Cheaper and more common alternatives to TV-8 high inrush rated relays? - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: May 27, 2025, 05:15:22 pm
content: I have a design where I have some relays switching the input and output of what effectively is a 24V inverter that charges large banks of batteries.The SMPS has quite a bit of input and output capacitance. The large inrush current from the capacitance will cause normal relays to weld shut.The purpose of the relays in this design is to (1) protect the inverter from certain situations like reverse connected batteries (2) to prolong the lifespan of the aluminum capacitors.Previously, I settled on using TE OJT series TV-8 (ie. 100 amp inrush current) rated relays with a small series resistance. It works, but TV-8 relays are uncommon and slightly expensive. I am looking for a way to do it with cheaper and more interchangeable commodity parts. Also, space (particularly height) is a concern.Previously, I had avoided doing solid state switching because I was not confident that I could made it as durable or simple as a relay might be for a given level of effort and complexity.This application passes 5A normally, but can have short circuits / transients / inrushes that are limited to no greater than 100 amps. (short circuits can occur while the output relays are opening or closing)Are there any mechanical relay alternatives that might be more interchangeable and cheaper? (Particularly from Asia)Occasionally one reads about tungsten contacts or similar used for certain applications. But I'm not confident that I've looked in the right places.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 27, 2025, 05:52:27 pm
content: On a related note, I have a potential application for normally-closed TV-8 rated relay - has anyone ever seen one ?

author: incf
author_rank: Frequent Contributor
timestamp: May 27, 2025, 06:34:38 pm
content: Quote from: mikeselectricstuff on May 27, 2025, 05:52:27 pmMe too! I can't believe they don't make them.I'd speculate solid state switching ate the market for these types of rugged relays.

author: MF-jockey
author_rank: Regular Contributor
timestamp: May 27, 2025, 10:11:40 pm
content: I use a high-current relay from AliX to turn on the inverter. They're available for 12V and 24V. The coil consumes about 1.8 W.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 28, 2025, 04:50:35 am
content: Just put a power resistor across one of the relays to make a precharge circuit?

author: dietert1
author_rank: Super Contributor
timestamp: May 28, 2025, 07:06:18 am
content: The first post describes an application for a mosfet switch. Mosfets exist with much lower on resistance than relays have after some cycles. Maybe relay price was mentioned because the relay didn't last.That switch would need two mosfets in order to provide complete control (unipolar on-off and polarity protection). Modules with back-to-back mosfet pairs driven by photovoltaic optocouplers have been used to replace speaker protection relays in audio. The combination already provides slow turn on (some milliseconds). Proposal is 2x 1 milliOhm Rdson mosfets with a PVI1050 opto or similar.Regards, Dieter

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 28, 2025, 09:15:54 am
content: Quote from: dietert1 on May 28, 2025, 07:06:18 amif you're looking at PV optoisolators, also check out the Si8751, which uses RF capacitive coupling.