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Author Topic: help! Effects of 250 - 260v too high voltage through pcb???  (Read 2007 times)

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rodders

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hi, just joined up today as i want to learn more about electronics, i'm a service engineer and a machine is now forcing my hand..

 A customer rang me with a coffee machine in a commercial outlet. The machine had blown a 3kw element, standard.. Put a new element in job done..

A few days later the element blew again which i thought was strange, this time all the water etc was contaminated so i suggested i put a spare machine in for them, have a propper rebuild of hers in the workshop which it was about due anyway, i fitted a spare machine of the same type and a day later or so the keypads died, new control board required.. This has happened ever since.

I fitted 2 boards to this machine thinking i just had a bad board or something out of line on my machine but it was fine in the workshop and taking the board out on site.

I got the customers own machine ready tested it well and returned it to site. That day the board was gone, fitted another board (getting expensive now and losing confidence in myself!) which went that same day..

I measured the voltage this morning about half 7, and was getting a steady 255.2 once it settled on a decent fluke meter.. This was the highest but i measured it at 3 different outlets and they were at lowest 248ish.. Could this be their problem?

The pcb has a hahn trasformer to take the voltage down to 12v 2.1va, a few relays etc and a bit of logic.. the board only really controlls water level and dosing, neither of which work any more.

I had suggested early on that maybe the electrics are causing an issue but the customer has had an electrician in twice who says it's not that, he put a socket to the machine now the customers convinced its the machine?

So thats 2 different machines, both taking out boards for fun but working perfectly in the workshop?? I'm not convinced, i thought the max voltage in the uk should be 254, but i'm not sure of the effects of this on a pcb,  so any suggestions would be much appreciated. I noted at one point on site the transformer on the board was red hot, i could feel it through the plastic casing. Sorry to go on but i wanted to make sure i didn't miss anything out..

Cheers in advance, rodders

kam

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Re: help! Effects of 250 - 260v too high voltage through pcb???
« Reply #1 on: June 10, 2012, 08:55:28 AM »
You've been going through the worst case scenario: a problem occurs periodically and never when you're on site. Whenever i experience such situations i feel totally helpless.

What exactly is it that explodes on the PCB? Is it near the transformer? Some capacitor or diode? A good point to start is to measure the voltage on the transformer input/output while you're in your workshop and while the machine is at the customer. And then try to locate the bridge rectifier and the voltage regulator that must exist right after the transformer, check their maximum specifications from their datasheets and see if the voltage exceeds those limits. I'm pretty sure though that their limits are way higher.

It could be that the power network experiences situations like voltage sinking or sudden voltage spikes as well... You an only prove this if you ask the power company of the area to install a 24-hours power logger which will record the event.

rodders

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Re: help! Effects of 250 - 260v too high voltage through pcb???
« Reply #2 on: June 10, 2012, 12:58:02 PM »
cheers, its made worse by being about 30 miles away, not too far but definitely far enough to be going back and forth constantly..

i found the voltage regulator, says it puts out 12v and 1.5a, and thermal protection cuts in at 1.8a, which is interesting because when i brought her machine back to the work shop with a duff board, i wired it in and it worked. Nothing had blown in any case on the boards, they just seemed to stop all functions, shut down..

The bridge rectifier says W08G 135C, found the datasheet but it was a bit confusing.. 1.5A output current so i'll measure this all on monday,

probably just doubled my knowledge on circuit boards already lol,

should i measure from the + to ground or -? On the rectifier, Someone told me to stick my meter on the + and - but i always assumed this could do damage with it all live. And the voltage regulator measure accross the 3rd pin and second?

Thanks again for your help

kam

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Re: help! Effects of 250 - 260v too high voltage through pcb???
« Reply #3 on: June 10, 2012, 20:12:50 PM »
First of all, regarding your measuring equipment... You said you have fluke multimeter: RESPECT! So unless you do something wrong, it is rather impossible that the multimeter will sort any contact whatsoever.

To measure the voltage you just hook the + and - to the output of the bridge rectifier. You will measure DC voltage there. Then, you hook the - to a negative DC portion of the PCB (no matter which), and with your + you can measure the output of the regulator. To find the output, you will need to see where the 3 pins go. One goes to the + of the rectifier, the other to the negative, and then there is another one that is the output. It should have also some sort of big electrolytic capacitors and 0.1uF capacitors connected to this pin. This is your second measurement.  Third measurement is to measure the AC output of the transformer.

You should also take these 3 measurements in your workshop before you leave on another machine that works. Compare them with the customer. You should not find critical changes. This is a good start.

Something more regarding the multimeter. As long as you measure voltage (AC or DC), the input of the multimeter has infinite resistance, which means that there is absolutely no way to damage the multimeter or the PCB, unless you radically exceed the safety limits of the instrument (if for example you try to measure 240V at the scale of 10 or 50Volts.
To measure current you must be very careful, because the theoritical resistance of the instrument becomes 0. This means that if you measure current (AC or DC) and you hook to the + an -, things will go really bad as huge amount of current will flow through the multimeter. Fluke has very good protection, but i do not know about your PCB.

You can measure voltage by connecting the instrument in parallel
TO MEASURE CURRENT ONLY CONNECT THE INSTRUMENT IN SERIES!!!!!!!