Date: Sun, 27 Jul 2014 15:28:04 -0700
Reply-To: Alistair Bell <albell@SHAW.CA>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Alistair Bell <albell@SHAW.CA>
Subject: Re: Fridge led glowing on its own again.
In-Reply-To: <vanagon%2014072717221672@GERRY.VANAGON.COM>
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Thanks David, and I'm happy to say I fixed the problem. Mark wrote to me explaining the circuitry (simply for my limited abilities) and I found the culprit immediately. It was, as he suggested it might be, the black wire that comes up from combustion chamber to the plastic connector on wire to panel. Just easing the wire and connector out of hole where panel sits was enough to turn the led off.
I reseated the connector and all is well. I'm guessing it was an iffy connection to begin with and perhaps packing stuff into closet under sink moved the wire bundle enough to make the connection bad.
Was so long ago that I replaced the led that I can't give you specs on it. But it is blue and it is fairly bright. And it does dim and brighten in response to flame size/ chamber temp.
The brighter led is one of those mods that pleases me every time I look at it. Easily amused aren't I?
Cheers
Alistair
> On Jul 27, 2014, at 2:21 PM, David Beierl <dbeierl@ATTGLOBAL.NET> wrote:
>
> At 12:36 PM 7/27/2014, Alistair Bell wrote:
>> I hope Mark chimes in here, I bet he knows the answer. I mentioned a
>> couple of weeks again that my blue led on indicator panel was
>> starting to glow when fridge not running. This started gradually
>> over the last year and culminated with it being quite bright. This
>> is a led I installed in panel about 13 years ago. It's a nice little
>> mod, really helps to tell at a glance if fridge is running. That is
>> if led working properly.
>
> Did you simply plug in a blue LED in place of the green one (if not, what)?
>
>> Until about a week ago, and now again the led is starting to glow. I
>> can't believe that it might be heat from the sun on van body
>> transferring to combustion chamber that is the cause, but I keep
>> forgetting to check at night or on cool days.
>
> That would be very^3 unlikely.
>
>> What could be the reason for this? Fridge stock except that 110 v
>> system not connect, nor is the high amperage 12v feed to resistive heater.
>
> So the only connections to the fridge are thermocouple and ground,
> yes? That eliminates a lot of questions right away. By any chance
> are the battery LEDs acting odd?
>
> My first guess based on field experience with this board is bad LM324
> (the right-hand one IIRC - at any rate the one that handles the
> battery LEDs). Unplug the black separate wire to the LED panel, which
> is the thermocouple output. If the light stays on that points pretty
> convincingly at the chip, unless something has gotten spilled on the
> board. If it goes out you have to start chasing things or bet the
> investment in changing the chip against hassle with
> measurements. This is a high-gain circuit where the difference
> between no LED and full brightness is in the small tens of millivolts.
>
> Here's a schematic of the flame detector with a few added goodies:
> http://pws.prserv.net/synergy/Vanagon/Pilotmod_schematic.gif The
> stock circuit is the part above the line, minus the 5-megohm
> pot. This uses the pin-one-end amplifier (out of the four on the
> chip). It's wired to have a gain of negative 370,** so when the
> input (the left end of the 2.7K resistor) goes negative 10 millivolts
> from the reference the output goes positive 3.7 volts which in this
> circuit would put about three milliamps through a typical green LED
> (and might or might not light a blue one at all). The reference is
> the LED-panel B- input, which is where the problems come in, because
> we're measuring millivolt differences and the thermocouple itself is
> referenced to the fridge gas valve and then by whatever path the
> fridge is grounded by. Now I have to stop talking, because either I
> made a mistake with the circuit all those years ago or I'm
> remembering wrong how it behaved when running the fridge on DC or I'm
> not understanding something now. My recollection is that running the
> DC would light the flame detector, and I justified that because the
> 7.5 amps would raise the fridge ground slightly compared to the panel
> ground. The problem is that if I'm understanding things correctly
> now, that should make the light go out rather than light it
> more. That would imply that the thermocouple output is positive to
> ground rather than negative, and that the op amp is wired with the
> inverting pin wired as reference and the noninverting as input. Or
> else I'm just confused. I've got no panel to look at.
>
> **This is an op-amp whose purpose in life is to keep its two input
> terminals at the same voltage. It does this by adjusting its output
> so that any current flowing through the 2.7K resistor is balanced by
> an equal current flowing through the 1M resistor, allowing the input
> terminal to stay at (almost) exactly the same potential as the
> reference terminal. The - and + on the diagram show that the
> non-inverting input is being used for reference and the inverting one
> for input, hence the negative gain. The battery and water tank
> circuits are wired without any feedback from output to input, so the
> effect is to slam the output to maximum with even a tiny difference
> between the inputs -- the LM324 has a DC gain of 100,000 when wired open loop.
>
> Yrs,
> d
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