Planning on modifying a crappy HV PSU. Member feedback wanted. - Page 1
Forum section: electronics / projects

author: msat
author_rank: Regular Contributor
timestamp: December 23, 2023, 12:58:46 am
content: Hi guys. I recently bought a cenco 79572 5kV power supply. Cenco appears to have been one of those equipment manufacturers targeted at university science departments used in lab demonstrations, thus not the pinnacle of design and quality. I didn't have particularly high expectations for it based on the fact that the outputs are basic banana jacks, but it looked to be in very good condition for the era it came from (60s or 70s I assume) and it was cheap. I also assumed it would be basic so that I could easily fix/modify it. Anyway, once I got it and opened it up, it ended up being way sketchier than I imagined - issues I plan on addressing anyway.So in it's original form, it can only output a positive high voltage referenced to ground, but I also need the option to output a negative HV instead. Here's the circuit. Note: the primary side of the transformer in the schematic below is actually fed by a variable autotransformer."Interestingly", the diode is actually 2 selenium rectifiers in series. So yeah, kinda old. Anyway, I played with a few options on how to accomplish what I wanted, but most of them would require expensive-ish components, relative to the price I paid for this, as well as the value that I'll get out of it (I only have a few experiments in mind), I didn't think I could justify it. So I've been thinking about just replacing the rectifier will a HV silicon bridge circuit, let the secondary side float, and then reference the high or low side to ground externally via the banana jacks as necessary such as in the circuit below.This is the cheapest and most flexible route at the cost of some connectors (probably go with sheathed banana jacks), diodes, and resistors. Since I've never delved into anything with this kind of voltage, I wanted to make sure I wouldn't do anything too egregious here, so I'd appreciate your feedback. Should I possibly short both outputs to ground via some high-ish value resistors? Thanks!

author: Andy Chee
author_rank: Super Contributor
timestamp: December 23, 2023, 02:05:13 am
content: Caution.If the transformer is anything like microwave oven transformers, then the earthed end of the secondary is either;a) connected to the transformer core, orb) has weak electrical insulation in the vicinity of the lower layer windings of the secondary.In both cases, lifting the bottom connection by using a bridge rectifier as you have, may result in electrical breakdown and arcing to the transformer core/chassis.

author: msat
author_rank: Regular Contributor
timestamp: December 23, 2023, 02:53:26 am
content: The transformer has a UL sticker listing it as a transformer for gas tube signs. Miraculously, I found a PDF of the manufacturer's catalog dated 1947(!), with the specific part number (720-361) listed (https://www.novioljourneys.com/wp-content/uploads/2018/04/Jefferson-Neon-Trans-Catalog-46-47.pdf). Though it does look different than the one illustrated. Otherwise the specs in the catalog match the stamping on the unit. So it would appear that this transformer is designed to be able to leave the secondary floating, and therefore is suitable for use in the way I described?Just as an aside, this power supply is apparently much older than I expected. Harvard lists one in their inventory being from circa 1940! UCSC also has one and can be seen here:https://ucscphysicsdemo.sites.ucsc.edu/physics-5c6c-demos/electromagnetism/electron-velocity-selector-finding-em-ratio/