What this diode is doing? Please help

Discussion in 'The Projects Forum' started by StewB, Jan 17, 2014.

  1. StewB

    Thread Starter New Member

    Jan 3, 2014
    17
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    Hello,

    PREAMBLE

    This is my first post so please forgive any protocol errors.

    On Christmas Eve my well-loved (old!) central heating boiler suddenly failed (post electric storm). Examination showed the PCB had a couple of baked switching diodes. None the less, I decided it would be better to get a replacement. That way I could de-solder / test etc at leisure.

    However, the replacement, though sporting an identical part number, features several new components and also differs in the number of spade connections. Moreover, the thing is OLD, so I decided attempts at reforming the caps might be in order. All of which led me to try and understand the circuit layout.

    The more I looked the more it became clear that I did not understand certain aspects of its operation. Which brings me to my question.

    QUESTION

    I've attached a photo of the PCB with an overlay of the new circuit components / connections. The grey path from the "Aquastat" seems to be entirely redundant as no sooner does it enter the board than its progress is blocked by a diode (labelled D1 - coloured blue) - at least this is what I believe given that everything to the right of this diode and the rectifier -ve has zero resistance. FYI, D1 is labelled T4148, which AFAIK is simply a switching diode (with a reverse breakdown of 100v - well in excess of the 21v put out by the transformer powering this circuit) with presumably no chance of zener breakdown.

    I've uploaded bmps so that members can easily colour parts of the circuit to trace paths easily.

    CAVEAT

    I should perhaps caution that the layout of paths coming into the "Aquastat" (which features two independent thermostatically controlled bi-state switches) shown in the attachment is how it used to connect to the original PCB. The new PCB has markings on its transformer alluding to connections that just might be different, though exactly what the transformer marking mean is not clear to me ( a lil photo is inset in the 52671 - new.zip attachment).

    ONE LAST THING

    Because it's an old boiler I've had precious little help from teh manufacturer, so any advice people here can give will be much appreciated. I include a pic of the old PCB with schematic overlay, and a pic of the circuit diagram of that unit (though slight differences exist even there - eg no reed-relay fly-back diode).

    Thanks in advance.
     
    Last edited: Jan 17, 2014
  2. wayneh

    Expert

    Sep 9, 2010
    12,154
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    I didn't see one yet. Are you hoping to have a complete description of the circuit?
     
  3. StewB

    Thread Starter New Member

    Jan 3, 2014
    17
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    Sorry if my question was not clear : I'm wondering if people agree that the diode, D1, makes the grey path to the aquastat redundant, or whether I'm missing something obvious about its function.

    As for a complete description in words of what the PCB is doing, that would not be fair to ask anyone. The CD is supposed to do that!
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    The two diodes act as a "blocking OR function". That means either diode (or both) can operate the relay coil A which has its other side at +24v DC.

    But if one diode operates the relay, it does not operate other things on the other circuit because the second diode "blocks" it.

    So either circuit will operate relay A, but will not operate the other circuit at the same time.
     
  5. StewB

    Thread Starter New Member

    Jan 3, 2014
    17
    1
    THE_RB,

    Really kind of you to respond, but I fear you are referring to the old schematic.

    Yes, indeed that does have a diode pair that serve this function, as does the new circuit, but my question concerns the diode I have labelled "D1" on the new circuit (that is being offered as a replacement by the manufacturer).

    I only included the old circuit because it's so much simpler and doubtless would allow peeps to get a quick handle on functionality - as you clearly have/ Sorry if this has caused its own confusion.

    If you could take a look at the new schematic and give me your opinion on what D1 is doing to the path B-B coming out of the Aquastat, I'd really be much obliged. Thanks for your time and input.
     
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Yeah I was talking about the old circuit (in your posted image).

    Sorry but I'm not unzipping something just to see what it is, and/or risking a zipped trojan or PDF trojan etc.

