portion of the circuit is safety critical, so you need to be careful about work there, as well as the fan airflow switch.
Clean the flame rod with steel wool and that's most of what maintenance I do.

[nali]

Yep, it's what you say. It's this


The cap next to the purple tx is 400v rated so 200v sounds about right. It's actually a pretty simple board, one LM324 is the most complex part on there.

Anyways, it's going back NFF...

[IanMacdonald]

My understanding is that gas regs do apply to caravans, and that although owners doing their own repairs is a bit of a grey area, doing a repair on someone else's gas boiler could get you into serious trouble if there was an accident. Like, imprisonment if someone died.

As far as what you can do is concerned my understanding is that the gas path through the boiler and exhaust flue are (obviously) off-limits to non-registered repairers, but that electronics may also be off-limits if they control the gas directly, as in ignition or flame fail.

Don't quote me on this though,. Just what I was told by a gas guy.

[james_s]

People are ridiculously paranoid about this sort of stuff, and the laws in some places are equally ridiculous. Be sensible about it and do tidy repair work and it will be fine. As long as you don't modify the circuit it's unlikely to cause any problems. It's not as if this stuff is ultra-reliable mil-spec avionics, it's just consumer grade gear. A lot of the laws surrounding things like this are as much about protecting the jobs of tradesmen as any safety aspect.

[nali]

I had a quick skim at the regs, it looks like private own-use caravans are exempt although accidents may be reportable. In any case for me it's more a case of "a bloke gave me a PCB to look at", the heater in question is 50+ miles from here (not sure how that would stand up as a legal defense though!).

But as it turns out it looks like there's nothing wrong anyway, just a bit of muppetry with the wiring, so it's going back untouched.