Biomass Boiler not driving cleaner motor relay. ENERTRES BIOMASA BI-650

[edugimeno]

Hi everyone!
I have this board that suddenly stopped supplying power to an output that drives an interchanger cleaner motor.
I checked while the board was in place with all wires attached and there's no AC voltage on the corresponding plug X5
This X5 is driven by fuse F8 (tested OK) which comes from the second orange relay
To make sure there are no charcoaled contacts inside the relay, I got a 24VDC power supply and got it ready to manually power up the coil side of these relays.
I respected the polarity, positive to cathode of protecting diode
So provided all the positives to all coils are common, I soldered + to this rail, and then applied negative to the other side of the relay coil.
With the first relay (not the faulty one), it clicked correctly on and off several times according to each time I touched it
With the second relay (faulty output), it clicked slowly and like "lazy" both for ON and OFF (some delay) and I got some smoke smell right away. Turned the board upside down and felt heat around both small Q's that drive this coil.
Can I have damaged the Q's that drive it? As the polarity is granted, they are all going + - + - + -....
Or is this just a sign that these transistors were the culprit for this issue?
I don't really understand the schematic (see images) for the driving Q's, it's like 2 different lines can drive independently the negative side?

Any help would be appreciated!
Thanks in advance

[edugimeno]

Oh also I can see in my own pictures.. .that the second Q at the second relay is burnt by the end of the letter W...And I measured resistance from E to C in this transistor and it has 30 ohm on both polarities, which is not happening on all the other Q's
Should I just replace this Q?

[edugimeno]

Anyone? Thanks

[petehall347]

yes replace relay driver and check the flyback diode

[truclacicr]

http://markingcodes.com/search/c/1gw

1GW = BC847CW or BC847C

[edugimeno]

Yes I have 5x BC547C ordered at the local store, arriving next week.
Will see how I manage to desolder/resolder them, Im not very proud of my smd soldering skills...neither my sight

[petehall347]

good magnifier with light. and hot air works a treat .

[edugimeno]

And does it sound reasonable that feeding 24V to the relay coil pins (correct polarity) the transistor started to smoke? Is this a hint that the Q was already blown?

Also can I have damaged any other electronics before the transistor driver (like an IC) maybe getting these 24V to get back to the IC?

Thanks

[edugimeno]

good magnifier with light. and hot air works a treat .

I will prpbaby get one of those magnifiers with a weight. I don't have hot air, only a gas powered soldering iron that has a point for hot air, but having some other tiny smd R's very close, I tend to belive that with hot air all the stuff around is going to get loose

[petehall347]

I will prpbaby get one of those magnifiers with a weight. I don't have hot air, only a gas powered soldering iron that has a point for hot air, but having some other tiny smd R's very close, I tend to belive that with hot air all the stuff around is going to get loose

i dont have trouble with components this size they are pretty big compared to the real small ones . thing is with an iron its difficult to hot up 3 legs at the same time . what i do is add new solder with my iron then hot air .

[edugimeno]

i dont have trouble with components this size they are pretty big compared to the real small ones . thing is with an iron its difficult to hot up 3 legs at the same time . what i do is add new solder with my iron then hot air .

Suppossedly flux around would help getting the heat spread around the whole transistor, that's my hope...Will see when they come

[edugimeno]

Im back, my local shop received the new SMD transistors. Instead of 1GW, I got K1M, do you believe they would fit?
Also I got one of the 2 transistors on another relay shorted too. My 2nd question is, being that I only applied direct voltage to the coild of these 2 relays (correct voltage, correct polarity), and there are only shorted transistors in the relays I checked, is there any chance that I could hve burned these transistors just by having tested the coils in circuit? Once I felt smoke at the second relay, I stopped checking all the other relays, but I don't know if it was me who was burnng them and the original issue was other...