    If you want people to respond on a public forum please post actual images (GIF/PNG/JPG etc) not files that make people open them and take risks. :)
     
  7. StewB

    Thread Starter New Member

    Jan 3, 2014
    17
    1

    Don't apologize for the security fears, there's absolutely no need. In fact it's me who should be apologizing: I've asked forum members to do something I would not do myself. Please understand I was only trying to make things easy for anyone kind enough to take an interest. The diagram as I use it is a BMP, which allows me to use the old "fill" tool to colour in particular circuit paths. It helps me as I'm far from used to looking at schematics. I wanted to make things easy by giving respondents that same functionality. Trouble is the BMP file size is maybe 5 times the forum limit, hence the zipping.

    I'm more than happy to post jpegs of the BMPs, which I do below.

    As to the BMPs, I can assure you that as far as Kaspersky Internet Security 2014 is concerned they are trojan free (there's a screen shot below of a scan). I also run Malwarebytes every evening. Of course you still have to trust my word - the word of a complete stranger with zero history on the forum, so I can hardly take offense if you choose not to.

    Either way, I'm grateful for any comments you can make on the D1 diode. Like I say, to me, as wired, the diode makes no sense: I strongly suspect I need to wire the Aquastat differently, and I'll be able to make a few simple polarity tests to confirm that's the case, once I have the nod from a second pair of eyes. Of course, there's still the worry of a possible need to reform the electrolytic to the right of the bridge rectifier, but I'm coming around to not worrying about that.

    Thanks again for your interest.
     
  8. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    Looks like another "or" function to me (at least the last attachement) The junction of the diodes connect to a coil, the other end of which is connected to +V, so applying a ground to either input turns on the relay.

    If I didn't understand the question, it's becuase I didn't look carefully at all the drawings. Please let me know which one I need to see and state the question explicitly.
     
  9. StewB

    Thread Starter New Member

    Jan 3, 2014
    17
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    Hi, thanks for the comment. I'm referring specifically to the diagram in the file "52671 - new.bmp", or if you don't want to unzip that, "52671 - new.jpg".

    In particular I wish to know if I'm right in thinking that progress beyond the path labelled "B" -> "B" from the Aquastat is totally blocked by the diode labelled D1 (coloured cyan and on the left of the diagram, directly below the potentiometer P1).

    Cheers
     
    Last edited: Jan 19, 2014
  10. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    No. When "less gas" is selected, +ve is connected to D1 anode, and the diode conducts.
     
  11. StewB

    Thread Starter New Member

    Jan 3, 2014
    17
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    Thanks.

    How would that situation change if P1 was diagrammatically accurate - ie it is slammed right over to the left as shown?

    Also, how would the situation change if "less Gas" can only ever be selected when the orange switch is closed?

    Thanks again for your input.
     
  12. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Thanks for the diagram/photo. :)

    There seems to be a few things wrong with it. The SCR pinouts are always KAG, yours says "KGA".

    Also your transistor that switches the motor relay seems to have issues? I think it should be an NPN, and the pinout reversed, ie ECB not your BCE. The collector switches the motor relay as you have shown, and the other pin of the relay coil is live at +21v. So the transistor switches to ground. Therefore its E would be the pin on the left, and its B would be on the right (and controlled from the RED limit thermistor, photo far right). I could be wrong with that, but something is definitely wrong with your drawing. ;)

    Also some of the diode orientations etc seem funky. Can you post a photo of the top of the PCB so we can see the parts? Being able to see the transistor body and number will help prove the pinout, likewise with the SCR.

    I think to diagnose this right will require separating the circuit into branches, each branch from +21v DC through to gnd, and drawing the branches out as a schematic.

    At the moment all we can see is a big mess and guess that the transistor drives the motor relay (but what controls the transistor base?) and that the SCR controls the gas solenoid, but its gate controlling mechanism looks like a real mess.
     
  13. StewB

    Thread Starter New Member

    Jan 3, 2014
    17
    1
    Thanks for your comments.

    It could well be that you are being slightly confused by another of my attempts to be helpful- this time to myself!

    The view shown is a left/right mirror image (as stated on the PCB to the left). This view came about because I wanted an easy way to superimpose components from above on the connections below, so I took pictures of both and flipped the green side.

