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I need help repairing a control board for a Bosch oven.

How it started: The oven would pop the fuse whenever it would be plugged in the socket -> short somewhere. I opened it and tested a few parts with my multimeter like heaters (2x), fans, lightbulb and all checked out fine (no short to ground.) After disconnecting piece by piece I narrowed it down to a control board.

The board itself did not show any signs of visible damage (blown parts) but a few components were suspicious. A power resistor (2 W I would guess) was measuring 150 Ω instead of the 1.5 Ω that was indicated by its color code (rings). Another was a Zener diode that was measuring s large resistance (unmeasurable by my multimeter) in both directions.

I desoldered both of them from the board and my plan to fix it is to just strip one of the many USB charger boards that I have lying around and use its 5 V output as a new power source. That is my approach because it would take me longer to get the proper parts ordered and with shipping it would be much more expensive. Please correct me if this is bad approach.

My real problem is to identify the "real" +/- (ground) points for the board. From what I gathered, the board is using a 5.1 V Zener diode as a voltage regulator. Please find the image of the board attached. Broken parts are desoldered.

Can you help me understand what would the ground be in this circuit? Because of the bridge rectifier, the circuit is not showing continuity with some parts that need ground and that is confusing me a lot.

Parts:

  • Zener 1SMA5913BT3 5.1 V by Onsemi
  • Electrolytic cap 25 V, 680 μF
  • Resistor 2 W, 150 Ω
  • Bridge rectifier (??) markings: 4 16G

Power supply area

Ground traced

EDIT: Whole board image is attached.

EDIT2: Front picture of the board added.

Front side

Thank you all for you input! I think I've found the problem when trying to test the relays' actuation. The problem seems to be with the flyback diodes that protect the relays. Both of them show a very low voltage of around ~0.07 V in both directions. I will replace them and test the relay actuation again.

ocrdu
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Stazh
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  • What's on the other side of the board? There must be something connected to the -ve of the bridge rectifier. – jonathanjo Jan 10 '23 at 09:55
  • @jonathanjo It's a single sided board. These dark areas are "second layer". They are some strange substance that links the traces exactly as shown with the darker traces, but they add 100ohms of resistance (that's the amount that my multimeter measured). – Stazh Jan 10 '23 at 10:05
  • This is the whole board. Only direct connection to - of the bridge rectifier that I've found is the one I've drawn. There are many points that show ~80ohm or ~170ohm resistance towards - of the bridge rectifier. [Whole board](https://imgur.com/a/BhkEd48) – Stazh Jan 10 '23 at 10:19
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    A 1.5 Ω resistor seem rather low in that position. Maybe you read the value wrong. 150Ω makes more sense. There is probably a big polyester through-hole dropping capacitor near the top- check that isn't shorted. Looks like you're working with a circuit that is 'hot' (connected to the mains directly). I suggest you toss the thing and buy a new one before you hurt yourself. Chances are it's a bad heater element and thus unlikely economical to repair (assuming it's a counter top thing). – Spehro Pefhany Jan 10 '23 at 10:24
  • @SpehroPefhany Yeah, resistor measures 150ohm. But its marking read 1.5ohm (used resistor marking decoder). There is additional capacitor on top layer: 0.68uF 275VAC X2 class [Link](https://www.iskra.eu/en/Components-for-radio-interference-suppression/Capacitors-KNB1560/) It reads ~0.9M ohm. This is board is for the oven... Replacement board is 150€ and new owen 500€ + ... I would really like to fix it if possible. – Stazh Jan 10 '23 at 10:36
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    It's unlikely that a short on a PCB will draw the 10-20A something needed to pop the fuse in your kitchen. The PCB and/or the faulty component would have burnt to a crisp long ago before that. Therefore suspect things like MOSFET drivers not working properly. – Lundin Jan 10 '23 at 10:45
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    As for what is ground and logic supply, measure each side of any decoupling cap near an IC. It will be sitting between Vdd and GND. – Lundin Jan 10 '23 at 10:46
  • With the above components removed, do you still measure a short across input terminals? – winny Jan 10 '23 at 11:03
  • @winny No. There is no short anymore. – Stazh Jan 10 '23 at 11:31
  • @Lundin Thanks for the advice! Board does not use MOSFETs for switching, it's using relays. They read proper resistances for their model. – Stazh Jan 10 '23 at 11:33
  • Is the (loose) capacitor shorted? – winny Jan 10 '23 at 12:44
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    @Stazh In that case maybe check that no relays have welded contacts. You don't mention checking relays anywhere in your post. – Lundin Jan 10 '23 at 13:32
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    please be very careful ... all parts of that board may be at powerline potential relative to earth ground – jsotola Jan 10 '23 at 17:53
  • @Lundin yeah, sry, i didnt write it in the original post. Will upda it and will test with lab power supply to see if they properly actuate (they are 6VDC ones). – Stazh Jan 10 '23 at 21:44
  • @jsotola thanks for the answer! I dindt even consider that. But the board does not have connection to the earth ground provided. It's all plastic that it touches so thats why probably. Will take a pic of the back and post. – Stazh Jan 10 '23 at 21:44
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    As usual when dealing with 230VAC stuff, electrolyte caps will stay charged for quite a while even after the board is unplugged. – Lundin Jan 11 '23 at 07:12
  • @Lundin yeah, been zapped few times in my history :) im using a variac to lower the ac voltage for testing. – Stazh Jan 11 '23 at 07:43
  • I've desoldered the flyback diodes and the relays actuate properly now, but I've tested their fw voltage and it's around 0.7V which seems right, and they are not shorted... I'm confused now additionally... With them on the board the relays would not actuate and the short circuit protection would kick in on the lab power supply at 300mA. Without them they actuate fine... – Stazh Jan 11 '23 at 09:05
  • And to answer my own question it is a Zener diode BZG05C7V5 with a breakdown voltage of 7V so it is broken. – Stazh Jan 11 '23 at 09:55
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    @Stazh Good! Please post it as an answer in the answer box below so you can mark it as accepted. – winny Jan 11 '23 at 09:56

1 Answers1

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So it was the 3 zeners that caused issues. The 5.1V regulator one and 7.5V flyback ones for the relays (i don't know why they use zeners for this but anyway...). Once replaced with new ones everything works fine now.

toolic
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Stazh
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    `[why use Zener diodes for flyback diodes]` There are different things to want from a *snubber*. An ideal diode would not dissipate any energy. You may want (most of) the energy dissipated before the next *on* period. – greybeard Jan 11 '23 at 16:25
  • @greybeard Thanks for the comment but can you help me understand how are zeners better here then ordinary diodes for flyback voltage protection? (Are you saying they will disipate heat faster?) – Stazh Jan 11 '23 at 18:26
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    Rather than writing the book on snubbers (I wish I knew one to refer/read), here is an [answer to When/why would you use a Zener diode as a flywheel diode?](https://electronics.stackexchange.com/a/388609) (sic) to an even older question with a schematic you can tinker with and simulate. – greybeard Jan 11 '23 at 20:16
  • @greybeard Thank you so much! – Stazh Jan 11 '23 at 23:05