800V DC power supply fast acting electronic fuses - Page 1
Forum section: electronics / testgear

author: cowasaki
author_rank: Frequent Contributor
timestamp: July 12, 2021, 03:24:17 pm
content: Hello all.I design and build valve amplifiers and valve effects units and I am modifying a large high voltage power supply for use with prototyping. I’m looking for resettable fast acting high voltage fuses or contact breakers. The fuses need to be happy with 1KV (the main transformer has a maximum secondary of 850v so maybe ev 1.2Kv/1.6Kv might be better. 800x1.4= 1120V so maybe 1.2Kv is a minimum) and I would like to have a rotary switch to select the output current before the device cuts the high voltage supply. I could do with 50mA, 100mA, 150mA and 200mA….. 120/125mA would also be helpful.Does anybody know of fast and slow acting resettable fuses that match this?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 12, 2021, 03:44:22 pm
content: What exactly are you expecting as "fast acting"? With so little current, it's going to take seconds of fault current to open such a thing, I think.I'm not aware of resettable fuses at high voltage DC, but I haven't looked, maybe they're available.Might you be better served with a current limiting circuit or electronic fuse instead?Tim

author: Cymaphore
author_rank: Regular Contributor
timestamp: July 12, 2021, 04:16:34 pm
content: There areABB S800Smodels that go down to .5A, lower is difficult and more expensive. Other types of high performance circuit breakers are available as well.What is your source impedance and how much current is separated usually?(Sorry, I posted a link to the german datasheet of the MCB series at first, corrected that)

author: tszaboo
author_rank: Super Contributor
timestamp: July 12, 2021, 04:26:51 pm
content: Just use a resistor for current limit for such things, it is a lot easier to do.I dont think you will find resettable fuses for voltages above 100V.

author: cowasaki
author_rank: Frequent Contributor
timestamp: July 12, 2021, 05:00:56 pm
content: Basically I have a large transformer with several fixed outputs plus 3x 6.3V, 290V and some others plus a switchable high voltage output which was for 0-150,50-200,200-350 & 350-500 the 350-500 comes from an AC tap of about 800V.This absolute beast is regulated by 7 valves and 5 selenium rectifiers. I have already removed the selenium rectifiers and will be replacing them with silicon bridges but I would like to re-create the options in a more modern way with safety paramount. Most current limiting and regulator circuits I find are not happy with these voltages. In reality the voltage regulation isn't that important but a smoothly adjustable voltage in these ranges would be VERY useful. I don't want a normal current limiter circuit but more of a shutdown if the draw is too high. I also need an adjustable 0-250V. negative supply (this is what the 290V secondary is for.My ideal would be a complete rebuild if I can create the adjustable output and current cut off. Regulation would be nice but not paramount. If I can do that I would fit my mains variac inside as well to create a comprehensive power supply. I will replace the analogue gauge with AC voltage in, AC voltage OUT & AC current for the mains supply then voltage and current digital gauges plus the same for the negative supply. The final image is my AC mains supply at the moment so I would want to incorporate this.

author: cowasaki
author_rank: Frequent Contributor
timestamp: July 12, 2021, 05:14:10 pm
content: Quote from: T3sl4co1l on July 12, 2021, 03:44:22 pmI'm open to any option. An electronic fuse would be ideal. I was thinking of having the output go through a 1 or 10 ohm resistor to measure the current then use that voltage to trigger a FET to cut the DC supply

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 12, 2021, 06:01:45 pm
content: Might not be too bad to add current limiting to it as-is. R18 and R25 (and the cathode ballast resistors to their own extent) are series resistors in the output, could add a BJT across that to pull down the grid drive. Which... I think would have to be a PNP in this case, E and B across the resistor(s) (with a current limiting resistor to base, and maybe a voltage divider to set the current range), collector to V5 grid or something like that. Will have to see about loop gain and stability stuff of course. It's not exactly the classic BJT circuit, because the pass tube grids are below cathode, whereas the classic BJT circuit, the current-limiting transistor pulls down on the output bases that are above their emitters. And adding the extra tube to the loop (V5) adds more phase shift, and BJTs have a lot of gain so it could oscillate is all. Likely treatment would be a compensation R+C from collector to base.That, and something about reducing, limiting or eliminating the large output caps (C11, C12). Those will deliver a huge wallop, enough to destroy say, small resistors, let alone transistors or whatever. Maybe some ESR can be added, enough to be practical while not increasing output impedance that much; maybe the loop compensation can be tweaked to improve Zo, allowing a reduction in Cout. Maybe you just make a point never to run anything that depends on external capacitance, i.e. always has enough local bypass to do the job; it's not like the fly leads to the power supply are any help (amounting to a couple uH easily; not a problem at audio, but useless at MF+).I mean, if you're only ever using this on other tube equipment, it probably doesn't matter much, it'll tolerate shorts and sparks for the most part. Avoid shorting it into grids I suppose, but other than that, right? It's not really going to be much of a safety feature as the voltage is already way more than enough (medium voltage!) and the nominal current is way over lethal. And sparks don't do much to flesh besides cratering I think, it's the longer term current that stops hearts. (I mean, that looks like enough capacitance to be a problem in that way too, but it's not that the risk will be eliminated by removing it.)Limiting is more of a problem for long term operation (don't be a dummy and leave it shorted, check the meter from time to time?), or for semiconductors (which fail in mere microseconds). If you're working with those much, it'll likely prove a welcome feature.Or also, CC mode for testing like, zeners and MOVs, or reforming/leakage testing of caps. Arguably not a big deal as resistors do well enough; maybe a nice-to-have?Tim

author: Fungus
author_rank: Super Contributor
timestamp: July 12, 2021, 06:15:27 pm
content: Quote from: cowasaki on July 12, 2021, 05:14:10 pmThis.