Identify zener in a SMPS feedback circuit?

[Pinball_Newf]

Hi, I've a blown zener in a plasma cutter with no markings.  Is SMD glass package and is used with a PC817 optoisolator to provide feedback [to a Viper22A PWM controller].  The input voltage to the zener measures 7.4V.  Typical forward voltage of PC817 is 1.2V The thing has zero markings except the blue stripe. 
Is there any way to identify this or any suitable replacement?

[magic]

Whatever you install, your new output voltage will be Vz+1.2V or something like that.

If my guess is right, you need to figure out exactly how the feedback circuit works and find what's the proper output voltage from this PSU, then select the right part to achieve it.

edit

The input voltage to the zener measures 7.4V.

What do you mean by this?
A schematic of the PSU would help.

[Pinball_Newf]

Hmmm, I can attempt to reverse engineer it. The diode is reverse biased to drive the optoisolator's LED...  So presumably I'd select one that has a Vz of whatever PSU voltage it should be [say 5V] - the 1.2V needed for the optoisolator. Am i thinking this right?   Basic digital electronics are more what i do, so i am a bit out of my element here...

[fzabkar]

Hmmm, I can attempt to reverse engineer it. The diode is reverse biased to drive the optoisolator's LED...  So presumably I'd select one that has a Vz of whatever PSU voltage it should be [say 5V] - the 1.2V needed for the optoisolator.

That would be correct ... if the zener is directly connected to the rail that it is trying to regulate.

[Pinball_Newf]

It's connected to a dedicated tap on the secondary side of the transformer.  That tap goes to just over 7V before the PWM controller cycles on undervoltage.  I'll need to determine the ratio of this tap vs. the one for the main rail and go from there.

[tom66]

It might be possible to monitor, under no load, the output voltages of other taps.  Then if you know the voltage required you can estimate Vz.  You could also experiment with different zener options though SMD makes this more difficult.

Also I would expect a zener to fail open circuit resulting in loss of feedback, high output voltage and usually smoke.  Sometimes the controller OVP stops things, but often it doesn't.

[DavidAlfa]

The viper22 regulates the output usign its own VDD and wants 12V IIRC. So you adding a zxener increases the voltage by the same amount.

[Pinball_Newf]

From what i can see it uses VDD gated through the optoisolator into the feedback pin... Useful voltage on the feedback pin is 0-1V according to datasheet IIRC.
The zener has failed open, so I'm presuming because it sees 0V on the FB pin, it's cycling...  [It does the same cycling even with the optoisolator out of circuit]
The zener is on the secondary size driving the other side of the optoisolator..

[Pinball_Newf]

I indeed ordered a box of zener's from amazon and intend to go this route.. Probably tack in some wires and make some sort of rig to test the various ones.
I replied to another comment why i think it's didn't totally grenade

[Jwillis]

The PC187 pin 1 and 2 are for a LED with a forward voltage of 1.2V. You can figure out the Zener voltage by the value of the limiting resistor. The LED has a Maximum forward current of 50mA but should limited to no more than 20mA. For normal operation 10mA is best.

Use the formula  R=V_s-V_f/I_f where V_s is the supply voltage, V_f is the forward voltage of led and I_f Is the forward current of LED.

R=V_s-1,2V/10mA depending on what value R is you should be able to find the value if the Zener. Where V_s is the value of Zener

If I understand the package description your probably looking for  SOD-80 miniMELF or QuadroMELF. Either depending on size. Although it is possible to mount an axial package with some dexterity.

[Pinball_Newf]

Thank you Jwillis, this helps immensely.  I can easily figure that out.. and Yes, it's a SOD80 miniMelf and I'm going to be trying an axial package 1st, they are easier to come by!