    I guess the way to view the diagram is this : although you are seeing green, just focus on the components as you are really looking from the top.

    (Not confusing at all :embarrassed smilie)

    I've attached a view of the top - not flipped honest - check the component markings!

    For the semiconductors:

    Transistor : TIP115 PNP Epitaxial Silicon Darlington Transistor.

    The SCR/Thyristor : 2N5061

    pdfs attached

    If you can bring yourself to unzip the zipped bmps, you can use MS paint's "fill" function to colour parts of the circuit and isolate certain elements.

    I really appreciate your time and effort. Thanks in advance for any further comments.
     
  14. StewB

    Thread Starter New Member

    Jan 3, 2014
    17
    1
    BIG OOPS here, as well as closeups of semiconductors, TR1 and TH1 and pot P1, I have to admit to discovering a mistake in what I thought was entirely resistors : there's a zener diode in the resistor bank!!!!!!!!!!

    I've included a pic of this too (it's a C55, of what type, well all I can say is its "#V*" where each of # and * have curved tops - ie 2, 3, 6, 8, or 9). The datasheet is also included.

    Really sorry for the mistake. I've also included an updated schematic showing the zener in place. I'll also include a zip file of a bmp below, just in case you can see your way to using it. I have no real excuse other than assuming it was just a small format resistor. What a pillock eh? It would not surprise me if you lose interest. All I can do is say I genuinely have tried my best to portray the PCB as is and apologize for wasting your time.

    I'm now going to see whether this change makes any difference to the function of D1.
     
    Last edited: Jan 20, 2014
  15. StewB

    Thread Starter New Member

    Jan 3, 2014
    17
    1
    Promised zip file of PCB BMP attached.
     
  16. StewB

    Thread Starter New Member

    Jan 3, 2014
    17
    1
    Despite the change I sill can't bring myself to understand the function of D1 or the path B-B.

    When the boiler is running, supplying central heating, the microswitch to the right is closed as shown. This brings PCB connections B and W to ground (along with the collector, C).

    Moreover, not only do both ends of D1 appear to be at the same potentail, , the connection B on the PCB has to be at the lowest potential, so the diode D1 will never conduct (with a max board voltage of 21v - well below D1's 100v breakdown).

    I have included an image of this situation with the ground potential path coloured in cyan. As usual I include a jpg and a zipped bmp. I cal them

    52671 - back - #7 - test D1 - less gas.jpg & bmp

    What do you think?
     
  17. StewB

    Thread Starter New Member

    Jan 3, 2014
    17
    1
    Further to much last post, I also attach a "much gas" version for the sake of completeness. As much gas uses the path P-P instead of B-B, it unsurprisingly has even less use for path B-B, or for that matter the diode D1. I'm pretty much convinced the Aquastat switch order has changed. I'm thinking the orientation has changed from the serial format illustrated, to a parallel format, one switch shorting path O-O to W-W, the other shorting P-P to B-B.
     
  18. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Thanks for providing more information.

    I apologise, the SCR is a KGA pinout, I didn't know it was a TO-92 package;

    [​IMG]

    However the "PNP" transistor is giving me grief. The circuit clearly shows the relay as being high-side, with one coil terminal at the +21v DC rail. Then the coil current must go to ground, so it goes through a series diode to the middle pin on the transistor (which as a TO-220 pack is definitely the Collector).

    The relay back-EMF diode and series driver diode both prove the relay coil is high-side and to turn the relay on it needs to sink current through the series diode into the collector of that transistor.

    That is all fine and makes sense if the transistor is an NPN. It will sink current through the collector, out the emitter to ground, and turn on the high side relay.

    But you don't use a PNP to sink current into it's collector! It sinks current into it's emitter, and sources the current out the collector. That is totally wrong for a high side relay driver.

    If you were to use a PNP to drive a high side relay (which would be MOST unusual in a commercial device) you would have to connect the emitter to the relay coil and the collector to ground. The PCB is not wired like that.

