5v rail in appliance has 240vac on it

Using Arduino General Electronics
#1

I've got an appliance that has two PCBs in it, it stopped turning on.
It has a 5v rail used for it's control circuit, it has a 12vdc circuit for a fan and a relay, and of course, 240 for it's primary function (heating).

I've taken apart and tested and found 240vac on the control circuit, so tested the 12v, and this also has 240 on it..

Unexpected results, and probably why it's not working.

I tested across the transformers (there are 4 of them), and they all read continuous on the primary side and a low to reasonable resistance on the secondary.

There isn't continuity from the secondary to the primary, but from the 5v side to the 240 side for example, I get a reading of 129kOhm (something I'd expect to be pretty high, but I never had known the result of that prior to the fault, so it's unknown).

Whatever the fault is, it's affected two boards inside (each board has a take off of the 240v feed, so they can operate independant, however, between both boards they have a control card and common 5v, gnd, and bus connections (so that the one control panel can turn on any of the 4).
I suspect that the fault in one board, caused the same sort of fault in the second?

Something like - a insect shorted one of the transformers secondary to 240, thus 240 is now in the secondary and through the control chips, over the 5v bus, and caused the same style of fault in the second board..

Naturally, I isolated both boards and both have identical results - 240 on the 12v, 240 on the 5v.

What aspects are worth testing further to find the source of this?

#2

None. Throw it away.

#3

aarg:
None. Throw it away.

I think what aarg is saying is that if you do not have the skills to completely figure out the wiring of this appliance and locate a fault, it is patently unsafe for us to suggest how to do this by "remote control".

Either you find someone locally who has those skills and can work through it with you, or consider it toast (it wasn't a toaster by chance, was it?). :grinning:

It is usual for control circuits in appliances such as dishwashers and washing machines, to be directly connected to the mains in order to use TRIACs or power FETs to control relays, solenoids and motors. This means that the display and control buttons all require to be fully insulated from the exterior.

#4

Ahh I realise that, i was more chasing pointers - i.e. what resistance should we expect between +5 and 240 Active?

It's not a toaster, but an induction cooktop actually!

The control board is normally 5V (it's labelled as such!).

What confuses me most is that there is high resistance, to 240, yet still presence of 240 on that rail. And the same being true for 12V, is telling me that something is taking both those points to 240.

Perhaps a more general question - with a typical primary / secondary transformer (with windings) - what typical resistance would you expect between the primary and secondary ? Is 129kOhm far too low?

#5

Open circuit is what I would expect. Infinite R.

Anything less than a qualified electronics tech should not play with it.

Weedpharma

#6

Yes, sometimes there are low voltage circuits that are internally connected to the mains, but insulated from the outside world. If your device is one of those, it might be a kind of red herring. In other words, nothing to do with the real cause of failure. But the bottom line is, it doesn't work.

You could investigate further to see whether there is another cause. But if you don't see anything, and you don't know what you're doing, it's not worth the time and risk.

#7

If you read 192k it is not isolated from the mains, whether by fault or design.

100M ohms is the sort of value you'd expect for isolation.

#8

Yeh I thought that they would be near infinite, it's low though.

The boards do have a clear fault on them, the terminals labelled 5V and 12V have 240 AC on them, that bit I am going to be sure is a fault condition.
There's a few ICs on the board that I can't find datasheets for :frowning: - the most interesting to me is related to the 5v, and is a 60351, from ST - but nothing shows in google for that, and it's no 7805.

I think I'm going to have to draw it out what they've got and see if there's any obvious reason, and likewise diag the caps and diodes at the 240v in and see if anything kicks there.

The safety concerns raised in the thread are noted, I'm working safe.

#9

the most interesting to me is related to the 5v, and is a 60351, from ST - but nothing shows in google for that, and it's no 7805.

Could be a buck controller. This non-isolated AC/DC buck converter circuit uses the TPS64203

#10

The fact a circuit uses 5V power and labelled as such is not a prohibition on it being floating
on mains live - I presume this device was double-insulated with no way to make contact with
any of the circuitry?

#11

tocpcs:
The boards do have a clear fault on them, the terminals labelled 5V and 12V have 240 AC on them, that bit I am going to be sure is a fault condition.

You cant be sure.

Other devices such as smart energy meters and lighting dimmers use low volts internally but work at mains potential.

Its pretty hopeless working on these things without manufacturer support.
Spare board can sometimes be had but not components on the board.

For if its a dangerous fault , take it to trading standards.
I assume its out of gaurantee.

#12

MarkT:
The fact a circuit uses 5V power and labelled as such is not a prohibition on it being floating
on mains live - I presume this device was double-insulated with no way to make contact with
any of the circuitry?

Single insulated (glass top comes off, metal enclosure, earthed.

Assumably, if the glass broke than you could make contact, but yes, ordinarily you wouldn't be able to.

I'm more curious as to contributing cause of fault, the cost of these appliances means buying another one and having a similar issue is stupid.

#13

Ok, it's an induction cooktop.

It is almost certain that the logic circuitry is directly connected to the live mains. In fact, if it uses touch control circuitry, that circuitry will deliberately use the mains as the sensor voltage for capacitive sensing. As no part of the control circuitry will be exposed to the user, there is no need to isolate it from the mains.

The fact that you measure a high resistance between the control circuitry and the mains suggests that they are linked by diodes which your meter does not detect as it uses a low voltage. The "diode test" function might indicate differently.

You are looking at the wrong "fault" if you are obsessively concerned about isolation from the mains.

And you have committed the cardinal sin - mucking about trying to describe the thing in words when a few decent photographs would make the situation substantially clearer.

#14

Somethin i would add.

I assume you are measuring across the 0 to 5 v with your meter and seeing 240 v.

If this as a dmm it may be incorrect.

I would check first with a moving coil meter, preferably 2000 ohm per volt or thereabouts.

Avos were 20000 ohm per volt.

I keep an old cheap multimeter specifically for these types of problem.

Even if you can identify what is wrong getting the required spares for these can be difficult.

I recently threw out a microwave as the call out charge was twice the cost of a new one.

#15

tocpcs:
I'm more curious as to contributing cause of fault, the cost of these appliances means buying another one and having a similar issue is stupid.

You have my sympathy.

Recently a relative had a broken water softener, replacement cost 1400.
Spares unobtanium to mortals.

However found a local immortal guy who had one on his shelf for some reason, 100 to repair or there about, inc labour.

#16

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..

#17

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).

#18

tocpcs:
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.

#19

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..

#20

tocpcs:
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..