Problem with one industrial controller

[Vancata]

Hello folks.
I have one industrial controller for controling some time relayed stuffs. The controller is powered directly from the network here 230V AC Central EU standart . I can't remember the name of it, because i forgot to take a picture or type it on note when i was at work, and now i dont have it with me.

With few swaps of boards with other same type controller i found that the Power supply curcuit had failed.
When i was testing it, I found I have one Zener diode that was pretty dead (open in both ways). Anyway I have TOP200YAI (Three terminal Off-line PWM Switch), so I suscpet the zener diode is the problem. Anyway the main problem is I cant find what Zener diode is there, 25V,50V75V? Tryed to measure the voltage drop on the working unit on that zener but for decent test i need to desolder it from the board, test it where it cuts the voltage et.c, but because of some other problems i really can't do that (really long to explain).

Anyway will go on the short version, i have TOP200YAI PWM switch, it have I'ts periphery on the board and i should have output - 24v. What zener diode do i need to replace the burned one? Did not found any formula or anything to calculate the zener in the TOP200YAI datesheet :/

btw: The zener have some color code pink+ blue band or something, you can see it in the uploaded photo.

Thanks in advance folks!

[Vancata]

Tryed some online calculators and its like 75V - Violet for 7 and green for 5, going the right way or?

[Rufus]

The zener diode clamps the voltage on the TOP200 drain to the dc supply voltage + the zener voltage.

The dc voltage on 240v ac will be around 340v and the TOP200 abs max drain voltage rating is 700v. The TOP200 data sheet shows example circuits will 100, 150, and 200v diodes.

I'm not sure what the trades offs are choosing the zener voltage, I guess it needs to be higher than the max dc output voltage multiplied by the transformer turns ratio. If you have a good board the zener could be measured in circuit but it will need a low current high voltage supply - an insulation tester would probably suffice. Otherwise search for zener or TVS diodes in melf package and see if you can find something with matching package and markings.

Because the zener limits the voltage on the TOP200 drain failing open circuit will likely have damaged the TOP200.

[SeanB]

Normally you have a 10-22n 400v rated capacitor there, and a parallel 22k 1W resistor to dissipate the stored charge from the turn off transient. You can use a 100-150v transient suppressor diode rated at 1W3 to replace the zener diode, as it will likely survive better.

[Vancata]

Thanks agein folks 
Will go with the transient diode tommorow, found one with 144 break down voltage in  SMB packet, will test it tommorow. Will go grab one TOP200 to be sure too.
 Hope to put this controller back in order  agein, otherwise will have to install new one and play like 1 year to set and calibrate the timings for the controled operations. 

P.S. Really sad when you mess up with old units where you can't find any spare parts for them on the market.

[128er]

Just for curiosity, is it something like a panel mount controller? For temperature or so? Manufacturer would be interesting. PCB style looks familiar.

[Vancata]

Just for curiosity, is it something like a panel mount controller? For temperature or so? Manufacturer would be interesting. PCB style looks familiar.

Panel mount yeap, for temperature control. The brand and model: "Eurotherm 2216"

[Neilm]

That is a fairly typical snubber circuit. I have seen various types, the zener configuration you have, an RC network and even a mixture of the two. When the switch turns off, the voltage on the FET rises. The rise of the voltage is due to the parasitic effects in the transformer, and the secondary side discharging.

Snubbers are designed to control this voltage rise, in this case the zener should clamp it to the rated voltage of the switch. The excess power is bled off as heat. In certain fault conditions it will end up conducting each cycle causing it to fail if the condition has not been thought about.

In other words, the zener blowing up could be a fault symptom of a fail on the secondary side (for instance part of the transformer shorting out, rectifier shorting etc.)

There is also another type of snubber known as a loss-less or resonant snubber. It uses an inductor, a capacitor and a pair of diodes to "store" the energy of the spike. This is then returned to the supply with only minor losses in the inductor and diodes. It is not often seen as the inductor is more expensive than a resistor or zener.