Identify zener in a SMPS feedback circuit? - Page 1
Forum section: electronics / repair

author: Pinball_Newf
author_rank: Newbie
timestamp: February 29, 2024, 04:40:43 am
content: 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?

author: magic
author_rank: Super Contributor
timestamp: February 29, 2024, 05:55:33 am
content: 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.editQuote from: Pinball_Newf on February 29, 2024, 04:40:43 amWhat do you mean by this?A schematic of the PSU would help.

author: Pinball_Newf
author_rank: Newbie
timestamp: February 29, 2024, 07:44:40 pm
content: 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...

author: fzabkar
author_rank: Super Contributor
timestamp: February 29, 2024, 08:03:52 pm
content: Quote from: Pinball_Newf on February 29, 2024, 07:44:40 pmThat would be correct ... if the zener is directly connected to the rail that it is trying to regulate.

author: Pinball_Newf
author_rank: Newbie
timestamp: February 29, 2024, 08:11:40 pm
content: 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.

author: tom66
author_rank: Super Contributor
timestamp: February 29, 2024, 08:37:21 pm
content: 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.

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 29, 2024, 09:01:14 pm
content: 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.

author: Pinball_Newf
author_rank: Newbie
timestamp: February 29, 2024, 09:48:58 pm
content: 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..

author: Pinball_Newf
author_rank: Newbie
timestamp: February 29, 2024, 09:50:28 pm
content: 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

author: Jwillis
author_rank: Super Contributor
timestamp: March 01, 2024, 12:12:51 am
content: 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=Vs-Vf/Ifwhere Vsis the supply voltage, Vfis the forward voltage of led and IfIs the forward current of LED.R=Vs-1,2V/10mA depending on what value R is you should be able to find the value if the Zener. Where Vsis the value of ZenerIf 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.

author: Pinball_Newf
author_rank: Newbie
timestamp: March 01, 2024, 03:35:51 am
content: 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!