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Topic: 5v rail in appliance has 240vac on it (Read 2495 times) previous topic - next topic

Wawa

Mains connected electronics could have a capacitive supply instead of an old transformer or switch mode supply.
If so, the big series capacitor could be bad. Value is usually ~220nF, 400 or 600volt.
Had several of them in various appliances go to 20% or less of their value.
A picture of the board could help.
I hope you know what you're doing. Mains power kills.
Leo..

tocpcs

It does have a few caps on the mains. Yes I will provide a picture of the boards, good idea.

I did think of a bad cap but I metered them (in circuit though!), and the readings found matched the caps as I expected for each one..

As noted before, I am working safe (safety includes RCD protection, meter probes 1000V rated, I am not  a part of the circuit under test).

Boardburner2

I am not  a part of the circuit under test).
If you are using a DMM with 10 Mohm impedance  you might actually be an unwitting part of the circuit.

tocpcs

#18
Feb 27, 2016, 11:40 am Last Edit: Feb 27, 2016, 12:22 pm by tocpcs
As much as I think I can add to provide some insight.

The system consists of two of the Induction PCBs - one each side each controlling two induction coils.
https://drive.google.com/file/d/0B7z2IPqvXbQud3VIZVlVNEo5S1k/view?usp=sharing

One one of the two boards, I went through and took out the bridge rectifier diodes and tested them with the diode test function on the DMM, they were all reading 0.450V in one direction only - good.
I checked the capacitors on the board (whilst they were still in circuit though) - they correlated to their values on the label correctly.

The same board I noticed there was obvious broken trace where an insect had shorted across the two traces (yay!)!

There's another board with some spring coils for the interface, this board has a BUS line, a 5V and ground terminal. It is this third board that I deduced a fault existed as the board had 240Vac on a terminal labelled 5V, the bus line doesn't have 240. I am aware the 5v line from the induction PCB is carrying 240vac and not 5Vdc.
I had a seperate power supply around and supplied it with 5vdc, and it still powers up, so the interface PCB is fine.

The two induction boards are linked together by 3 core cables, these contain 5v, ground, bus.
https://drive.google.com/file/d/0B7z2IPqvXbQuOE9KNklLZGZVVzg/view?usp=sharing

You can see in the left, below half way the two white connectors and two ICs these interfaced to.
The two ICs - look to be optoisolators - These are part number: CNY17-3.
My initial suspicion was that the two boards suffered from 240v on the 5v circuit (see insect joined two traces together), however! they both take 240v direct in and must rectify to produce 12v and 5v each side.

The optoisolators looks to be there for the bus comms only, as I continuity tested from the pins and found 0ohms to the 5v and gnd links on the PCB to the socket pins.

If there was 240V on one board on 5V, there would also be 240V on 5v on the other board....

In the top of the picture you can see on the left where active and neutral (in that order) are wired to that one board to terminals. There's a 30ohm resistor from the active. There's an inductor with a label CC05 on it, I thought that was one of the transformers initially, but it looks only to be an isolation transformer or similar? 240in, 0out?

All transformers on the board were tested to see if they were shorted across from primary to secondary.. not found.
Next to the yellow transformer at top right is a ST Viper17 IC.
Theres a plug nearby for a 12vdc fan as well. I deduce 12vdc must originate or be derived from that transformer. The 4 pin IC below the yellow right transformer is an opto- sfh617a-2

On the back of the board:
https://drive.google.com/file/d/0B7z2IPqvXbQucGFrSTZhdWZRbGM/view?usp=sharing

You can see around where the bridge rectifier diodes are the short the insect created, it goes from primary to secondary?? of the CC05 part / transformer near mains in.
That I suspect caused the alledged fault of 240v on the 5vdc and 12vdc (yes, it affects both 12vdc and 5vdc)..

Testing the transformers on the other board, I got the same for the "CC05" transformer - I got 240 on the primary side, and 0 on the secondary. The relay will be off as the control circuit is off.

But testing to where the 3 pin connectors are, there's now 19Vac there on all 3 terminals (from Earth, to each pin, 19Vac, 0Vdc). I expect to find, 5vdc, ground and bus..

Wawa

You can see around where the bridge rectifier diodes are the short the insect created, it goes from primary to secondary??
It seems to be between phase and neutral of the mains.
A  spark there shouldn't be abe to do any harm to the rest of the board.
Clean the board with a sharp tool, and fix the track with a piece of bare wire.
Leo..

tocpcs

#20
Feb 27, 2016, 11:36 pm Last Edit: Feb 27, 2016, 11:37 pm by tocpcs
Wow... It is all good now.
Yes, it was simply a matter of decently cleaning that side up and joining enameled wire where the trace was!

The 240 on the 5v was occurring on the other PCB (i.e. the one that is undamaged) that must be normal though when the appliance is off (the large relay must change that when the cooktop is on vs off)..

Thanks for the help, I had an inkling to do exactly that before posting up on here but didn't believe it would actually work and got stuck on the ICs must be stuffed theory..

I had to really clean all the burnt insect out of the PCB too, the resistance was 200k with the guts still there, and the good board was 5 million. Cleaning it up nice and neat restored it to 5 million and all works good.

Now to take off my electronics hat and put on my pest control hat.

Paul__B

So ...

It is no surprise that the logic is attached to the mains.  This is normal in dishwashers, brushless motor washing machines, but not generally microwave ovens (well, certainly not the older ones).

You really did have a "bug".  Not at all uncommon.

One such (in the dishwasher) attempted to burn down my kitchen some years ago when it bridged a solenoid.  Even before that, I adopted the practice of completely covering either all possible entry points (including surrounding the control knobs) or covering the control board (or both) of kitchen appliances - microwave, dishwasher, cooktop - with flyscreen mesh, secured with hot melt glue (away from the hot parts, of course).  A necessary alternative (but could equally be, addition) to frequent applications of insecticide everywhere.

tocpcs

So ...

It is no surprise that the logic is attached to the mains.  This is normal in dishwashers, brushless motor washing machines, but not generally microwave ovens (well, certainly not the older ones).

You really did have a "bug".  Not at all uncommon.

One such (in the dishwasher) attempted to burn down my kitchen some years ago when it bridged a solenoid.  Even before that, I adopted the practice of completely covering either all possible entry points (including surrounding the control knobs) or covering the control board (or both) of kitchen appliances - microwave, dishwasher, cooktop - with flyscreen mesh, secured with hot melt glue (away from the hot parts, of course).  A necessary alternative (but could equally be, addition) to frequent applications of insecticide everywhere.

Yeh, I have a bad habit of getting fixated on small issues without taking in the big picture.

The other trick that caught me was the fact that both boards must be working for it to power on, and it just happens to be the left board is the 'master' and that's the one that suffered the 'bug'.

I have a strong feeling those insects aren't going to be renting for free much longer.. They've been given an eviction notice and the bags are packed for them.

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