Need help to identify Melf diode  - Page 1
Forum section: electronics / beginners

author: Albert2011
author_rank: Newbie
timestamp: August 05, 2024, 09:50:17 am
content: Hi, appreciate it if anyone could advise what is the spec of the attached Melf diode?The first band is blue color, second band is grey.This Melf diode is used in switch mode power supply. I could not find the same type of spec in vishay website.https://www.vishay.com/docs/88580/egl34.pdf

author: RayRay
author_rank: Frequent Contributor
timestamp: August 05, 2024, 10:54:35 am
content: I'd say the only way for you to identify it would be by searching for the service manual/schematics of the particular device by it's model number. That said, I can tell you from experience that in-circuit component testing doesn't always provide accurate results! I've had cases where a diode read a short or provided two way voltage drop in diode testing mode (in circuit) but tested fine out of it! Point is, if you get a bad reading from an in-circuit component, it's not at all definitive that it's faulty! It's 50/50 and you have to remove it from the circuit in order to verify if it's indeed faulty or it was just something in the circuit that was affecting the reading. It's entirely possible that this diode is perfectly fine.

author: Albert2011
author_rank: Newbie
timestamp: August 06, 2024, 04:05:48 am
content: Hi, I could not find the schematic diagram for this power supply. This Schroff maxpower MAX180 PSU is working fine.I need the spec of the Melf diode to use in a custom-made Schroff maxpower MAX180 PSU for special broadcast equipment. The only difference is the output sensing circuit and both + / - sense circuits were internally connected with one Melf diode and jumper resistor.The radio audio routing equipment manufacturer does not want to sell us this new PSU and has not authorized us to purchase a custom-made PSU from the Schroff. Schroff could not sell us a custom-made PSU without an authorized letter from the equipment manufacturer. This equipment manufacturer wants us and forces us to spend millions of dollars to upgrade to their new system but we refuse due to budget costing.So, I have bought a similar maxpower MAX180 PSU from Schroff. I want to modify the output sensing circuit according to the custom-made Schroff maxpower MAX180 PSU design.At the custom-made Schroff maxpower MAX180 PSU, I measured both ends were short circuits with a Melf diode on the pcb board sense circuit. So, I believe this Melf diode works as a protection circuit and only allows current flow in one direction or purpose as a surge protection.I really hope an expert could give me advice on this matter. What would happen if I did not put this Melf diode on the custom-made Schroff maxpower MAX180 PSU?https://schroff.nvent.com/en-gb/products/enc13100-152Thanks

author: floobydust
author_rank: Super Contributor
timestamp: August 08, 2024, 01:54:15 am
content: Best I could find for the plastic MELF colour codes that Vishay uses. Nothing for two stripe blue, greyOne stripe blue (cathode mark) isVishay Tranzorb TVS plastic MELF1st band white is polarity, 2nd band gray is 50V for BYM10-50.It seems to connect to TL431B pin 8 (REF)? That might imply a zener for OVP or something.I did not look at other manufacturers.

author: Albert2011
author_rank: Newbie
timestamp: August 08, 2024, 03:29:37 am
content: Hi, thank you for sharingI encounter the Melf diode was parallel with a zero ohm resistor as refer to the attached photo.So, do you think whether it is necessary to put this Melf diode into the circuit?Do you know the purpose of adding the Melf diode with parallel with zero ohm resistor?This diode is not connect to TL431B pin 8 (REF).

author: floobydust
author_rank: Super Contributor
timestamp: August 08, 2024, 04:10:29 am
content: It's weird if the diode is actually shorted by 0R resistor. TheUCC29002-1IC is a load sharing controller, the transistor "1GM" SOT-23 E connects to pin 5 the current-adjust pin. C goes to TL431.I would guess it's a design mistake or load-sharing option this PSU has. They might have a model of this PSU with advanced load-sharing, and the 0R is removed for that model?But why bother populate all the other parts like the UCC29002? A strange bird.

author: Albert2011
author_rank: Newbie
timestamp: August 08, 2024, 08:51:22 am
content: Our Lawo audio equipment comes with dual redundant power supply.So, I have to internal connect the sense input according to their custom-made design PSU.Shall I not placing the Melf diode into the circuit base your knowledge? Which means I only remove the SMD ceramic capacitor and changed the 220R resistor to 0R resistor.Do you think necessary to use Melf diode BYM10-50?Thanks