Thanks

PS: I was able to remove the old transistors with no much effort just by using a normal solder iron, switching from one side to another quickly and pushing gently with tweezers. I removed the relay to make some room, which was way harder to remove than the Q's using a basic desoldering pump

[harp]

My 2nd question is, being that I only applied direct voltage to the coild of these 2 relays (correct voltage, correct polarity), and there are only shorted transistors in the relays I checked, is there any chance that I could hve burned these transistors just by having tested the coils in circuit? Once I felt smoke at the second relay, I stopped checking all the other relays, but I don't know if it was me who was burnng them and the original issue was other...

So on what points you are injecting voltage, 24v?
Have you inspect conections from positive rail through relay and transistor to negative rail? Are negative rail ground? Coil resistance, driving voltage?
It is general bad idea to inject voltage without limit the current, but if you inject directly on coil, that should be the smallest resistance in this circuit, so the most current should pass through coil, not transistor...

Use multimeter in continuity mode and traceback schematic of that part of circuit.

And... 1GW = BC847CW or BC847C is not 5x BC547C... I not check datasheet.
Make sure that transistor is same type, voltage and current capabilityes.

[edugimeno]

Thanks. I directly injected 24V to the relay coils (across protetion diodes). I checked polarity, in this case the common point is the positive side, and then GND is switched by the driver transistors
I haven't checked all those parameters. All I have checked is the protetion diode and it tests OK
Yes the K1N transistor is an BT3904 NPN transistor which has Vce=40v (this board is powered at 24v), and has Ic=0.2 and P=350mW while the old one 1GW has Ic=0.1 and P=250mW, so it looks like it would work fine
Thanks!

[redwire]

Injecting 24V across the relay coils causes back-feed through the transistors and can damage them. It's not a good idea.
I think what happens is reverse current flow throught the (zener) E-B junction and forward C-B junction diodes.

[edugimeno]

Im a little confused on this. Per the schematic, it looks like these relays have 2 transistors that will drive the coils, mounted in parallel, like different sources will be able to independently drive them high.
If having fed 24V manually at the coil lead would damage the transistors, what if one transistor drives 24 to the coils? It would be also getting 24V in reverse to the other transistor in paralel right? So this leads me to think that if it's me who applies the 24V across the coils leads, the voltage seen by the transistors would be the same as if the other transistor would be providing it during normal circuit operation, and I wrong?

Thanks again!

[redwire]

What schematic? Current flow normally is from emitter(s) to GND, leaving the coil(-). But it goes backwards when putting only 24V power on the coil, it is trying to power up the entire board! Flows into the emitter(s).

For functional safety, sometimes a relay driver needs consensus from two microcontrollers or supervisor systems. As a redundant "AND" gate. So it will have two inputs- even though one is all you need and see on other control boards.
But you seem to have an "OR" gate with the two transistors.

[edugimeno]

Im realizing... when I applied 24V across the protecting diode, all I did was applying +24v to the upper rail (already common +24v for all relays) and 24v GND to the other side of the coil, which is directly fed by both transistors, and driven to gnd by one of them when their source signals becomes high. Is putting their collectors to GND a source to backcurrent? I don't see how this can damage them. Not trying to argue, just not able to see it.
And like I said, when one transistor switches on, the other, in parallel like an OR gate, would be "suffering" this backcurrent too, like when I manually applied this voltage to the whole pair, right?
See attached reduced schematic...
Thanks!

[redwire]

It's your fault. You blew it up lol. This is how I look at it...

A BJT consists of two diode junctions inside. For a transistor to work, the C-B junction must be reverse-biased, and the E-B junction is forward-biased.
But when you apply 24V across the coil, current flows backwards through the transistor's two diode junctions - the C-B junction is forward-biased on, and the E-B junction avalanches as a zener because they are around -7V only.
Then current flow is from the GND, as the board also draws power. You end up supplying the board with 24V power flowing backwards through the transistor(s) and if the current is too high, it will damage them. Note the transistor base will appear negative V and the resistor protects the MCU output pin hopefully.

If it was a 5V relay/power the transistor(s) E-B junction would not avalanche and conduct, so maybe this is why it can work then.
I just remove relays that I suspect are bad and bench test them.

[petehall347]

can always short relay driver c to e with board powered . or get real technical and monitor voltage on it base