    Maybe the transistor drives the OTHER solenoid, the gas solenoid? Then it makes a bit more sense, if that solenoid is connected high side, current then goes into the E of the PNP. Still seems weird as heck.

    Does the transistor have another number, ie; TIP115x ??

    Are your hand-drawn wires in the black box and the far right areas correct? They look funky too and make it hard to trace the PCB circuit.

    I think you need to measure some voltages on that PCB (and external components) when powered, and start breaking it down into circuit branches as a drawn schematic that actually makes sense. :)

    You need to understand exactly what the transistor connects to, and what voltages are on its pins when operating, to see what that trimpot does and how those diodes in your question affect it.
     
  19. StewB

    Thread Starter New Member

    Jan 3, 2014
    17
    1
    Thanks so much for your time and thoughts. I'm always struck by how generous capable people are with their time. It often strikes me that the less people really know or can really deliver, the more they want (yeah, I am thinking of estate agents, equity analysts, central bankers, ratings agency staff .. there, now you've got me all wound up!)

    I believe this to be so. It must be like hell coming at this problem over a forum, and I've really tried to provide just the right amount of information (and yes, I've obviously got the balance wrong), but when I posted the "old" pcb layout, my intention was to point people in the direction you are, I believe arriving at, w/o influencing them too much.

    Looking at the "old" pcb, it's clear that the on-pcb solenoid - or relay (labelled "A" on teh original schematic) - acts merely as a by-pass to the resistor labelled "10" (on the old schematic).

    Energising of this relay takes place via routes that have no in-line thermistors. The only controls (Microswitch and Aquastat) are simple binary, on/off, affairs. Clearly transistors find use in running relays, but I think I'm right in saying that they act as amplifiers in such circumstances. Here, no amplification is required to energise the relay, as the switches in the Aquastat and Microswitch Assembly are mechanically closed / opened.

    To this end, I view the switching of the relay A as totally separate from the rest of the circuit, and I believe that the "new" circuit would have, and likely does, follow this behaviour. To that end I've included an updated "old" pcb (changed the potentiometer a bit) with relay A paths energised, together with a coloured "old" schematic and a simplified schematic of the isolated circuit for motor-relay control. I've also attached two view of the new pcb with the same relay A path (as it's known on the old circuit) energised, to show how little it has to do with the rest of the circuit. I've decluttered this view in the 2nd pic by colouring other paths in a colour similar to the pcb.

    As you say, that leaves the transistor, TR1, to control the gas solenoid. This to my mind makes sense, as the gas solenoid requires analogue type control (in contrast to the relay A on/off functionality)


    No. It's just TIP115.


    When I first received this PCB I thought it a 1 for 1 replacement for the old PCB, largely as it had the same manufacturer part number (and was told it would be by the manufacturer). HOWEVER, the facts :

    - that it has only one white flying lead (as opposed to two on the original); - that the transformer markings suggest an alternative wiring (see right inset pic on "new" pcb diagram and;
    - the direction of the diode D1;

    suggest to me that the Aquastat must almost certainly be wired differently. I will include what I now believe to be this new Aquastat wiring as attachments (they have the word "guess" in their titles) in the psot following this one.

    ... and there's the rub! This board may well be 18 years old. I've done my best to reform those capacitors that I can, but the tantalums the non-rectifier filtering cap aren't sufficiently isolated. Moreover, I'm gonna have to think whether incorrect Aquastat wiring might send the transistor connection potentials into reverse and wreck them.

    Before getting into all that I was hoping to nail down whether the Aquastat's connections made sense, and that's why I have focused on the polarity of diode D1. I really do see it as an acid test. Maybe you would not mind, at this juncture, reviewing its function, as portrayed? I'd really appreciate it. If I could just get somebody with experience to conclude it made no sense, it would be a big plus.

    Thanks again for your interest and time.
     
  20. StewB

    Thread Starter New Member

    Jan 3, 2014
    17
    1
    Please refer to above post for comments.

    Attached : guesses at how the Aquastat should be connected. One shows the Aquastat in the "much gas" config, the other in "less gas" (see filenames for which is which)
     
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